N1570                      Committee Draft -- April 12, 2011          ISO/IEC 9899:201x




INTERNATIONAL STANDARD                         (C)ISO/IEC              ISO/IEC 9899:201x




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Programming languages -- C



                                       ABSTRACT



                     (Cover sheet to be provided by ISO Secretariat.)

This International Standard specifies the form and establishes the interpretation of
programs expressed in the programming language C. Its purpose is to promote
portability, reliability, maintainability, and efficient execution of C language programs on
a variety of computing systems.

Clauses are included that detail the C language itself and the contents of the C language
execution library. Annexes summarize aspects of both of them, and enumerate factors
that influence the portability of C programs.

Although this International Standard is intended to guide knowledgeable C language
programmers as well as implementors of C language translation systems, the document
itself is not designed to serve as a tutorial.

Recipients of this draft are invited to submit, with their comments, notification of any
relevant patent rights of which they are aware and to provide supporting documentation.

Changes from the previous draft (N1539) are indicated by ''diff marks'' in the right
margin: deleted text is marked with ''*'', new or changed text with '' ''.


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Foreword

1 ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are member of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work.

2 International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. This International Standard was drafted in accordance with the fifth edition (2004).

3 In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75% of the national bodies casting a vote.

4 Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

5 This International Standard was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 22, Programming languages, their environments and system software interfaces. The Working Group responsible for this standard (WG 14) maintains a site on the World Wide Web at http://www.open- std.org/JTC1/SC22/WG14/ containing additional information relevant to this standard such as a Rationale for many of the decisions made during its preparation and a log of Defect Reports and Responses.

6 This third edition cancels and replaces the second edition, ISO/IEC 9899:1999, as corrected by ISO/IEC 9899:1999/Cor 1:2001, ISO/IEC 9899:1999/Cor 2:2004, and ISO/IEC 9899:1999/Cor 3:2007. Major changes from the previous edition include:

7 Major changes in the second edition included:

8 Annexes D, F, G, K, and L form a normative part of this standard; annexes A, B, C, E, H, I, J, the bibliography, and the index are for information only. In accordance with Part 2 of the ISO/IEC Directives, this foreword, the introduction, notes, footnotes, and examples are also for information only.

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Introduction

1 With the introduction of new devices and extended character sets, new features may be added to this International Standard. Subclauses in the language and library clauses warn implementors and programmers of usages which, though valid in themselves, may conflict with future additions.

2 Certain features are obsolescent, which means that they may be considered for withdrawal in future revisions of this International Standard. They are retained because of their widespread use, but their use in new implementations (for implementation features) or new programs (for language [6.11] or library features [7.31]) is discouraged.

3 This International Standard is divided into four major subdivisions:

4 Examples are provided to illustrate possible forms of the constructions described. Footnotes are provided to emphasize consequences of the rules described in that subclause or elsewhere in this International Standard. References are used to refer to other related subclauses. Recommendations are provided to give advice or guidance to implementors. Annexes provide additional information and summarize the information contained in this International Standard. A bibliography lists documents that were referred to during the preparation of the standard.

5 The language clause (clause 6) is derived from ''The C Reference Manual''.

6 The library clause (clause 7) is based on the 1984 /usr/group Standard.

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Programming languages -- C

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1. Scope

1 This International Standard specifies the form and establishes the interpretation of programs written in the C programming language.1) It specifies

2 This International Standard does not specify

Footnotes

1) This International Standard is designed to promote the portability of C programs among a variety of data-processing systems. It is intended for use by implementors and programmers.

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2. Normative references

1 The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

2 ISO 31-11:1992, Quantities and units -- Part 11: Mathematical signs and symbols for use in the physical sciences and technology.

3 ISO/IEC 646, Information technology -- ISO 7-bit coded character set for information interchange.

4 ISO/IEC 2382-1:1993, Information technology -- Vocabulary -- Part 1: Fundamental terms.

5 ISO 4217, Codes for the representation of currencies and funds.

6 ISO 8601, Data elements and interchange formats -- Information interchange -- Representation of dates and times.

7 ISO/IEC 10646 (all parts), Information technology -- Universal Multiple-Octet Coded Character Set (UCS).

8 IEC 60559:1989, Binary floating-point arithmetic for microprocessor systems (previously designated IEC 559:1989).

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3. Terms, definitions, and symbols

1 For the purposes of this International Standard, the following definitions apply. Other terms are defined where they appear in italic type or on the left side of a syntax rule. Terms explicitly defined in this International Standard are not to be presumed to refer implicitly to similar terms defined elsewhere. Terms not defined in this International Standard are to be interpreted according to ISO/IEC 2382-1. Mathematical symbols not defined in this International Standard are to be interpreted according to ISO 31-11.

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3.1

1 access
<execution-time action> to read or modify the value of an object

2 NOTE 1 Where only one of these two actions is meant, ''read'' or ''modify'' is used.

3 NOTE 2 ''Modify'' includes the case where the new value being stored is the same as the previous value.

4 NOTE 3 Expressions that are not evaluated do not access objects.

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3.2

1 alignment
requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address

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3.3

1 argument
actual argument
actual parameter (deprecated)
expression in the comma-separated list bounded by the parentheses in a function call expression, or a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a function-like macro invocation

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3.4

1 behavior
external appearance or action

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3.4.1

1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made

2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.

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3.4.2

1 locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each implementation documents

2 EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lowercase Latin letters.

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3.4.3

1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

3 EXAMPLE An example of undefined behavior is the behavior on integer overflow.

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3.4.4

1 unspecified behavior
use of an unspecified value, or other behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance

2 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

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3.5

1 bit
unit of data storage in the execution environment large enough to hold an object that may have one of two values

2 NOTE It need not be possible to express the address of each individual bit of an object.

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3.6

1 byte
addressable unit of data storage large enough to hold any member of the basic character set of the execution environment

2 NOTE 1 It is possible to express the address of each individual byte of an object uniquely.

3 NOTE 2 A byte is composed of a contiguous sequence of bits, the number of which is implementation- defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit.

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3.7

1 character
<abstract> member of a set of elements used for the organization, control, or representation of data

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3.7.1

1 character
single-byte character <C> bit representation that fits in a byte

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3.7.2

1 multibyte character
sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment

2 NOTE The extended character set is a superset of the basic character set.

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3.7.3

1 wide character
value representable by an object of type wchar_t, capable of representing any character in the current locale

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3.8

1 constraint
restriction, either syntactic or semantic, by which the exposition of language elements is to be interpreted

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3.9

1 correctly rounded result
representation in the result format that is nearest in value, subject to the current rounding mode, to what the result would be given unlimited range and precision

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3.10

1 diagnostic message
message belonging to an implementation-defined subset of the implementation's message output

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3.11

1 forward reference
reference to a later subclause of this International Standard that contains additional information relevant to this subclause

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3.12

1 implementation
particular set of software, running in a particular translation environment under particular control options, that performs translation of programs for, and supports execution of functions in, a particular execution environment

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3.13

1 implementation limit
restriction imposed upon programs by the implementation

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3.14

1 memory location
either an object of scalar type, or a maximal sequence of adjacent bit-fields all having nonzero width

2 NOTE 1 Two threads of execution can update and access separate memory locations without interfering with each other.

3 NOTE 2 A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration. It is not safe to concurrently update two non-atomic bit-fields in the same structure if all members declared between them are also (non-zero-length) bit-fields, no matter what the sizes of those intervening bit-fields happen to be.

4 EXAMPLE A structure declared as

          struct {
                char a;
                int b:5, c:11, :0, d:8;
                struct { int ee:8; } e;
          }
contains four separate memory locations: The member a, and bit-fields d and e.ee are each separate memory locations, and can be modified concurrently without interfering with each other. The bit-fields b and c together constitute the fourth memory location. The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be.

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3.15

1 object
region of data storage in the execution environment, the contents of which can represent values

2 NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.

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3.16

1 parameter
formal parameter formal argument (deprecated) object declared as part of a function declaration or definition that acquires a value on entry to the function, or an identifier from the comma-separated list bounded by the parentheses immediately following the macro name in a function-like macro definition

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3.17

1 recommended practice
specification that is strongly recommended as being in keeping with the intent of the standard, but that may be impractical for some implementations

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3.18

1 runtime-constraint
requirement on a program when calling a library function

2 NOTE 1 Despite the similar terms, a runtime-constraint is not a kind of constraint as defined by 3.8, and need not be diagnosed at translation time.

3 NOTE 2 Implementations that support the extensions in annex K are required to verify that the runtime- constraints for a library function are not violated by the program; see K.3.1.4.

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3.19

1 value
precise meaning of the contents of an object when interpreted as having a specific type

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3.19.1

1 implementation-defined value
unspecified value where each implementation documents how the choice is made

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3.19.2

1 indeterminate value
either an unspecified value or a trap representation

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3.19.3

1 unspecified value
valid value of the relevant type where this International Standard imposes no requirements on which value is chosen in any instance

2 NOTE An unspecified value cannot be a trap representation.

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3.19.4

1 trap representation
an object representation that need not represent a value of the object type

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3.19.5

1 perform a trap
interrupt execution of the program such that no further operations are performed

2 NOTE In this International Standard, when the word ''trap'' is not immediately followed by ''representation'', this is the intended usage.2)

Footnotes

2) For example, ''Trapping or stopping (if supported) is disabled...'' (F.8.2). Note that fetching a trap representation might perform a trap but is not required to (see 6.2.6.1).

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3.20

1 [^ x ^]
ceiling of x: the least integer greater than or equal to x

2 EXAMPLE [^2.4^] is 3, [^-2.4^] is -2.

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3.21

1 [_ x _]
floor of x: the greatest integer less than or equal to x

2 EXAMPLE [_2.4_] is 2, [_-2.4_] is -3.

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4. Conformance

1 In this International Standard, ''shall'' is to be interpreted as a requirement on an implementation or on a program; conversely, ''shall not'' is to be interpreted as a prohibition.

2 If a ''shall'' or ''shall not'' requirement that appears outside of a constraint or runtime- constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ''undefined behavior'' or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ''behavior that is undefined''.

3 A program that is correct in all other aspects, operating on correct data, containing unspecified behavior shall be a correct program and act in accordance with 5.1.2.3.

4 The implementation shall not successfully translate a preprocessing translation unit containing a #error preprocessing directive unless it is part of a group skipped by conditional inclusion.

5 A strictly conforming program shall use only those features of the language and library specified in this International Standard.3) It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit.

6 The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers <float.h>, <iso646.h>, <limits.h>, <stdalign.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>, and <stdnoreturn.h>. A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program.4)

7 A conforming program is one that is acceptable to a conforming implementation.5)

8 An implementation shall be accompanied by a document that defines all implementation- defined and locale-specific characteristics and all extensions.

Forward references: conditional inclusion (6.10.1), error directive (6.10.5), characteristics of floating types <float.h> (7.7), alternative spellings <iso646.h> (7.9), sizes of integer types <limits.h> (7.10), alignment <stdalign.h> (7.15), variable arguments <stdarg.h> (7.16), boolean type and values <stdbool.h> (7.18), common definitions <stddef.h> (7.19), integer types <stdint.h> (7.20), <stdnoreturn.h> (7.23).

Footnotes

3) A strictly conforming program can use conditional features (see 6.10.8.3) provided the use is guarded by an appropriate conditional inclusion preprocessing directive using the related macro. For example:

         #ifdef __STDC_IEC_559__ /* FE_UPWARD defined */
            /* ... */
            fesetround(FE_UPWARD);
            /* ... */
         #endif

4) This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard.

5) Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming programs may depend upon nonportable features of a conforming implementation.

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5. Environment

1 An implementation translates C source files and executes C programs in two data- processing-system environments, which will be called the translation environment and the execution environment in this International Standard. Their characteristics define and constrain the results of executing conforming C programs constructed according to the syntactic and semantic rules for conforming implementations.

Forward references: In this clause, only a few of many possible forward references have been noted.

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5.1 Conceptual models

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5.1.1 Translation environment

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5.1.1.1 Program structure

1 A C program need not all be translated at the same time. The text of the program is kept in units called source files, (or preprocessing files) in this International Standard. A source file together with all the headers and source files included via the preprocessing directive #include is known as a preprocessing translation unit. After preprocessing, a preprocessing translation unit is called a translation unit. Previously translated translation units may be preserved individually or in libraries. The separate translation units of a program communicate by (for example) calls to functions whose identifiers have external linkage, manipulation of objects whose identifiers have external linkage, or manipulation of data files. Translation units may be separately translated and then later linked to produce an executable program.

Forward references: linkages of identifiers (6.2.2), external definitions (6.9), preprocessing directives (6.10).

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5.1.1.2 Translation phases

1 The precedence among the syntax rules of translation is specified by the following phases.6)

  1. Physical source file multibyte characters are mapped, in an implementation- defined manner, to the source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences are replaced by corresponding single-character internal representations.
  2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.
  3. The source file is decomposed into preprocessing tokens7) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined.
  4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.
  5. Each source character set member and escape sequence in character constants and string literals is converted to the corresponding member of the execution character set; if there is no corresponding member, it is converted to an implementation- defined member other than the null (wide) character.8)
  6. Adjacent string literal tokens are concatenated.
  7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.
  8. All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.

Forward references: universal character names (6.4.3), lexical elements (6.4), preprocessing directives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).

Footnotes

6) Implementations shall behave as if these separate phases occur, even though many are typically folded together in practice. Source files, translation units, and translated translation units need not necessarily be stored as files, nor need there be any one-to-one correspondence between these entities and any external representation. The description is conceptual only, and does not specify any particular implementation.

7) As described in 6.4, the process of dividing a source file's characters into preprocessing tokens is context-dependent. For example, see the handling of < within a #include preprocessing directive.

8) An implementation need not convert all non-corresponding source characters to the same execution character.

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5.1.1.3 Diagnostics

1 A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. Diagnostic messages need not be produced in other circumstances.9)

2 EXAMPLE An implementation shall issue a diagnostic for the translation unit:

          char i;
          int i;
because in those cases where wording in this International Standard describes the behavior for a construct as being both a constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.

Footnotes

9) The intent is that an implementation should identify the nature of, and where possible localize, each violation. Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may also successfully translate an invalid program.

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5.1.2 Execution environments

1 Two execution environments are defined: freestanding and hosted. In both cases, program startup occurs when a designated C function is called by the execution environment. All objects with static storage duration shall be initialized (set to their initial values) before program startup. The manner and timing of such initialization are otherwise unspecified. Program termination returns control to the execution environment.

Forward references: storage durations of objects (6.2.4), initialization (6.7.9).

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5.1.2.1 Freestanding environment

1 In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

2 The effect of program termination in a freestanding environment is implementation- defined.

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5.1.2.2 Hosted environment

1 A hosted environment need not be provided, but shall conform to the following specifications if present.

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5.1.2.2.1 Program startup

1 The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

         int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):
         int main(int argc, char *argv[]) { /* ... */ }
or equivalent;10) or in some other implementation-defined manner.

2 If they are declared, the parameters to the main function shall obey the following constraints:

Footnotes

10) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char ** argv, and so on.

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5.1.2.2.2 Program execution

1 In a hosted environment, a program may use all the functions, macros, type definitions, and objects described in the library clause (clause 7).

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5.1.2.2.3 Program termination

1 If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument;11) reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

Forward references: definition of terms (7.1.1), the exit function (7.22.4.4).

Footnotes

11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main will have ended in the former case, even where they would not have in the latter.

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5.1.2.3 Program execution

1 The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant.

2 Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects,12) which are changes in the state of the execution environment. Evaluation of an expression in general includes both value computations and initiation of side effects. Value computation for an lvalue expression includes determining the identity of the designated object.

3 Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a single thread, which induces a partial order among those evaluations. Given any two evaluations A and B, if A is sequenced before B, then the execution of A shall precede the execution of B. (Conversely, if A is sequenced before B, then B is sequenced after A.) If A is not sequenced before or after B, then A and B are unsequenced. Evaluations A and B are indeterminately sequenced when A is sequenced either before or after B, but it is unspecified which.13) The presence of a sequence point between the evaluation of expressions A and B implies that every value computation and side effect associated with A is sequenced before every value computation and side effect associated with B. (A summary of the sequence points is given in annex C.)

4 In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or accessing a volatile object).

5 When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, as is the state of the floating-point environment. The value of any object modified by the handler that is neither a lock-free atomic object nor of type volatile sig_atomic_t becomes indeterminate when the handler exits, as does the state of the floating-point environment if it is modified by the handler and not restored to its original state.

6 The least requirements on a conforming implementation are:

This is the observable behavior of the program.

7 What constitutes an interactive device is implementation-defined.

8 More stringent correspondences between abstract and actual semantics may be defined by each implementation.

9 EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual semantics: at every sequence point, the values of the actual objects would agree with those specified by the abstract semantics. The keyword volatile would then be redundant.

10 Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual semantics would agree with the abstract semantics only when making function calls across translation unit boundaries. In such an implementation, at the time of each function entry and function return where the calling function and the called function are in different translation units, the values of all externally linked objects and of all objects accessible via pointers therein would agree with the abstract semantics. Furthermore, at the time of each such function entry the values of the parameters of the called function and of all objects accessible via pointers therein would agree with the abstract semantics. In this type of implementation, objects referred to by interrupt service routines activated by the signal function would require explicit specification of volatile storage, as well as other implementation-defined restrictions.

11 EXAMPLE 2 In executing the fragment

          char c1, c2;
          /* ... */
          c1 = c1 + c2;
the ''integer promotions'' require that the abstract machine promote the value of each variable to int size and then add the two ints and truncate the sum. Provided the addition of two chars can be done without overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only produce the same result, possibly omitting the promotions.

12 EXAMPLE 3 Similarly, in the fragment

          float f1, f2;
          double d;
          /* ... */
          f1 = f2 * d;
the multiplication may be executed using single-precision arithmetic if the implementation can ascertain that the result would be the same as if it were executed using double-precision arithmetic (for example, if d were replaced by the constant 2.0, which has type double).

13 EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate semantics. Values are independent of whether they are represented in a register or in memory. For example, an implicit spilling of a register is not permitted to alter the value. Also, an explicit store and load is required to round to the precision of the storage type. In particular, casts and assignments are required to perform their specified conversion. For the fragment

          double d1, d2;
          float f;
          d1 = f = expression;
          d2 = (float) expression;
the values assigned to d1 and d2 are required to have been converted to float.

14 EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as range. The implementation cannot generally apply the mathematical associative rules for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of overflow and underflow. Likewise, implementations cannot generally replace decimal constants in order to rearrange expressions. In the following fragment, rearrangements suggested by mathematical rules for real numbers are often not valid (see F.9).

          double x, y, z;
          /* ... */
          x = (x * y) * z;            //   not equivalent to x   *= y * z;
          z = (x - y) + y ;           //   not equivalent to z   = x;
          z = x + x * y;              //   not equivalent to z   = x * (1.0 + y);
          y = x / 5.0;                //   not equivalent to y   = x * 0.2;

15 EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment

          int a, b;
          /* ... */
          a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
          a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in which overflows produce an explicit trap and in which the range of values representable by an int is [-32768, +32767], the implementation cannot rewrite this expression as
          a = ((a + b) + 32765);
since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce a trap while the original expression would not; nor can the expression be rewritten either as
          a = ((a + 32765) + b);
or
          a = (a + (b + 32765));
since the values for a and b might have been, respectively, 4 and -8 or -17 and 12. However, on a machine in which overflow silently generates some value and where positive and negative overflows cancel, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur.

16 EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the following fragment

          #include <stdio.h>
          int sum;
          char *p;
          /* ... */
          sum = sum * 10 - '0' + (*p++ = getchar());
the expression statement is grouped as if it were written as
          sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));
but the actual increment of p can occur at any time between the previous sequence point and the next sequence point (the ;), and the call to getchar can occur at any point prior to the need of its returned value.

Forward references: expressions (6.5), type qualifiers (6.7.3), statements (6.8), floating- point environment <fenv.h> (7.6), the signal function (7.14), files (7.21.3).

Footnotes

12) The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status flags and control modes. Floating-point operations implicitly set the status flags; modes affect result values of floating-point operations. Implementations that support such floating-point state are required to regard changes to it as side effects -- see annex F for details. The floating-point environment library <fenv.h> provides a programming facility for indicating when these side effects matter, freeing the implementations in other cases.

13) The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations cannot interleave, but can be executed in any order.

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5.1.2.4 Multi-threaded executions and data races

1 Under a hosted implementation, a program can have more than one thread of execution (or thread) running concurrently. The execution of each thread proceeds as defined by the remainder of this standard. The execution of the entire program consists of an execution of all of its threads.14) Under a freestanding implementation, it is implementation-defined whether a program can have more than one thread of execution.

2 The value of an object visible to a thread T at a particular point is the initial value of the object, a value stored in the object by T , or a value stored in the object by another thread, according to the rules below.

3 NOTE 1 In some cases, there may instead be undefined behavior. Much of this section is motivated by the desire to support atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for more restricted programs.

4 Two expression evaluations conflict if one of them modifies a memory location and the other one reads or modifies the same memory location.

5 The library defines a number of atomic operations (7.17) and operations on mutexes (7.26.4) that are specially identified as synchronization operations. These operations play a special role in making assignments in one thread visible to another. A synchronization operation on one or more memory locations is either an acquire operation, a release operation, both an acquire and release operation, or a consume operation. A synchronization operation without an associated memory location is a fence and can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write operations, which have special characteristics.

6 NOTE 2 For example, a call that acquires a mutex will perform an acquire operation on the locations composing the mutex. Correspondingly, a call that releases the same mutex will perform a release operation on those same locations. Informally, performing a release operation on A forces prior side effects on other memory locations to become visible to other threads that later perform an acquire or consume operation on A. We do not include relaxed atomic operations as synchronization operations although, like synchronization operations, they cannot contribute to data races.

7 All modifications to a particular atomic object M occur in some particular total order, called the modification order of M. If A and B are modifications of an atomic object M, and A happens before B, then A shall precede B in the modification order of M, which is defined below.

8 NOTE 3 This states that the modification orders must respect the ''happens before'' relation.

9 NOTE 4 There is a separate order for each atomic object. There is no requirement that these can be combined into a single total order for all objects. In general this will be impossible since different threads may observe modifications to different variables in inconsistent orders.

10 A release sequence headed by a release operation A on an atomic object M is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first operation is A and every subsequent operation either is performed by the same thread that performed the release or is an atomic read-modify-write operation.

11 Certain library calls synchronize with other library calls performed by another thread. In particular, an atomic operation A that performs a release operation on an object M synchronizes with an atomic operation B that performs an acquire operation on M and reads a value written by any side effect in the release sequence headed by A.

12 NOTE 5 Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a requirement would sometimes interfere with efficient implementation.

13 NOTE 6 The specifications of the synchronization operations define when one reads the value written by another. For atomic variables, the definition is clear. All operations on a given mutex occur in a single total order. Each mutex acquisition ''reads the value written'' by the last mutex release.

14 An evaluation A carries a dependency 15) to an evaluation B if:

15 An evaluation A is dependency-ordered before16) an evaluation B if:

16 An evaluation A inter-thread happens before an evaluation B if A synchronizes with B, A is dependency-ordered before B, or, for some evaluation X:

17 NOTE 7 The ''inter-thread happens before'' relation describes arbitrary concatenations of ''sequenced before'', ''synchronizes with'', and ''dependency-ordered before'' relationships, with two exceptions. The first exception is that a concatenation is not permitted to end with ''dependency-ordered before'' followed by ''sequenced before''. The reason for this limitation is that a consume operation participating in a ''dependency-ordered before'' relationship provides ordering only with respect to operations to which this consume operation actually carries a dependency. The reason that this limitation applies only to the end of such a concatenation is that any subsequent release operation will provide the required ordering for a prior consume operation. The second exception is that a concatenation is not permitted to consist entirely of ''sequenced before''. The reasons for this limitation are (1) to permit ''inter-thread happens before'' to be transitively closed and (2) the ''happens before'' relation, defined below, provides for relationships consisting entirely of ''sequenced before''.

18 An evaluation A happens before an evaluation B if A is sequenced before B or A inter- thread happens before B.

19 A visible side effect A on an object M with respect to a value computation B of M satisfies the conditions:

The value of a non-atomic scalar object M, as determined by evaluation B, shall be the value stored by the visible side effect A.

20 NOTE 8 If there is ambiguity about which side effect to a non-atomic object is visible, then there is a data race and the behavior is undefined.

21 NOTE 9 This states that operations on ordinary variables are not visibly reordered. This is not actually detectable without data races, but it is necessary to ensure that data races, as defined here, and with suitable restrictions on the use of atomics, correspond to data races in a simple interleaved (sequentially consistent) execution.

22 The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every subsequent side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B. Furthermore, if a value computation A of an atomic object M happens before a value computation B of M, and the value computed by A corresponds to the value stored by side effect X, then the value computed by B shall either equal the value computed by A, or be the value stored by side effect Y , where Y follows X in the modification order of M.

23 NOTE 10 This effectively disallows compiler reordering of atomic operations to a single object, even if both operations are ''relaxed'' loads. By doing so, we effectively make the ''cache coherence'' guarantee provided by most hardware available to C atomic operations.

24 NOTE 11 The visible sequence depends on the ''happens before'' relation, which in turn depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the ''happens before'' relation derived as described above, satisfy the resulting constraints as imposed here.

25 The execution of a program contains a data race if it contains two conflicting actions in different threads, at least one of which is not atomic, and neither happens before the other. Any such data race results in undefined behavior.

26 NOTE 12 It can be shown that programs that correctly use simple mutexes and memory_order_seq_cst operations to prevent all data races, and use no other synchronization operations, behave as though the operations executed by their constituent threads were simply interleaved, with each value computation of an object being the last value stored in that interleaving. This is normally referred to as ''sequential consistency''. However, this applies only to data-race-free programs, and data- race-free programs cannot observe most program transformations that do not change single-threaded program semantics. In fact, most single-threaded program transformations continue to be allowed, since any program that behaves differently as a result must contain undefined behavior.

27 NOTE 13 Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. We also generally preclude reordering of atomic loads in cases in which the atomics in question may alias, since this may violate the "visible sequence" rules.

28 NOTE 14 Transformations that introduce a speculative read of a potentially shared memory location may not preserve the semantics of the program as defined in this standard, since they potentially introduce a data race. However, they are typically valid in the context of an optimizing compiler that targets a specific machine with well-defined semantics for data races. They would be invalid for a hypothetical machine that is not tolerant of races or provides hardware race detection.

Footnotes

14) The execution can usually be viewed as an interleaving of all of the threads. However, some kinds of atomic operations, for example, allow executions inconsistent with a simple interleaving as described below.

15) The ''carries a dependency'' relation is a subset of the ''sequenced before'' relation, and is similarly strictly intra-thread.

16) The ''dependency-ordered before'' relation is analogous to the ''synchronizes with'' relation, but uses release/consume in place of release/acquire.

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5.2 Environmental considerations

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5.2.1 Character sets

1 Two sets of characters and their associated collating sequences shall be defined: the set in which source files are written (the source character set), and the set interpreted in the execution environment (the execution character set). Each set is further divided into a basic character set, whose contents are given by this subclause, and a set of zero or more locale-specific members (which are not members of the basic character set) called extended characters. The combined set is also called the extended character set. The values of the members of the execution character set are implementation-defined.

2 In a character constant or string literal, members of the execution character set shall be represented by corresponding members of the source character set or by escape sequences consisting of the backslash \ followed by one or more characters. A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string.

3 Both the basic source and basic execution character sets shall have the following members: the 26 uppercase letters of the Latin alphabet

         A   B   C   D   E   F   G   H   I   J   K   L   M
         N   O   P   Q   R   S   T   U   V   W   X   Y   Z
the 26 lowercase letters of the Latin alphabet
         a   b   c   d   e   f   g   h   i   j   k   l   m
         n   o   p   q   r   s   t   u   v   w   x   y   z
the 10 decimal digits
         0   1   2   3   4   5   6   7   8   9
the following 29 graphic characters
         !   "   #   %   &   '   (   )   *   +   ,   -   .   /   :
         ;   <   =   >   ?   [   \   ]   ^   _   {   |   }   ~
the space character, and control characters representing horizontal tab, vertical tab, and form feed. The representation of each member of the source and execution basic character sets shall fit in a byte. In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous. In source files, there shall be some way of indicating the end of each line of text; this International Standard treats such an end-of-line indicator as if it were a single new-line character. In the basic execution character set, there shall be control characters representing alert, backspace, carriage return, and new line. If any other characters are encountered in a source file (except in an identifier, a character constant, a string literal, a header name, a comment, or a preprocessing token that is never converted to a token), the behavior is undefined.

4 A letter is an uppercase letter or a lowercase letter as defined above; in this International Standard the term does not include other characters that are letters in other alphabets.

5 The universal character name construct provides a way to name other characters.

Forward references: universal character names (6.4.3), character constants (6.4.4.4), preprocessing directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).

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5.2.1.1 Trigraph sequences

1 Before any other processing takes place, each occurrence of one of the following sequences of three characters (called trigraph sequences17)) is replaced with the corresponding single character.

        ??=      #                       ??)      ]                       ??!     |
        ??(      [                       ??'      ^                       ??>     }
        ??/      \                       ??<      {                       ??-     ~
No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed above is not changed.

2 EXAMPLE 1

           ??=define arraycheck(a, b) a??(b??) ??!??! b??(a??)
becomes
           #define arraycheck(a, b) a[b] || b[a]

3 EXAMPLE 2 The following source line

           printf("Eh???/n");
becomes (after replacement of the trigraph sequence ??/)
           printf("Eh?\n");

Footnotes

17) The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as described in ISO/IEC 646, which is a subset of the seven-bit US ASCII code set.

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5.2.1.2 Multibyte characters

1 The source character set may contain multibyte characters, used to represent members of the extended character set. The execution character set may also contain multibyte characters, which need not have the same encoding as for the source character set. For both character sets, the following shall hold:

2 For source files, the following shall hold:

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5.2.2 Character display semantics

1 The active position is that location on a display device where the next character output by the fputc function would appear. The intent of writing a printing character (as defined by the isprint function) to a display device is to display a graphic representation of that character at the active position and then advance the active position to the next position on the current line. The direction of writing is locale-specific. If the active position is at the final position of a line (if there is one), the behavior of the display device is unspecified.

2 Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows:

\a
(alert) Produces an audible or visible alert without changing the active position.
\b
(backspace) Moves the active position to the previous position on the current line. If the active position is at the initial position of a line, the behavior of the display device is unspecified.
\f
(form feed) Moves the active position to the initial position at the start of the next logical page.
\n
(new line) Moves the active position to the initial position of the next line.
\r
(carriage return) Moves the active position to the initial position of the current line.
\t
(horizontal tab) Moves the active position to the next horizontal tabulation position on the current line. If the active position is at or past the last defined horizontal tabulation position, the behavior of the display device is unspecified.
\v
(vertical tab) Moves the active position to the initial position of the next vertical tabulation position. If the active position is at or past the last defined vertical tabulation position, the behavior of the display device is unspecified.

3 Each of these escape sequences shall produce a unique implementation-defined value which can be stored in a single char object. The external representations in a text file need not be identical to the internal representations, and are outside the scope of this International Standard.

Forward references: the isprint function (7.4.1.8), the fputc function (7.21.7.3).

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5.2.3 Signals and interrupts

1 Functions shall be implemented such that they may be interrupted at any time by a signal, or may be called by a signal handler, or both, with no alteration to earlier, but still active, invocations' control flow (after the interruption), function return values, or objects with automatic storage duration. All such objects shall be maintained outside the function image (the instructions that compose the executable representation of a function) on a per-invocation basis.

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5.2.4 Environmental limits

1 Both the translation and execution environments constrain the implementation of language translators and libraries. The following summarizes the language-related environmental limits on a conforming implementation; the library-related limits are discussed in clause 7.

Contents

5.2.4.1 Translation limits

1 The implementation shall be able to translate and execute at least one program that contains at least one instance of every one of the following limits:18)

Footnotes

18) Implementations should avoid imposing fixed translation limits whenever possible.

19) See ''future language directions'' (6.11.3).

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5.2.4.2 Numerical limits

1 An implementation is required to document all the limits specified in this subclause, which are specified in the headers <limits.h> and <float.h>. Additional limits are specified in <stdint.h>.

Forward references: integer types <stdint.h> (7.20).

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5.2.4.2.1 Sizes of integer types <limits.h>

1 The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.

2 If the value of an object of type char is treated as a signed integer when used in an expression, the value of CHAR_MIN shall be the same as that of SCHAR_MIN and the value of CHAR_MAX shall be the same as that of SCHAR_MAX. Otherwise, the value of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX.20) The value UCHAR_MAX shall equal 2CHAR_BIT - 1.

Forward references: representations of types (6.2.6), conditional inclusion (6.10.1).

Footnotes

20) See 6.2.5.

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5.2.4.2.2 Characteristics of floating types <float.h>

1 The characteristics of floating types are defined in terms of a model that describes a representation of floating-point numbers and values that provide information about an implementation's floating-point arithmetic.21) The following parameters are used to define the model for each floating-point type:

        s          sign ((+-)1)
        b          base or radix of exponent representation (an integer > 1)
        e          exponent (an integer between a minimum emin and a maximum emax )
        p          precision (the number of base-b digits in the significand)
        fk         nonnegative integers less than b (the significand digits)

2 A floating-point number (x) is defined by the following model:

                    p
        x = s be (Sum) fk b-k ,   emin <= e <= emax
                   k=1

3 In addition to normalized floating-point numbers ( f1 > 0 if x != 0), floating types may be able to contain other kinds of floating-point numbers, such as subnormal floating-point numbers (x != 0, e = emin , f1 = 0) and unnormalized floating-point numbers (x != 0, e > emin , f1 = 0), and values that are not floating-point numbers, such as infinities and NaNs. A NaN is an encoding signifying Not-a-Number. A quiet NaN propagates through almost every arithmetic operation without raising a floating-point exception; a signaling NaN generally raises a floating-point exception when occurring as an arithmetic operand.22)

4 An implementation may give zero and values that are not floating-point numbers (such as infinities and NaNs) a sign or may leave them unsigned. Wherever such values are unsigned, any requirement in this International Standard to retrieve the sign shall produce an unspecified sign, and any requirement to set the sign shall be ignored.

5 The minimum range of representable values for a floating type is the most negative finite floating-point number representable in that type through the most positive finite floating- point number representable in that type. In addition, if negative infinity is representable in a type, the range of that type is extended to all negative real numbers; likewise, if positive infinity is representable in a type, the range of that type is extended to all positive real numbers.

6 The accuracy of the floating-point operations (+, -, *, /) and of the library functions in <math.h> and <complex.h> that return floating-point results is implementation- defined, as is the accuracy of the conversion between floating-point internal representations and string representations performed by the library functions in <stdio.h>, <stdlib.h>, and <wchar.h>. The implementation may state that the accuracy is unknown.

7 All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant expressions suitable for use in #if preprocessing directives; all floating values shall be constant expressions. All except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX, and FLT_ROUNDS have separate names for all three floating-point types. The floating-point model representation is provided for all values except FLT_EVAL_METHOD and FLT_ROUNDS.

8 The rounding mode for floating-point addition is characterized by the implementation- defined value of FLT_ROUNDS:23)

       -1      indeterminable
        0      toward zero
        1      to nearest
        2      toward positive infinity
        3      toward negative infinity
All other values for FLT_ROUNDS characterize implementation-defined rounding behavior.

9 Except for assignment and cast (which remove all extra range and precision), the values yielded by operators with floating operands and values subject to the usual arithmetic conversions and of floating constants are evaluated to a format whose range and precision may be greater than required by the type. The use of evaluation formats is characterized by the implementation-defined value of FLT_EVAL_METHOD:24)

        -1        indeterminable;
         0        evaluate all operations and constants just to the range and precision of the
                  type;
         1        evaluate operations and constants of type float and double to the
                  range and precision of the double type, evaluate long double
                  operations and constants to the range and precision of the long double
                  type;
         2        evaluate all operations and constants to the range and precision of the
                  long double type.
All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior.

10 The presence or absence of subnormal numbers is characterized by the implementation- defined values of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and LDBL_HAS_SUBNORM:

        -1       indeterminable25)
         0       absent26) (type does not support subnormal numbers)
         1       present (type does support subnormal numbers)

11 The values given in the following list shall be replaced by constant expressions with implementation-defined values that are greater or equal in magnitude (absolute value) to those shown, with the same sign:

12 The values given in the following list shall be replaced by constant expressions with implementation-defined values that are greater than or equal to those shown:

13 The values given in the following list shall be replaced by constant expressions with implementation-defined (positive) values that are less than or equal to those shown:

Recommended practice

14 Conversion from (at least) double to decimal with DECIMAL_DIG digits and back should be the identity function.

15 EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum requirements of this International Standard, and the appropriate values in a <float.h> header for type float:

                    6
       x = s 16e (Sum) fk 16-k ,   -31 <= e <= +32
                   k=1
         FLT_RADIX                                    16
         FLT_MANT_DIG                                  6
         FLT_EPSILON                     9.53674316E-07F
         FLT_DECIMAL_DIG                               9
         FLT_DIG                                       6
         FLT_MIN_EXP                                 -31
         FLT_MIN                         2.93873588E-39F
         FLT_MIN_10_EXP                              -38
         FLT_MAX_EXP                                 +32
         FLT_MAX                         3.40282347E+38F
         FLT_MAX_10_EXP                              +38

16 EXAMPLE 2 The following describes floating-point representations that also meet the requirements for single-precision and double-precision numbers in IEC 60559,28) and the appropriate values in a <float.h> header for types float and double:

                   24
       xf = s 2e (Sum) fk 2-k ,   -125 <= e <= +128
                   k=1
                   53
       xd = s 2e (Sum) fk 2-k ,   -1021 <= e <= +1024
                   k=1
         FLT_RADIX                                     2
         DECIMAL_DIG                                  17
         FLT_MANT_DIG                                 24
         FLT_EPSILON                     1.19209290E-07F // decimal constant
         FLT_EPSILON                            0X1P-23F // hex constant
         FLT_DECIMAL_DIG                               9
         FLT_DIG                             6
         FLT_MIN_EXP                      -125
         FLT_MIN               1.17549435E-38F               //   decimal constant
         FLT_MIN                     0X1P-126F               //   hex constant
         FLT_TRUE_MIN          1.40129846E-45F               //   decimal constant
         FLT_TRUE_MIN                0X1P-149F               //   hex constant
         FLT_HAS_SUBNORM                     1
         FLT_MIN_10_EXP                    -37
         FLT_MAX_EXP                      +128
         FLT_MAX               3.40282347E+38F               // decimal constant
         FLT_MAX               0X1.fffffeP127F               // hex constant
         FLT_MAX_10_EXP                    +38
         DBL_MANT_DIG                       53
         DBL_EPSILON    2.2204460492503131E-16               // decimal constant
         DBL_EPSILON                   0X1P-52               // hex constant
         DBL_DECIMAL_DIG                    17
         DBL_DIG                            15
         DBL_MIN_EXP                     -1021
         DBL_MIN      2.2250738585072014E-308                //   decimal constant
         DBL_MIN                     0X1P-1022               //   hex constant
         DBL_TRUE_MIN 4.9406564584124654E-324                //   decimal constant
         DBL_TRUE_MIN                0X1P-1074               //   hex constant
         DBL_HAS_SUBNORM                     1
         DBL_MIN_10_EXP                   -307
         DBL_MAX_EXP                     +1024
         DBL_MAX      1.7976931348623157E+308                // decimal constant
         DBL_MAX        0X1.fffffffffffffP1023               // hex constant
         DBL_MAX_10_EXP                   +308
If a type wider than double were supported, then DECIMAL_DIG would be greater than 17. For example, if the widest type were to use the minimal-width IEC 60559 double-extended format (64 bits of precision), then DECIMAL_DIG would be 21.

Forward references: conditional inclusion (6.10.1), complex arithmetic <complex.h> (7.3), extended multibyte and wide character utilities <wchar.h> (7.29), floating-point environment <fenv.h> (7.6), general utilities <stdlib.h> (7.22), input/output <stdio.h> (7.21), mathematics <math.h> (7.12).

Footnotes

21) The floating-point model is intended to clarify the description of each floating-point characteristic and does not require the floating-point arithmetic of the implementation to be identical.

22) IEC 60559:1989 specifies quiet and signaling NaNs. For implementations that do not support IEC 60559:1989, the terms quiet NaN and signaling NaN are intended to apply to encodings with similar behavior.

23) Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through the function fesetround in <fenv.h>.

24) The evaluation method determines evaluation formats of expressions involving all floating types, not just real types. For example, if FLT_EVAL_METHOD is 1, then the product of two float _Complex operands is represented in the double _Complex format, and its parts are evaluated to double.

25) Characterization as indeterminable is intended if floating-point operations do not consistently interpret subnormal representations as zero, nor as nonzero.

26) Characterization as absent is intended if no floating-point operations produce subnormal results from non-subnormal inputs, even if the type format includes representations of subnormal numbers.

27) If the presence or absence of subnormal numbers is indeterminable, then the value is intended to be a positive number no greater than the minimum normalized positive number for the type.

28) The floating-point model in that standard sums powers of b from zero, so the values of the exponent limits are one less than shown here.

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6. Language

Contents

6.1 Notation

1 In the syntax notation used in this clause, syntactic categories (nonterminals) are indicated by italic type, and literal words and character set members (terminals) by bold type. A colon (:) following a nonterminal introduces its definition. Alternative definitions are listed on separate lines, except when prefaced by the words ''one of''. An optional symbol is indicated by the subscript ''opt'', so that

          { expressionopt }
indicates an optional expression enclosed in braces.

2 When syntactic categories are referred to in the main text, they are not italicized and words are separated by spaces instead of hyphens.

3 A summary of the language syntax is given in annex A.

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6.2 Concepts

Contents

6.2.1 Scopes of identifiers

1 An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter. The same identifier can denote different entities at different points in the program. A member of an enumeration is called an enumeration constant. Macro names and macro parameters are not considered further here, because prior to the semantic phase of program translation any occurrences of macro names in the source file are replaced by the preprocessing token sequences that constitute their macro definitions.

2 For each different entity that an identifier designates, the identifier is visible (i.e., can be used) only within a region of program text called its scope. Different entities designated by the same identifier either have different scopes, or are in different name spaces. There are four kinds of scopes: function, file, block, and function prototype. (A function prototype is a declaration of a function that declares the types of its parameters.)

3 A label name is the only kind of identifier that has function scope. It can be used (in a goto statement) anywhere in the function in which it appears, and is declared implicitly by its syntactic appearance (followed by a : and a statement).

4 Every other identifier has scope determined by the placement of its declaration (in a declarator or type specifier). If the declarator or type specifier that declares the identifier appears outside of any block or list of parameters, the identifier has file scope, which terminates at the end of the translation unit. If the declarator or type specifier that declares the identifier appears inside a block or within the list of parameter declarations in a function definition, the identifier has block scope, which terminates at the end of the associated block. If the declarator or type specifier that declares the identifier appears within the list of parameter declarations in a function prototype (not part of a function definition), the identifier has function prototype scope, which terminates at the end of the function declarator. If an identifier designates two different entities in the same name space, the scopes might overlap. If so, the scope of one entity (the inner scope) will end strictly before the scope of the other entity (the outer scope). Within the inner scope, the identifier designates the entity declared in the inner scope; the entity declared in the outer scope is hidden (and not visible) within the inner scope.

5 Unless explicitly stated otherwise, where this International Standard uses the term ''identifier'' to refer to some entity (as opposed to the syntactic construct), it refers to the entity in the relevant name space whose declaration is visible at the point the identifier occurs.

6 Two identifiers have the same scope if and only if their scopes terminate at the same point.

7 Structure, union, and enumeration tags have scope that begins just after the appearance of the tag in a type specifier that declares the tag. Each enumeration constant has scope that begins just after the appearance of its defining enumerator in an enumerator list. Any other identifier has scope that begins just after the completion of its declarator.

8 As a special case, a type name (which is not a declaration of an identifier) is considered to have a scope that begins just after the place within the type name where the omitted identifier would appear were it not omitted.

Forward references: declarations (6.7), function calls (6.5.2.2), function definitions (6.9.1), identifiers (6.4.2), macro replacement (6.10.3), name spaces of identifiers (6.2.3), source file inclusion (6.10.2), statements (6.8).

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6.2.2 Linkages of identifiers

1 An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage.29) There are three kinds of linkage: external, internal, and none.

2 In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function. Each declaration of an identifier with no linkage denotes a unique entity.

3 If the declaration of a file scope identifier for an object or a function contains the storage- class specifier static, the identifier has internal linkage.30)

4 For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,31) if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.

5 If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

6 The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; a block scope identifier for an object declared without the storage-class specifier extern.

7 If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.

Forward references: declarations (6.7), expressions (6.5), external definitions (6.9), statements (6.8).

Footnotes

29) There is no linkage between different identifiers.

30) A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.7.1.

31) As specified in 6.2.1, the later declaration might hide the prior declaration.

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6.2.3 Name spaces of identifiers

1 If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1), structure and union specifiers (6.7.2.1), structure and union members (6.5.2.3), tags (6.7.2.3), the goto statement (6.8.6.1).

Footnotes

32) There is only one name space for tags even though three are possible.

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6.2.4 Storage durations of objects

1 An object has a storage duration that determines its lifetime. There are four storage durations: static, thread, automatic, and allocated. Allocated storage is described in 7.22.3.

2 The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address,33) and retains its last-stored value throughout its lifetime.34) If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.

3 An object whose identifier is declared without the storage-class specifier _Thread_local, and either with external or internal linkage or with the storage-class specifier static, has static storage duration. Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.

4 An object whose identifier is declared with the storage-class specifier _Thread_local has thread storage duration. Its lifetime is the entire execution of the thread for which it is created, and its stored value is initialized when the thread is started. There is a distinct object per thread, and use of the declared name in an expression refers to the object associated with the thread evaluating the expression. The result of attempting to indirectly access an object with thread storage duration from a thread other than the one with which the object is associated is implementation-defined.

5 An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration, as do some compound literals. The result of attempting to indirectly access an object with automatic storage duration from a thread other than the one with which the object is associated is implementation-defined.

6 For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. (Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.) If the block is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration or compound literal is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.

7 For such an object that does have a variable length array type, its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration.35) If the scope is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate.

8 A non-lvalue expression with structure or union type, where the structure or union contains a member with array type (including, recursively, members of all contained structures and unions) refers to an object with automatic storage duration and temporary lifetime.36) Its lifetime begins when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends when the evaluation of the containing full expression or full declarator ends. Any attempt to modify an object with temporary lifetime results in undefined behavior.

Forward references: array declarators (6.7.6.2), compound literals (6.5.2.5), declarators (6.7.6), function calls (6.5.2.2), initialization (6.7.9), statements (6.8).

Footnotes

33) The term ''constant address'' means that two pointers to the object constructed at possibly different times will compare equal. The address may be different during two different executions of the same program.

34) In the case of a volatile object, the last store need not be explicit in the program.

35) Leaving the innermost block containing the declaration, or jumping to a point in that block or an embedded block prior to the declaration, leaves the scope of the declaration.

36) The address of such an object is taken implicitly when an array member is accessed.

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6.2.5 Types

1 The meaning of a value stored in an object or returned by a function is determined by the type of the expression used to access it. (An identifier declared to be an object is the simplest such expression; the type is specified in the declaration of the identifier.) Types are partitioned into object types (types that describe objects) and function types (types that describe functions). At various points within a translation unit an object type may be incomplete (lacking sufficient information to determine the size of objects of that type) or complete (having sufficient information).37)

2 An object declared as type _Bool is large enough to store the values 0 and 1.

3 An object declared as type char is large enough to store any member of the basic execution character set. If a member of the basic execution character set is stored in a char object, its value is guaranteed to be nonnegative. If any other character is stored in a char object, the resulting value is implementation-defined but shall be within the range of values that can be represented in that type.

4 There are five standard signed integer types, designated as signed char, short int, int, long int, and long long int. (These and other types may be designated in several additional ways, as described in 6.7.2.) There may also be implementation-defined extended signed integer types.38) The standard and extended signed integer types are collectively called signed integer types.39)

5 An object declared as type signed char occupies the same amount of storage as a ''plain'' char object. A ''plain'' int object has the natural size suggested by the architecture of the execution environment (large enough to contain any value in the range INT_MIN to INT_MAX as defined in the header <limits.h>).

6 For each of the signed integer types, there is a corresponding (but different) unsigned integer type (designated with the keyword unsigned) that uses the same amount of storage (including sign information) and has the same alignment requirements. The type _Bool and the unsigned integer types that correspond to the standard signed integer types are the standard unsigned integer types. The unsigned integer types that correspond to the extended signed integer types are the extended unsigned integer types. The standard and extended unsigned integer types are collectively called unsigned integer types.40)

7 The standard signed integer types and standard unsigned integer types are collectively called the standard integer types, the extended signed integer types and extended unsigned integer types are collectively called the extended integer types.

8 For any two integer types with the same signedness and different integer conversion rank (see 6.3.1.1), the range of values of the type with smaller integer conversion rank is a subrange of the values of the other type.

9 The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned integer type, and the representation of the same value in each type is the same.41) A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.

10 There are three real floating types, designated as float, double, and long double.42) The set of values of the type float is a subset of the set of values of the type double; the set of values of the type double is a subset of the set of values of the type long double.

11 There are three complex types, designated as float _Complex, double _Complex, and long double _Complex.43) (Complex types are a conditional feature that implementations need not support; see 6.10.8.3.) The real floating and complex types are collectively called the floating types.

12 For each floating type there is a corresponding real type, which is always a real floating type. For real floating types, it is the same type. For complex types, it is the type given by deleting the keyword _Complex from the type name.

13 Each complex type has the same representation and alignment requirements as an array type containing exactly two elements of the corresponding real type; the first element is equal to the real part, and the second element to the imaginary part, of the complex number.

14 The type char, the signed and unsigned integer types, and the floating types are collectively called the basic types. The basic types are complete object types. Even if the implementation defines two or more basic types to have the same representation, they are nevertheless different types.44)

15 The three types char, signed char, and unsigned char are collectively called the character types. The implementation shall define char to have the same range, representation, and behavior as either signed char or unsigned char.45)

16 An enumeration comprises a set of named integer constant values. Each distinct enumeration constitutes a different enumerated type.

17 The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types.

18 Integer and floating types are collectively called arithmetic types. Each arithmetic type belongs to one type domain: the real type domain comprises the real types, the complex type domain comprises the complex types.

19 The void type comprises an empty set of values; it is an incomplete object type that cannot be completed.

20 Any number of derived types can be constructed from the object and function types, as follows:

These methods of constructing derived types can be applied recursively.

21 Arithmetic types and pointer types are collectively called scalar types. Array and structure types are collectively called aggregate types.46)

22 An array type of unknown size is an incomplete type. It is completed, for an identifier of that type, by specifying the size in a later declaration (with internal or external linkage). A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all declarations of that type, by declaring the same structure or union tag with its defining content later in the same scope.

23 A type has known constant size if the type is not incomplete and is not a variable length array type.

24 Array, function, and pointer types are collectively called derived declarator types. A declarator type derivation from a type T is the construction of a derived declarator type from T by the application of an array-type, a function-type, or a pointer-type derivation to T.

25 A type is characterized by its type category, which is either the outermost derivation of a derived type (as noted above in the construction of derived types), or the type itself if the type consists of no derived types.

26 Any type so far mentioned is an unqualified type. Each unqualified type has several qualified versions of its type,47) corresponding to the combinations of one, two, or all three of the const, volatile, and restrict qualifiers. The qualified or unqualified versions of a type are distinct types that belong to the same type category and have the same representation and alignment requirements.48) A derived type is not qualified by the qualifiers (if any) of the type from which it is derived.

27 Further, there is the _Atomic qualifier. The presence of the _Atomic qualifier designates an atomic type. The size, representation, and alignment of an atomic type need not be the same as those of the corresponding unqualified type. Therefore, this Standard explicitly uses the phrase ''atomic, qualified or unqualified type'' whenever the atomic version of a type is permitted along with the other qualified versions of a type. The phrase ''qualified or unqualified type'', without specific mention of atomic, does not include the atomic types.

28 A pointer to void shall have the same representation and alignment requirements as a pointer to a character type.48) Similarly, pointers to qualified or unqualified versions of compatible types shall have the same representation and alignment requirements. All pointers to structure types shall have the same representation and alignment requirements as each other. All pointers to union types shall have the same representation and alignment requirements as each other. Pointers to other types need not have the same representation or alignment requirements.

29 EXAMPLE 1 The type designated as ''float *'' has type ''pointer to float''. Its type category is pointer, not a floating type. The const-qualified version of this type is designated as ''float * const'' whereas the type designated as ''const float *'' is not a qualified type -- its type is ''pointer to const- qualified float'' and is a pointer to a qualified type.

30 EXAMPLE 2 The type designated as ''struct tag (*[5])(float)'' has type ''array of pointer to function returning struct tag''. The array has length five and the function has a single parameter of type float. Its type category is array.

Forward references: compatible type and composite type (6.2.7), declarations (6.7).

Footnotes

37) A type may be incomplete or complete throughout an entire translation unit, or it may change states at different points within a translation unit.

38) Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3.

39) Therefore, any statement in this Standard about signed integer types also applies to the extended signed integer types.

40) Therefore, any statement in this Standard about unsigned integer types also applies to the extended unsigned integer types.

41) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

42) See ''future language directions'' (6.11.1).

43) A specification for imaginary types is in annex G.

44) An implementation may define new keywords that provide alternative ways to designate a basic (or any other) type; this does not violate the requirement that all basic types be different. Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3.

45) CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.

46) Note that aggregate type does not include union type because an object with union type can only contain one member at a time.

47) See 6.7.3 regarding qualified array and function types.

48) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

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6.2.6 Representations of types

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6.2.6.1 General

1 The representations of all types are unspecified except as stated in this subclause.

2 Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined.

3 Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a pure binary notation.49)

4 Values stored in non-bit-field objects of any other object type consist of n x CHAR_BIT bits, where n is the size of an object of that type, in bytes. The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value. Values stored in bit-fields consist of m bits, where m is the size specified for the bit-field. The object representation is the set of m bits the bit-field comprises in the addressable storage unit holding it. Two values (other than NaNs) with the same object representation compare equal, but values that compare equal may have different object representations.

5 Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined.50) Such a representation is called a trap representation.

6 When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.51) The value of a structure or union object is never a trap representation, even though the value of a member of the structure or union object may be a trap representation.

7 When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values.

8 Where an operator is applied to a value that has more than one object representation, which object representation is used shall not affect the value of the result.52) Where a value is stored in an object using a type that has more than one object representation for that value, it is unspecified which representation is used, but a trap representation shall not be generated.

9 Loads and stores of objects with atomic types are done with memory_order_seq_cst semantics.

Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function designators (6.3.2.1), order and consistency (7.17.3).

Footnotes

49) A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest position. (Adapted from the American National Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to 2CHAR_BIT - 1.

50) Thus, an automatic variable can be initialized to a trap representation without causing undefined behavior, but the value of the variable cannot be used until a proper value is stored in it.

51) Thus, for example, structure assignment need not copy any padding bits.

52) It is possible for objects x and y with the same effective type T to have the same value when they are accessed as objects of type T, but to have different values in other contexts. In particular, if == is defined for type T, then x == y does not imply that memcmp(&x, &y, sizeof (T)) == 0. Furthermore, x == y does not necessarily imply that x and y have the same value; other operations on values of type T may distinguish between them.

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6.2.6.2 Integer types

1 For unsigned integer types other than unsigned char, the bits of the object representation shall be divided into two groups: value bits and padding bits (there need not be any of the latter). If there are N value bits, each bit shall represent a different power of 2 between 1 and 2N - 1, so that objects of that type shall be capable of representing values from 0 to 2N - 1 using a pure binary representation; this shall be known as the value representation. The values of any padding bits are unspecified.53)

2 For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; signed char shall not have any padding bits. There shall be exactly one sign bit. Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type and N in the unsigned type, then M <= N ). If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, the value shall be modified in one of the following ways:

Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones' complement), is a trap representation or a normal value. In the case of sign and magnitude and ones' complement, if this representation is a normal value it is called a negative zero.

3 If the implementation supports negative zeros, they shall be generated only by:

It is unspecified whether these cases actually generate a negative zero or a normal zero, and whether a negative zero becomes a normal zero when stored in an object.

4 If the implementation does not support negative zeros, the behavior of the &, |, ^, ~, <<, and >> operators with operands that would produce such a value is undefined.

5 The values of any padding bits are unspecified.54) A valid (non-trap) object representation of a signed integer type where the sign bit is zero is a valid object representation of the corresponding unsigned type, and shall represent the same value. For any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type.

6 The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits. The width of an integer type is the same but including any sign bit; thus for unsigned integer types the two values are the same, while for signed integer types the width is one greater than the precision.

Footnotes

53) Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow, and this cannot occur with unsigned types. All other combinations of padding bits are alternative object representations of the value specified by the value bits.

54) Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow. All other combinations of padding bits are alternative object representations of the value specified by the value bits.

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6.2.7 Compatible type and composite type

1 Two types have compatible type if their types are the same. Additional rules for determining whether two types are compatible are described in 6.7.2 for type specifiers, in 6.7.3 for type qualifiers, and in 6.7.6 for declarators.55) Moreover, two structure, union, or enumerated types declared in separate translation units are compatible if their tags and members satisfy the following requirements: If one is declared with a tag, the other shall be declared with the same tag. If both are completed anywhere within their respective translation units, then the following additional requirements apply: there shall be a one-to-one correspondence between their members such that each pair of corresponding members are declared with compatible types; if one member of the pair is declared with an alignment specifier, the other is declared with an equivalent alignment specifier; and if one member of the pair is declared with a name, the other is declared with the same name. For two structures, corresponding members shall be declared in the same order. For two structures or unions, corresponding bit-fields shall have the same widths. For two enumerations, corresponding members shall have the same values.

2 All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

3 A composite type can be constructed from two types that are compatible; it is a type that is compatible with both of the two types and satisfies the following conditions:

These rules apply recursively to the types from which the two types are derived.

4 For an identifier with internal or external linkage declared in a scope in which a prior declaration of that identifier is visible,56) if the prior declaration specifies internal or external linkage, the type of the identifier at the later declaration becomes the composite type.

Forward references: array declarators (6.7.6.2).

5 EXAMPLE Given the following two file scope declarations:

          int f(int (*)(), double (*)[3]);
          int f(int (*)(char *), double (*)[]);
The resulting composite type for the function is:
          int f(int (*)(char *), double (*)[3]);

Footnotes

55) Two types need not be identical to be compatible.

56) As specified in 6.2.1, the later declaration might hide the prior declaration.

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6.2.8 Alignment of objects

1 Complete object types have alignment requirements which place restrictions on the addresses at which objects of that type may be allocated. An alignment is an implementation-defined integer value representing the number of bytes between successive addresses at which a given object can be allocated. An object type imposes an alignment requirement on every object of that type: stricter alignment can be requested using the _Alignas keyword.

2 A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to _Alignof (max_align_t).

3 An extended alignment is represented by an alignment greater than _Alignof (max_align_t). It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported. A type having an extended alignment requirement is an over-aligned type.57)

4 Alignments are represented as values of the type size_t. Valid alignments include only those values returned by an _Alignof expression for fundamental types, plus an additional implementation-defined set of values, which may be empty. Every valid alignment value shall be a nonnegative integral power of two.

5 Alignments have an order from weaker to stronger or stricter alignments. Stricter alignments have larger alignment values. An address that satisfies an alignment requirement also satisfies any weaker valid alignment requirement.

6 The alignment requirement of a complete type can be queried using an _Alignof expression. The types char, signed char, and unsigned char shall have the weakest alignment requirement.

7 Comparing alignments is meaningful and provides the obvious results:

Footnotes

57) Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has been applied.

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6.3 Conversions

1 Several operators convert operand values from one type to another automatically. This subclause specifies the result required from such an implicit conversion, as well as those that result from a cast operation (an explicit conversion). The list in 6.3.1.8 summarizes the conversions performed by most ordinary operators; it is supplemented as required by the discussion of each operator in 6.5.

2 Conversion of an operand value to a compatible type causes no change to the value or the representation.

Forward references: cast operators (6.5.4).

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6.3.1 Arithmetic operands

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6.3.1.1 Boolean, characters, and integers

1 Every integer type has an integer conversion rank defined as follows:

2 The following may be used in an expression wherever an int or unsigned int may be used:

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.58) All other types are unchanged by the integer promotions.

3 The integer promotions preserve value including sign. As discussed earlier, whether a ''plain'' char is treated as signed is implementation-defined.

Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1).

Footnotes

58) The integer promotions are applied only: as part of the usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses.

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6.3.1.2 Boolean type

1 When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.59)

Footnotes

59) NaNs do not compare equal to 0 and thus convert to 1.

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6.3.1.3 Signed and unsigned integers

1 When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

2 Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.60)

3 Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

Footnotes

60) The rules describe arithmetic on the mathematical value, not the value of a given type of expression.

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6.3.1.4 Real floating and integer

1 When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.61)

2 When a value of integer type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined. Results of some implicit conversions may be represented in greater range and precision than that required by the new type (see 6.3.1.8 and 6.8.6.4).

Footnotes

61) The remaindering operation performed when a value of integer type is converted to unsigned type need not be performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is (-1, Utype_MAX+1).

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6.3.1.5 Real floating types

1 When a value of real floating type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined. Results of some implicit conversions may be represented in greater range and precision than that required by the new type (see 6.3.1.8 and 6.8.6.4).

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6.3.1.6 Complex types

1 When a value of complex type is converted to another complex type, both the real and imaginary parts follow the conversion rules for the corresponding real types.

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6.3.1.7 Real and complex

1 When a value of real type is converted to a complex type, the real part of the complex result value is determined by the rules of conversion to the corresponding real type and the imaginary part of the complex result value is a positive zero or an unsigned zero.

2 When a value of complex type is converted to a real type, the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.

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6.3.1.8 Usual arithmetic conversions

1 Many operators that expect operands of arithmetic type cause conversions and yield result types in a similar way. The purpose is to determine a common real type for the operands and result. For the specified operands, each operand is converted, without change of type domain, to a type whose corresponding real type is the common real type. Unless explicitly stated otherwise, the common real type is also the corresponding real type of the result, whose type domain is the type domain of the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic conversions:

2 The values of floating operands and of the results of floating expressions may be represented in greater range and precision than that required by the type; the types are not changed thereby.63)

Footnotes

62) For example, addition of a double _Complex and a float entails just the conversion of the float operand to double (and yields a double _Complex result).

63) The cast and assignment operators are still required to remove extra range and precision.

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6.3.2 Other operands

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6.3.2.1 Lvalues, arrays, and function designators

1 An lvalue is an expression (with an object type other than void) that potentially designates an object;64) if an lvalue does not designate an object when it is evaluated, the behavior is undefined. When an object is said to have a particular type, the type is specified by the lvalue used to designate the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete type, does not have a const- qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const- qualified type.

2 Except when it is the operand of the sizeof operator, the _Alignof operator, the unary & operator, the ++ operator, the -- operator, or the left operand of the . operator or an assignment operator, an lvalue that does not have array type is converted to the value stored in the designated object (and is no longer an lvalue); this is called lvalue conversion. If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the lvalue has an incomplete type and does not have array type, the behavior is undefined. If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.

3 Except when it is the operand of the sizeof operator, the _Alignof operator, or the unary & operator, or is a string literal used to initialize an array, an expression that has type ''array of type'' is converted to an expression with type ''pointer to type'' that points to the initial element of the array object and is not an lvalue. If the array object has register storage class, the behavior is undefined.

4 A function designator is an expression that has function type. Except when it is the operand of the sizeof operator, the _Alignof operator,65) or the unary & operator, a function designator with type ''function returning type'' is converted to an expression that has type ''pointer to function returning type''.

Forward references: address and indirection operators (6.5.3.2), assignment operators (6.5.16), common definitions <stddef.h> (7.19), initialization (6.7.9), postfix increment and decrement operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), the sizeof and _Alignof operators (6.5.3.4), structure and union members (6.5.2.3).

Footnotes

64) The name ''lvalue'' comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object ''locator value''. What is sometimes called ''rvalue'' is in this International Standard described as the ''value of an expression''. An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a pointer to an object, *E is an lvalue that designates the object to which E points.

65) Because this conversion does not occur, the operand of the sizeof or _Alignof operator remains a function designator and violates the constraints in 6.5.3.4.

Contents

6.3.2.2 void

1 The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any way, and implicit or explicit conversions (except to void) shall not be applied to such an expression. If an expression of any other type is evaluated as a void expression, its value or designator is discarded. (A void expression is evaluated for its side effects.)

Contents

6.3.2.3 Pointers

1 A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

2 For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to the q-qualified version of the type; the values stored in the original and converted pointers shall compare equal.

3 An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

4 Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

5 An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.67)

6 Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.

7 A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned68) for the referenced type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer. When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object. Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object.

8 A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the referenced type, the behavior is undefined.

Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types capable of holding object pointers (7.20.1.4), simple assignment (6.5.16.1).

Footnotes

66) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19.

67) The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with the addressing structure of the execution environment.

68) In general, the concept ''correctly aligned'' is transitive: if a pointer to type A is correctly aligned for a pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.

Contents

6.4 Lexical elements

Syntax

1

          token:
                   keyword
                   identifier
                   constant
                   string-literal
                   punctuator
          preprocessing-token:
                 header-name
                 identifier
                 pp-number
                 character-constant
                 string-literal
                 punctuator
                 each non-white-space character that cannot be one of the above

Constraints

2 Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an identifier, a constant, a string literal, or a punctuator.

Semantics

3 A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of tokens are: keywords, identifiers, constants, string literals, and punctuators. A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing tokens are: header names, identifiers, preprocessing numbers, character constants, string literals, punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories.69) If a ' or a " character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments (described later), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in 6.10, in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space may appear within a preprocessing token only as part of a header name or between the quotation characters in a character constant or string literal.

4 If the input stream has been parsed into preprocessing tokens up to a given character, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token. There is one exception to this rule: header name preprocessing tokens are recognized only within #include preprocessing directives and in implementation-defined locations within #pragma directives. In such contexts, a sequence of characters that could be either a header name or a string literal is recognized as the former.

5 EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer constant token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating constant token), whether or not E is a macro name.

6 EXAMPLE 2 The program fragment x+++++y is parsed as x ++ ++ + y, which violates a constraint on increment operators, even though the parse x ++ + ++ y might yield a correct expression.

Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5), floating constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), postfix increment and decrement operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), preprocessing directives (6.10), preprocessing numbers (6.4.8), string literals (6.4.5).

Footnotes

69) An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot occur in source files.

Contents

6.4.1 Keywords

Syntax

1

          keyword: one of
                auto                          if                             unsigned
                break                         inline                         void
                case                          int                            volatile
                char                          long                           while
                const                         register                       _Alignas
                continue                      restrict                       _Alignof
                default                       return                         _Atomic
                do                            short                          _Bool
                double                        signed                         _Complex
                else                          sizeof                         _Generic
                enum                          static                         _Imaginary
                extern                        struct                         _Noreturn
                float                         switch                         _Static_assert
                for                           typedef                        _Thread_local
                goto                          union

Semantics

2 The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords, and shall not be used otherwise. The keyword _Imaginary is reserved for specifying imaginary types.70)

Footnotes

70) One possible specification for imaginary types appears in annex G.

Contents

6.4.2 Identifiers

Contents

6.4.2.1 General

Syntax

1

          identifier:
                 identifier-nondigit
                 identifier identifier-nondigit
                 identifier digit
          identifier-nondigit:
                 nondigit
                 universal-character-name
                 other implementation-defined characters
          nondigit: one of
                 _   a   b   c   d   e   f   g   h   i   j   k   l   m
                     n   o   p   q   r   s   t   u   v   w   x   y   z
                     A   B   C   D   E   F   G   H   I   J   K   L   M
                     N   O   P   Q   R   S   T   U   V   W   X   Y   Z
          digit: one of
                 0   1   2   3   4   5   6   7   8   9

Semantics

2 An identifier is a sequence of nondigit characters (including the underscore _, the lowercase and uppercase Latin letters, and other characters) and digits, which designates one or more entities as described in 6.2.1. Lowercase and uppercase letters are distinct. There is no specific limit on the maximum length of an identifier.

3 Each universal character name in an identifier shall designate a character whose encoding in ISO/IEC 10646 falls into one of the ranges specified in D.1.71) The initial character shall not be a universal character name designating a character whose encoding falls into one of the ranges specified in D.2. An implementation may allow multibyte characters that are not part of the basic source character set to appear in identifiers; which characters and their correspondence to universal character names is implementation-defined.

4 When preprocessing tokens are converted to tokens during translation phase 7, if a preprocessing token could be converted to either a keyword or an identifier, it is converted to a keyword.

Implementation limits

5 As discussed in 5.2.4.1, an implementation may limit the number of significant initial characters in an identifier; the limit for an external name (an identifier that has external linkage) may be more restrictive than that for an internal name (a macro name or an identifier that does not have external linkage). The number of significant characters in an identifier is implementation-defined.

6 Any identifiers that differ in a significant character are different identifiers. If two identifiers differ only in nonsignificant characters, the behavior is undefined.

Forward references: universal character names (6.4.3), macro replacement (6.10.3).

Footnotes

71) On systems in which linkers cannot accept extended characters, an encoding of the universal character name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal character name. Extended characters may produce a long external identifier.

Contents

6.4.2.2 Predefined identifiers

Semantics

1 The identifier __func__ shall be implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration

          static const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-enclosing function.72)

2 This name is encoded as if the implicit declaration had been written in the source character set and then translated into the execution character set as indicated in translation phase 5.

3 EXAMPLE Consider the code fragment:

          #include <stdio.h>
          void myfunc(void)
          {
                printf("%s\n", __func__);
                /* ... */
          }
Each time the function is called, it will print to the standard output stream:
          myfunc

Forward references: function definitions (6.9.1).

Footnotes

72) Since the name __func__ is reserved for any use by the implementation (7.1.3), if any other identifier is explicitly declared using the name __func__, the behavior is undefined.

Contents

6.4.3 Universal character names

Syntax

1

          universal-character-name:
                 \u hex-quad
                 \U hex-quad hex-quad
          hex-quad:
                 hexadecimal-digit hexadecimal-digit
                              hexadecimal-digit hexadecimal-digit

Constraints

2 A universal character name shall not specify a character whose short identifier is less than 00A0 other than 0024 ($), 0040 (@), or 0060 ('), nor one in the range D800 through DFFF inclusive.73)

Description

3 Universal character names may be used in identifiers, character constants, and string literals to designate characters that are not in the basic character set.

Semantics

4 The universal character name \Unnnnnnnn designates the character whose eight-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn.74) Similarly, the universal character name \unnnn designates the character whose four-digit short identifier is nnnn (and whose eight-digit short identifier is 0000nnnn).

Footnotes

73) The disallowed characters are the characters in the basic character set and the code positions reserved by ISO/IEC 10646 for control characters, the character DELETE, and the S-zone (reserved for use by UTF-16).

74) Short identifiers for characters were first specified in ISO/IEC 10646-1/AMD9:1997.

Contents

6.4.4 Constants

Syntax

1

          constant:
                 integer-constant
                 floating-constant
                 enumeration-constant
                 character-constant

Constraints

2 Each constant shall have a type and the value of a constant shall be in the range of representable values for its type.

Semantics

3 Each constant has a type, determined by its form and value, as detailed later.

Contents

6.4.4.1 Integer constants

Syntax

1

          integer-constant:
                decimal-constant integer-suffixopt
                octal-constant integer-suffixopt
                hexadecimal-constant integer-suffixopt
          decimal-constant:
                nonzero-digit
                decimal-constant digit
          octal-constant:
                0
                octal-constant octal-digit
          hexadecimal-constant:
                hexadecimal-prefix hexadecimal-digit
                hexadecimal-constant hexadecimal-digit
          hexadecimal-prefix: one of
                0x   0X
          nonzero-digit: one of
                1   2   3   4   5   6   7   8   9
          octal-digit: one of
                0   1   2   3   4   5   6   7
          hexadecimal-digit: one of
                0   1   2   3   4   5   6   7   8   9
                a   b   c   d   e   f
                A   B   C   D   E   F
          integer-suffix:
                unsigned-suffix long-suffixopt
                unsigned-suffix long-long-suffix
                long-suffix unsigned-suffixopt
                long-long-suffix unsigned-suffixopt
          unsigned-suffix: one of
                u   U
          long-suffix: one of
                l   L
          long-long-suffix: one of
                ll   LL

Description

2 An integer constant begins with a digit, but has no period or exponent part. It may have a prefix that specifies its base and a suffix that specifies its type.

3 A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and the letters a (or A) through f (or F) with values 10 through 15 respectively.

Semantics

4 The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a hexadecimal constant, base 16. The lexically first digit is the most significant.

5 The type of an integer constant is the first of the corresponding list in which its value can be represented.
Suffix Decimal Constant Octal or Hexadecimal Constant
none
int
long int
long long int
int
unsigned int
long int
unsigned long int
long long int
unsigned long long int
u or U
unsigned int
unsigned long int
unsigned long long int
unsigned int
unsigned long int
unsigned long long int
l or L
long int
long long int
long int
unsigned long int
long long int
unsigned long long int
Both u or U and l or L
unsigned long int
unsigned long long int
unsigned long int
unsigned long long int
ll or LL
long long int
long long int
unsigned long long int
Both u or U and ll or LL
unsigned long long int
unsigned long long int

6 If an integer constant cannot be represented by any type in its list, it may have an extended integer type, if the extended integer type can represent its value. If all of the types in the list for the constant are signed, the extended integer type shall be signed. If all of the types in the list for the constant are unsigned, the extended integer type shall be unsigned. If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned. If an integer constant cannot be represented by any type in its list and has no extended integer type, then the integer constant has no type.

Contents

6.4.4.2 Floating constants

Syntax

1

          floating-constant:
                 decimal-floating-constant
                 hexadecimal-floating-constant
          decimal-floating-constant:
                fractional-constant exponent-partopt floating-suffixopt
                digit-sequence exponent-part floating-suffixopt
          hexadecimal-floating-constant:
                hexadecimal-prefix hexadecimal-fractional-constant
                               binary-exponent-part floating-suffixopt
                hexadecimal-prefix hexadecimal-digit-sequence
                               binary-exponent-part floating-suffixopt
          fractional-constant:
                  digit-sequenceopt . digit-sequence
                  digit-sequence .
          exponent-part:
                e signopt digit-sequence
                E signopt digit-sequence
          sign: one of
                 + -
          digit-sequence:
                  digit
                  digit-sequence digit
          hexadecimal-fractional-constant:
                hexadecimal-digit-sequenceopt .
                               hexadecimal-digit-sequence
                hexadecimal-digit-sequence .
          binary-exponent-part:
                 p signopt digit-sequence
                 P signopt digit-sequence
          hexadecimal-digit-sequence:
                hexadecimal-digit
                hexadecimal-digit-sequence hexadecimal-digit
          floating-suffix: one of
                 f l F L

Description

2 A floating constant has a significand part that may be followed by an exponent part and a suffix that specifies its type. The components of the significand part may include a digit sequence representing the whole-number part, followed by a period (.), followed by a digit sequence representing the fraction part. The components of the exponent part are an e, E, p, or P followed by an exponent consisting of an optionally signed digit sequence. Either the whole-number part or the fraction part has to be present; for decimal floating constants, either the period or the exponent part has to be present.

Semantics

3 The significand part is interpreted as a (decimal or hexadecimal) rational number; the digit sequence in the exponent part is interpreted as a decimal integer. For decimal floating constants, the exponent indicates the power of 10 by which the significand part is to be scaled. For hexadecimal floating constants, the exponent indicates the power of 2 by which the significand part is to be scaled. For decimal floating constants, and also for hexadecimal floating constants when FLT_RADIX is not a power of 2, the result is either the nearest representable value, or the larger or smaller representable value immediately adjacent to the nearest representable value, chosen in an implementation-defined manner. For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is correctly rounded.

4 An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has type float. If suffixed by the letter l or L, it has type long double.

5 Floating constants are converted to internal format as if at translation-time. The conversion of a floating constant shall not raise an exceptional condition or a floating- point exception at execution time. All floating constants of the same source form75) shall convert to the same internal format with the same value.

Recommended practice

6 The implementation should produce a diagnostic message if a hexadecimal constant cannot be represented exactly in its evaluation format; the implementation should then proceed with the translation of the program.

7 The translation-time conversion of floating constants should match the execution-time conversion of character strings by library functions, such as strtod, given matching inputs suitable for both conversions, the same result format, and default execution-time rounding.76)

Footnotes

75) 1.23, 1.230, 123e-2, 123e-02, and 1.23L are all different source forms and thus need not convert to the same internal format and value.

76) The specification for the library functions recommends more accurate conversion than required for floating constants (see 7.22.1.3).

Contents

6.4.4.3 Enumeration constants

Syntax

1

          enumeration-constant:
                identifier

Semantics

2 An identifier declared as an enumeration constant has type int.

Forward references: enumeration specifiers (6.7.2.2).

Contents

6.4.4.4 Character constants

Syntax

1

          character-constant:
                 ' c-char-sequence '
                 L' c-char-sequence '
                 u' c-char-sequence '
                 U' c-char-sequence '
          c-char-sequence:
                 c-char
                 c-char-sequence c-char
          c-char:
                 any member of the source character set except
                              the single-quote ', backslash \, or new-line character
                 escape-sequence
          escape-sequence:
                 simple-escape-sequence
                 octal-escape-sequence
                 hexadecimal-escape-sequence
                 universal-character-name
          simple-escape-sequence: one of
                 \'  \"  \?  \\
                 \a  \b  \f  \n  \r  \t  \v
          octal-escape-sequence:
                 \ octal-digit
                 \ octal-digit octal-digit
                 \ octal-digit octal-digit octal-digit
          hexadecimal-escape-sequence:
                 \x hexadecimal-digit
                 hexadecimal-escape-sequence hexadecimal-digit

Description

2 An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x'. A wide character constant is the same, except prefixed by the letter L, u, or U. With a few exceptions detailed later, the elements of the sequence are any members of the source character set; they are mapped in an implementation-defined manner to members of the execution character set.

3 The single-quote ', the double-quote ", the question-mark ?, the backslash \, and arbitrary integer values are representable according to the following table of escape sequences:

       single quote '            \'
       double quote "            \"
       question mark ?           \?
       backslash \               \\
       octal character           \octal digits
       hexadecimal character     \x hexadecimal digits

4 The double-quote " and question-mark ? are representable either by themselves or by the escape sequences \" and \?, respectively, but the single-quote ' and the backslash \ shall be represented, respectively, by the escape sequences \' and \\.

5 The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the octal integer so formed specifies the value of the desired character or wide character.

6 The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the hexadecimal integer so formed specifies the value of the desired character or wide character.

7 Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence.

8 In addition, characters not in the basic character set are representable by universal character names and certain nongraphic characters are representable by escape sequences consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v.77)

Constraints

9 The value of an octal or hexadecimal escape sequence shall be in the range of representable values for the corresponding type:
Prefix Corresponding Type
noneunsigned char
Lthe unsigned type corresponding to wchar_t
uchar16_t
Uchar32_t

Semantics

10 An integer character constant has type int. The value of an integer character constant containing a single character that maps to a single-byte execution character is the numerical value of the representation of the mapped character interpreted as an integer. The value of an integer character constant containing more than one character (e.g., 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined. If an integer character constant contains a single character or escape sequence, its value is the one that results when an object with type char whose value is that of the single character or escape sequence is converted to type int.

11 A wide character constant prefixed by the letter L has type wchar_t, an integer type defined in the <stddef.h> header; a wide character constant prefixed by the letter u or U has type char16_t or char32_t, respectively, unsigned integer types defined in the <uchar.h> header. The value of a wide character constant containing a single multibyte character that maps to a single member of the extended execution character set is the wide character corresponding to that multibyte character, as defined by the mbtowc, mbrtoc16, or mbrtoc32 function as appropriate for its type, with an implementation-defined current locale. The value of a wide character constant containing more than one multibyte character or a single multibyte character that maps to multiple members of the extended execution character set, or containing a multibyte character or escape sequence not represented in the extended execution character set, is implementation-defined.

12 EXAMPLE 1 The construction '\0' is commonly used to represent the null character.

13 EXAMPLE 2 Consider implementations that use two's complement representation for integers and eight bits for objects that have type char. In an implementation in which type char has the same range of values as signed char, the integer character constant '\xFF' has the value -1; if type char has the same range of values as unsigned char, the character constant '\xFF' has the value +255.

14 EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction '\x123' specifies an integer character constant containing only one character, since a hexadecimal escape sequence is terminated only by a non-hexadecimal character. To specify an integer character constant containing the two characters whose values are '\x12' and '3', the construction '\0223' may be used, since an octal escape sequence is terminated after three octal digits. (The value of this two-character integer character constant is implementation-defined.)

15 EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_t, the construction L'\1234' specifies the implementation-defined value that results from the combination of the values 0123 and '4'.

Forward references: common definitions <stddef.h> (7.19), the mbtowc function (7.22.7.2), Unicode utilities <uchar.h> (7.28).

Footnotes

77) The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash, the result is not a token and a diagnostic is required. See ''future language directions'' (6.11.4).

Contents

6.4.5 String literals

Syntax

1

          string-literal:
                 encoding-prefixopt " s-char-sequenceopt "
          encoding-prefix:
                 u8
                 u
                 U
                 L
          s-char-sequence:
                 s-char
                 s-char-sequence s-char
          s-char:
                 any member of the source character set except
                              the double-quote ", backslash \, or new-line character
                 escape-sequence

Constraints

2 A sequence of adjacent string literal tokens shall not include both a wide string literal and a UTF-8 string literal.

Description

3 A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in "xyz". A UTF-8 string literal is the same, except prefixed by u8. A wide string literal is the same, except prefixed by the letter L, u, or U.

4 The same considerations apply to each element of the sequence in a string literal as if it were in an integer character constant (for a character or UTF-8 string literal) or a wide character constant (for a wide string literal), except that the single-quote ' is representable either by itself or by the escape sequence \', but the double-quote " shall be represented by the escape sequence \".

Semantics

5 In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and identically-prefixed string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens has an encoding prefix, the resulting multibyte character sequence is treated as having the same prefix; otherwise, it is treated as a character string literal. Whether differently-prefixed wide string literal tokens can be concatenated and, if so, the treatment of the resulting multibyte character sequence are implementation-defined.

6 In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals.78) The multibyte character sequence is then used to initialize an array of static storage duration and length just sufficient to contain the sequence. For character string literals, the array elements have type char, and are initialized with the individual bytes of the multibyte character sequence. For UTF-8 string literals, the array elements have type char, and are initialized with the characters of the multibyte character sequence, as encoded in UTF-8. For wide string literals prefixed by the letter L, the array elements have type wchar_t and are initialized with the sequence of wide characters corresponding to the multibyte character sequence, as defined by the mbstowcs function with an implementation- defined current locale. For wide string literals prefixed by the letter u or U, the array elements have type char16_t or char32_t, respectively, and are initialized with the sequence of wide characters corresponding to the multibyte character sequence, as defined by successive calls to the mbrtoc16, or mbrtoc32 function as appropriate for its type, with an implementation-defined current locale. The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set is implementation-defined.

7 It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.

8 EXAMPLE 1 This pair of adjacent character string literals

          "\x12" "3"
produces a single character string literal containing the two characters whose values are '\x12' and '3', because escape sequences are converted into single members of the execution character set just prior to adjacent string literal concatenation.

9 EXAMPLE 2 Each of the sequences of adjacent string literal tokens

          "a" "b" L"c"
          "a" L"b" "c"
          L"a" "b" L"c"
          L"a" L"b" L"c"
is equivalent to the string literal
          L"abc"
Likewise, each of the sequences
          "a" "b" u"c"
          "a" u"b" "c"
          u"a" "b" u"c"
          u"a" u"b" u"c"
is equivalent to
          u"abc"

Forward references: common definitions <stddef.h> (7.19), the mbstowcs function (7.22.8.1), Unicode utilities <uchar.h> (7.28).

Footnotes

78) A string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a \0 escape sequence.

Contents

6.4.6 Punctuators

Syntax

1

          punctuator: one of
                 [   ]   (   )   {   }   .   ->
                 ++   --   &   *   +   -   ~   !
                 /   %   <<   >>   <   >   <=   >=   ==   !=   ^   |   &&   ||
                 ?   :   ;   ...
                 =   *=   /=   %=   +=   -=   <<=   >>=   &=   ^=   |=
                 ,   #   ##
                 <:   :>   <%   %>   %:   %:%:

Semantics

2 A punctuator is a symbol that has independent syntactic and semantic significance. Depending on context, it may specify an operation to be performed (which in turn may yield a value or a function designator, produce a side effect, or some combination thereof) in which case it is known as an operator (other forms of operator also exist in some contexts). An operand is an entity on which an operator acts.

3 In all aspects of the language, the six tokens79)

          <:    :>      <%    %>     %:     %:%:
behave, respectively, the same as the six tokens
          [     ]       {     }      #      ##
except for their spelling.80)

Forward references: expressions (6.5), declarations (6.7), preprocessing directives (6.10), statements (6.8).

Footnotes

79) These tokens are sometimes called ''digraphs''.

80) Thus [ and <: behave differently when ''stringized'' (see 6.10.3.2), but can otherwise be freely interchanged.

Contents

6.4.7 Header names

Syntax

1

          header-name:
                 < h-char-sequence >
                 " q-char-sequence "
          h-char-sequence:
                 h-char
                 h-char-sequence h-char
          h-char:
                    any member of the source character set except
                                 the new-line character and >
          q-char-sequence:
                 q-char
                 q-char-sequence q-char
          q-char:
                    any member of the source character set except
                                 the new-line character and "

Semantics

2 The sequences in both forms of header names are mapped in an implementation-defined manner to headers or external source file names as specified in 6.10.2.

3 If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the sequence between the " delimiters, the behavior is undefined.81) Header name preprocessing tokens are recognized only within #include preprocessing directives and in implementation-defined locations within #pragma directives.82)

4 EXAMPLE The following sequence of characters:

          0x3<1/a.h>1e2
          #include <1/a.h>
          #define const.member@$
forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited by a { on the left and a } on the right).
          {0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
          {#}{include} {<1/a.h>}
          {#}{define} {const}{.}{member}{@}{$}

Forward references: source file inclusion (6.10.2).

Footnotes

81) Thus, sequences of characters that resemble escape sequences cause undefined behavior.

82) For an example of a header name preprocessing token used in a #pragma directive, see 6.10.9.

Contents

6.4.8 Preprocessing numbers

Syntax

1

          pp-number:
                digit
                . digit
                pp-number       digit
                pp-number       identifier-nondigit
                pp-number       e sign
                pp-number       E sign
                pp-number       p sign
                pp-number       P sign
                pp-number       .

Description

2 A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed by valid identifier characters and the character sequences e+, e-, E+, E-, p+, p-, P+, or P-.

3 Preprocessing number tokens lexically include all floating and integer constant tokens.

Semantics

4 A preprocessing number does not have type or a value; it acquires both after a successful conversion (as part of translation phase 7) to a floating constant token or an integer constant token.

Contents

6.4.9 Comments

1 Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters */ that terminate it.83)

2 Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the terminating new-line character.

3 EXAMPLE

          "a//b"                             //   four-character string literal
          #include "//e"                     //   undefined behavior
          // */                              //   comment, not syntax error
          f = g/**//h;                       //   equivalent to f = g / h;
          //\
          i();                               // part of a two-line comment
          /\
          / j();                             // part of a two-line comment
          #define glue(x,y) x##y
          glue(/,/) k();                     // syntax error, not comment
          /*//*/ l();                        // equivalent to l();
          m = n//**/o
             + p;                            // equivalent to m = n + p;

Footnotes

83) Thus, /* ... */ comments do not nest.

Contents

6.5 Expressions

1 An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof. The value computations of the operands of an operator are sequenced before the value computation of the result of the operator.

2 If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined. If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.84)

3 The grouping of operators and operands is indicated by the syntax.85) Except as specified later, side effects and value computations of subexpressions are unsequenced.86)

4 Some operators (the unary operator ~, and the binary operators <<, >>, &, ^, and |, collectively described as bitwise operators) are required to have operands that have integer type. These operators yield values that depend on the internal representations of integers, and have implementation-defined and undefined aspects for signed types.

5 If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.

6 The effective type of an object for an access to its stored value is the declared type of the object, if any.87) If a value is stored into an object having no declared type through an lvalue having a type that is not a character type, then the type of the lvalue becomes the effective type of the object for that access and for subsequent accesses that do not modify the stored value. If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one. For all other accesses to an object having no declared type, the effective type of the object is simply the type of the lvalue used for the access.

7 An object shall have its stored value accessed only by an lvalue expression that has one of the following types:88)

8 A floating expression may be contracted, that is, evaluated as though it were a single operation, thereby omitting rounding errors implied by the source code and the expression evaluation method.89) The FP_CONTRACT pragma in <math.h> provides a way to disallow contracted expressions. Otherwise, whether and how expressions are contracted is implementation-defined.90)

Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.24.2).

Footnotes

84) This paragraph renders undefined statement expressions such as

           i = ++i + 1;
           a[i++] = i;
while allowing
           i = i + 1;
           a[i] = i;

85) The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the major subclauses of this subclause, highest precedence first. Thus, for example, the expressions allowed as the operands of the binary + operator (6.5.6) are those expressions defined in 6.5.1 through 6.5.6. The exceptions are cast expressions (6.5.4) as operands of unary operators (6.5.3), and an operand contained between any of the following pairs of operators: grouping parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and the conditional operator ? : (6.5.15). Within each major subclause, the operators have the same precedence. Left- or right-associativity is indicated in each subclause by the syntax for the expressions discussed therein.

86) In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed consistently in different evaluations.

87) Allocated objects have no declared type.

88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.

89) The intermediate operations in the contracted expression are evaluated as if to infinite range and precision, while the final operation is rounded to the format determined by the expression evaluation method. A contracted expression might also omit the raising of floating-point exceptions.

90) This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions, their use needs to be well-defined and clearly documented.

Contents

6.5.1 Primary expressions

Syntax

1

          primary-expression:
                 identifier
                 constant
                 string-literal
                 ( expression )
                 generic-selection

Semantics

2 An identifier is a primary expression, provided it has been declared as designating an object (in which case it is an lvalue) or a function (in which case it is a function designator).91)

3 A constant is a primary expression. Its type depends on its form and value, as detailed in 6.4.4.

4 A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.

5 A parenthesized expression is a primary expression. Its type and value are identical to those of the unparenthesized expression. It is an lvalue, a function designator, or a void expression if the unparenthesized expression is, respectively, an lvalue, a function designator, or a void expression.

6 A generic selection is a primary expression. Its type and value depend on the selected generic association, as detailed in the following subclause.

Forward references: declarations (6.7).

Footnotes

91) Thus, an undeclared identifier is a violation of the syntax.

Contents

6.5.1.1 Generic selection

Syntax

1

          generic-selection:
                 _Generic ( assignment-expression , generic-assoc-list )
          generic-assoc-list:
                 generic-association
                 generic-assoc-list , generic-association
          generic-association:
                 type-name : assignment-expression
                 default : assignment-expression

Constraints

2 A generic selection shall have no more than one default generic association. The type name in a generic association shall specify a complete object type other than a variably modified type. No two generic associations in the same generic selection shall specify compatible types. The controlling expression of a generic selection shall have type compatible with at most one of the types named in its generic association list. If a generic selection has no default generic association, its controlling expression shall have type compatible with exactly one of the types named in its generic association list.

Semantics

3 The controlling expression of a generic selection is not evaluated. If a generic selection has a generic association with a type name that is compatible with the type of the controlling expression, then the result expression of the generic selection is the expression in that generic association. Otherwise, the result expression of the generic selection is the expression in the default generic association. None of the expressions from any other generic association of the generic selection is evaluated.

4 The type and value of a generic selection are identical to those of its result expression. It is an lvalue, a function designator, or a void expression if its result expression is, respectively, an lvalue, a function designator, or a void expression.

5 EXAMPLE The cbrt type-generic macro could be implemented as follows:

          #define cbrt(X) _Generic((X),                                      \
                                  long double: cbrtl,                        \
                                  default: cbrt,                             \
                                  float: cbrtf                               \
                                  )(X)

Contents

6.5.2 Postfix operators

Syntax

1

          postfix-expression:
                 primary-expression
                 postfix-expression [ expression ]
                 postfix-expression ( argument-expression-listopt )
                 postfix-expression . identifier
                 postfix-expression -> identifier
                 postfix-expression ++
                 postfix-expression --
                 ( type-name ) { initializer-list }
                 ( type-name ) { initializer-list , }
          argument-expression-list:
                assignment-expression
                argument-expression-list , assignment-expression

Contents

6.5.2.1 Array subscripting

Constraints

1 One of the expressions shall have type ''pointer to complete object type'', the other expression shall have integer type, and the result has type ''type''.

Semantics

2 A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

3 Successive subscript operators designate an element of a multidimensional array object. If E is an n-dimensional array (n >= 2) with dimensions i x j x . . . x k, then E (used as other than an lvalue) is converted to a pointer to an (n - 1)-dimensional array with dimensions j x . . . x k. If the unary * operator is applied to this pointer explicitly, or implicitly as a result of subscripting, the result is the referenced (n - 1)-dimensional array, which itself is converted into a pointer if used as other than an lvalue. It follows from this that arrays are stored in row-major order (last subscript varies fastest).

4 EXAMPLE Consider the array object defined by the declaration

          int x[3][5];
Here x is a 3 x 5 array of ints; more precisely, x is an array of three element objects, each of which is an array of five ints. In the expression x[i], which is equivalent to (*((x)+(i))), x is first converted to a pointer to the initial array of five ints. Then i is adjusted according to the type of x, which conceptually entails multiplying i by the size of the object to which the pointer points, namely an array of five int objects. The results are added and indirection is applied to yield an array of five ints. When used in the expression x[i][j], that array is in turn converted to a pointer to the first of the ints, so x[i][j] yields an int.

Forward references: additive operators (6.5.6), address and indirection operators (6.5.3.2), array declarators (6.7.6.2).

Contents

6.5.2.2 Function calls

Constraints

1 The expression that denotes the called function92) shall have type pointer to function returning void or returning a complete object type other than an array type.

2 If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

Semantics

3 A postfix expression followed by parentheses () containing a possibly empty, comma- separated list of expressions is a function call. The postfix expression denotes the called function. The list of expressions specifies the arguments to the function.

4 An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned the value of the corresponding argument.93)

5 If the expression that denotes the called function has type pointer to function returning an object type, the function call expression has the same type as that object type, and has the value determined as specified in 6.8.6.4. Otherwise, the function call has type void.

6 If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions. If the number of arguments does not equal the number of parameters, the behavior is undefined. If the function is defined with a type that includes a prototype, and either the prototype ends with an ellipsis (, ...) or the types of the arguments after promotion are not compatible with the types of the parameters, the behavior is undefined. If the function is defined with a type that does not include a prototype, and the types of the arguments after promotion are not compatible with those of the parameters after promotion, the behavior is undefined, except for the following cases:

7 If the expression that denotes the called function has a type that does include a prototype, the arguments are implicitly converted, as if by assignment, to the types of the corresponding parameters, taking the type of each parameter to be the unqualified version of its declared type. The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments.

8 No other conversions are performed implicitly; in particular, the number and types of arguments are not compared with those of the parameters in a function definition that does not include a function prototype declarator.

9 If the function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function, the behavior is undefined.

10 There is a sequence point after the evaluations of the function designator and the actual arguments but before the actual call. Every evaluation in the calling function (including other function calls) that is not otherwise specifically sequenced before or after the execution of the body of the called function is indeterminately sequenced with respect to the execution of the called function.94)

11 Recursive function calls shall be permitted, both directly and indirectly through any chain of other functions.

12 EXAMPLE In the function call

          (*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may be called in any order. All side effects have to be completed before the function pointed to by pf[f1()] is called.

Forward references: function declarators (including prototypes) (6.7.6.3), function definitions (6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).

Footnotes

92) Most often, this is the result of converting an identifier that is a function designator.

93) A function may change the values of its parameters, but these changes cannot affect the values of the arguments. On the other hand, it is possible to pass a pointer to an object, and the function may change the value of the object pointed to. A parameter declared to have array or function type is adjusted to have a pointer type as described in 6.9.1.

94) In other words, function executions do not ''interleave'' with each other.

Contents

6.5.2.3 Structure and union members

Constraints

1 The first operand of the . operator shall have an atomic, qualified, or unqualified structure or union type, and the second operand shall name a member of that type.

2 The first operand of the -> operator shall have type ''pointer to atomic, qualified, or unqualified structure'' or ''pointer to atomic, qualified, or unqualified union'', and the second operand shall name a member of the type pointed to.

Semantics

3 A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member,95) and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member.

4 A postfix expression followed by the -> operator and an identifier designates a member of a structure or union object. The value is that of the named member of the object to which the first expression points, and is an lvalue.96) If the first expression is a pointer to a qualified type, the result has the so-qualified version of the type of the designated member.

5 Accessing a member of an atomic structure or union object results in undefined behavior.97)

6 One special guarantee is made in order to simplify the use of unions: if a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the completed type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.

7 EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid postfix expression but is not an lvalue.

8 EXAMPLE 2 In:

          struct s { int i; const int ci; };
          struct s s;
          const struct s cs;
          volatile struct s vs;
the various members have the types:
          s.i        int
          s.ci       const int
          cs.i       const int
          cs.ci      const int
          vs.i       volatile int
          vs.ci      volatile const int

9 EXAMPLE 3 The following is a valid fragment:

          union {
                  struct {
                        int      alltypes;
                  } n;
                  struct {
                        int      type;
                        int      intnode;
                  } ni;
                  struct {
                        int      type;
                        double doublenode;
                  } nf;
          } u;
          u.nf.type = 1;
          u.nf.doublenode = 3.14;
          /* ... */
          if (u.n.alltypes == 1)
                  if (sin(u.nf.doublenode) == 0.0)
                        /* ... */
The following is not a valid fragment (because the union type is not visible within function f):
          struct t1 { int m; };
          struct t2 { int m; };
          int f(struct t1 *p1, struct t2 *p2)
          {
                if (p1->m < 0)
                        p2->m = -p2->m;
                return p1->m;
          }
          int g()
          {
                union {
                        struct t1 s1;
                        struct t2 s2;
                } u;
                /* ... */
                return f(&u.s1, &u.s2);
          }

Forward references: address and indirection operators (6.5.3.2), structure and union specifiers (6.7.2.1).

Footnotes

95) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ''type punning''). This might be a trap representation.

96) If &E is a valid pointer expression (where & is the ''address-of '' operator, which generates a pointer to its operand), the expression (&E)->MOS is the same as E.MOS.

97) For example, a data race would occur if access to the entire structure or union in one thread conflicts with access to a member from another thread, where at least one access is a modification. Members can be safely accessed using a non-atomic object which is assigned to or from the atomic object.

Contents

6.5.2.4 Postfix increment and decrement operators

Constraints

1 The operand of the postfix increment or decrement operator shall have atomic, qualified, or unqualified real or pointer type, and shall be a modifiable lvalue.

Semantics

2 The result of the postfix ++ operator is the value of the operand. As a side effect, the value of the operand object is incremented (that is, the value 1 of the appropriate type is added to it). See the discussions of additive operators and compound assignment for information on constraints, types, and conversions and the effects of operations on pointers. The value computation of the result is sequenced before the side effect of updating the stored value of the operand. With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation. Postfix ++ on an object with atomic type is a read-modify-write operation with memory_order_seq_cst memory order semantics.98)

3 The postfix -- operator is analogous to the postfix ++ operator, except that the value of the operand is decremented (that is, the value 1 of the appropriate type is subtracted from it).

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

Footnotes

98) Where a pointer to an atomic object can be formed and E has integer type, E++ is equivalent to the following code sequence where T is the type of E:

           T *addr = &E;
           T old = *addr;
           T new;
           do {
                  new = old + 1;
           } while (!atomic_compare_exchange_strong(addr, &old, new));
with old being the result of the operation. Special care must be taken if E has floating type; see 6.5.16.2.

Contents

6.5.2.5 Compound literals

Constraints

1 The type name shall specify a complete object type or an array of unknown size, but not a variable length array type.

2 All the constraints for initializer lists in 6.7.9 also apply to compound literals.

Semantics

3 A postfix expression that consists of a parenthesized type name followed by a brace- enclosed list of initializers is a compound literal. It provides an unnamed object whose value is given by the initializer list.99)

4 If the type name specifies an array of unknown size, the size is determined by the initializer list as specified in 6.7.9, and the type of the compound literal is that of the completed array type. Otherwise (when the type name specifies an object type), the type of the compound literal is that specified by the type name. In either case, the result is an lvalue.

5 The value of the compound literal is that of an unnamed object initialized by the initializer list. If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.

6 All the semantic rules for initializer lists in 6.7.9 also apply to compound literals.100)

7 String literals, and compound literals with const-qualified types, need not designate distinct objects.101)

8 EXAMPLE 1 The file scope definition

          int *p = (int []){2, 4};
initializes p to point to the first element of an array of two ints, the first having the value two and the second, four. The expressions in this compound literal are required to be constant. The unnamed object has static storage duration.

9 EXAMPLE 2 In contrast, in

          void f(void)
          {
                int *p;
                /*...*/
                p = (int [2]){*p};
                /*...*/
          }
p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and the second, zero. The expressions in this compound literal need not be constant. The unnamed object has automatic storage duration.

10 EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order:

          drawline((struct point){.x=1, .y=1},
                (struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct point:
          drawline(&(struct point){.x=1, .y=1},
                &(struct point){.x=3, .y=4});

11 EXAMPLE 4 A read-only compound literal can be specified through constructions like:

          (const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}

12 EXAMPLE 5 The following three expressions have different meanings:

          "/tmp/fileXXXXXX"
          (char []){"/tmp/fileXXXXXX"}
          (const char []){"/tmp/fileXXXXXX"}
The first always has static storage duration and has type array of char, but need not be modifiable; the last two have automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.

13 EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. For example,

          (const char []){"abc"} == "abc"
might yield 1 if the literals' storage is shared.

14 EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For example, there is no way to write a self-referential compound literal that could be used as the function argument in place of the named object endless_zeros below:

          struct int_list { int car; struct int_list *cdr; };
          struct int_list endless_zeros = {0, &endless_zeros};
          eval(endless_zeros);

15 EXAMPLE 8 Each compound literal creates only a single object in a given scope:

          struct s { int i; };
          int f (void)
          {
                struct s *p = 0, *q;
                int j = 0;
          again:
                    q = p, p = &((struct s){ j++ });
                    if (j < 2) goto again;
                    return p == q && q->i == 1;
          }
The function f() always returns the value 1.

16 Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would have an indeterminate value, which would result in undefined behavior.

Forward references: type names (6.7.7), initialization (6.7.9).

Footnotes

99) Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and the result of a cast expression is not an lvalue.

100) For example, subobjects without explicit initializers are initialized to zero.

101) This allows implementations to share storage for string literals and constant compound literals with the same or overlapping representations.

Contents

6.5.3 Unary operators

Syntax

1

          unary-expression:
                 postfix-expression
                 ++ unary-expression
                 -- unary-expression
                 unary-operator cast-expression
                 sizeof unary-expression
                 sizeof ( type-name )
                 _Alignof ( type-name )
          unary-operator: one of
                 & * + - ~             !

Contents

6.5.3.1 Prefix increment and decrement operators

Constraints

1 The operand of the prefix increment or decrement operator shall have atomic, qualified, or unqualified real or pointer type, and shall be a modifiable lvalue.

Semantics

2 The value of the operand of the prefix ++ operator is incremented. The result is the new value of the operand after incrementation. The expression ++E is equivalent to (E+=1). See the discussions of additive operators and compound assignment for information on constraints, types, side effects, and conversions and the effects of operations on pointers.

3 The prefix -- operator is analogous to the prefix ++ operator, except that the value of the operand is decremented.

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

Contents

6.5.3.2 Address and indirection operators

Constraints

1 The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

2 The operand of the unary * operator shall have pointer type.

Semantics

3 The unary & operator yields the address of its operand. If the operand has type ''type'', the result has type ''pointer to type''. If the operand is the result of a unary * operator, neither that operator nor the & operator is evaluated and the result is as if both were omitted, except that the constraints on the operators still apply and the result is not an lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result is as if the & operator were removed and the [] operator were changed to a + operator. Otherwise, the result is a pointer to the object or function designated by its operand.

4 The unary * operator denotes indirection. If the operand points to a function, the result is a function designator; if it points to an object, the result is an lvalue designating the object. If the operand has type ''pointer to type'', the result has type ''type''. If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.102)

Forward references: storage-class specifiers (6.7.1), structure and union specifiers (6.7.2.1).

Footnotes

102) Thus, &*E is equivalent to E (even if E is a null pointer), and &(E1[E2]) to ((E1)+(E2)). It is always true that if E is a function designator or an lvalue that is a valid operand of the unary & operator, *&E is a function designator or an lvalue equal to E. If *P is an lvalue and T is the name of an object pointer type, *(T)P is an lvalue that has a type compatible with that to which T points. Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately aligned for the type of object pointed to, and the address of an object after the end of its lifetime.

Contents

6.5.3.3 Unary arithmetic operators

Constraints

1 The operand of the unary + or - operator shall have arithmetic type; of the ~ operator, integer type; of the ! operator, scalar type.

Semantics

2 The result of the unary + operator is the value of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.

3 The result of the unary - operator is the negative of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.

4 The result of the ~ operator is the bitwise complement of its (promoted) operand (that is, each bit in the result is set if and only if the corresponding bit in the converted operand is not set). The integer promotions are performed on the operand, and the result has the promoted type. If the promoted type is an unsigned type, the expression ~E is equivalent to the maximum value representable in that type minus E.

5 The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

Contents

6.5.3.4 The sizeof and _Alignof operators

Constraints

1 The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member. The _Alignof operator shall not be applied to a function type or an incomplete type.

Semantics

2 The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

3 The _Alignof operator yields the alignment requirement of its operand type. The operand is not evaluated and the result is an integer constant. When applied to an array type, the result is the alignment requirement of the element type.

4 When sizeof is applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1. When applied to an operand that has array type, the result is the total number of bytes in the array.103) When applied to an operand that has structure or union type, the result is the total number of bytes in such an object, including internal and trailing padding.

5 The value of the result of both operators is implementation-defined, and its type (an unsigned integer type) is size_t, defined in <stddef.h> (and other headers).

6 EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/O systems. A storage-allocation function might accept a size (in bytes) of an object to allocate and return a pointer to void. For example:

         extern void *alloc(size_t);
         double *dp = alloc(sizeof *dp);
The implementation of the alloc function should ensure that its return value is aligned suitably for conversion to a pointer to double.

7 EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:

         sizeof array / sizeof array[0]

8 EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:

         #include <stddef.h>
          size_t fsize3(int n)
          {
                char b[n+3];                  // variable length array
                return sizeof b;              // execution time sizeof
          }
          int main()
          {
                size_t size;
                size = fsize3(10); // fsize3 returns 13
                return 0;
          }

Forward references: common definitions <stddef.h> (7.19), declarations (6.7), structure and union specifiers (6.7.2.1), type names (6.7.7), array declarators (6.7.6.2).

Footnotes

103) When applied to a parameter declared to have array or function type, the sizeof operator yields the size of the adjusted (pointer) type (see 6.9.1).

Contents

6.5.4 Cast operators

Syntax

1

          cast-expression:
                 unary-expression
                 ( type-name ) cast-expression

Constraints

2 Unless the type name specifies a void type, the type name shall specify atomic, qualified, or unqualified scalar type, and the operand shall have scalar type.

3 Conversions that involve pointers, other than where permitted by the constraints of 6.5.16.1, shall be specified by means of an explicit cast.

4 A pointer type shall not be converted to any floating type. A floating type shall not be converted to any pointer type.

Semantics

5 Preceding an expression by a parenthesized type name converts the value of the expression to the named type. This construction is called a cast.104) A cast that specifies no conversion has no effect on the type or value of an expression.

6 If the value of the expression is represented with greater range or precision than required by the type named by the cast (6.3.1.8), then the cast specifies a conversion even if the type of the expression is the same as the named type and removes any extra range and precision.

Forward references: equality operators (6.5.9), function declarators (including prototypes) (6.7.6.3), simple assignment (6.5.16.1), type names (6.7.7).

Footnotes

104) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the unqualified version of the type.

Contents

6.5.5 Multiplicative operators

Syntax

1

          multiplicative-expression:
                  cast-expression
                  multiplicative-expression * cast-expression
                  multiplicative-expression / cast-expression
                  multiplicative-expression % cast-expression

Constraints

2 Each of the operands shall have arithmetic type. The operands of the % operator shall have integer type.

Semantics

3 The usual arithmetic conversions are performed on the operands.

4 The result of the binary * operator is the product of the operands.

5 The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder. In both operations, if the value of the second operand is zero, the behavior is undefined.

6 When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.105) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a; otherwise, the behavior of both a/b and a%b is undefined.

Footnotes

105) This is often called ''truncation toward zero''.

Contents

6.5.6 Additive operators

Syntax

1

          additive-expression:
                 multiplicative-expression
                 additive-expression + multiplicative-expression
                 additive-expression - multiplicative-expression

Constraints

2 For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

3 For subtraction, one of the following shall hold:

(Decrementing is equivalent to subtracting 1.)

Semantics

4 If both operands have arithmetic type, the usual arithmetic conversions are performed on them.

5 The result of the binary + operator is the sum of the operands.

6 The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first.

7 For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

8 When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i-n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

9 When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array elements. The size of the result is implementation-defined, and its type (a signed integer type) is ptrdiff_t defined in the <stddef.h> header. If the result is not representable in an object of that type, the behavior is undefined. In other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of an array object, the expression (P)-(Q) has the value i-j provided the value fits in an object of type ptrdiff_t. Moreover, if the expression P points either to an element of an array object or one past the last element of an array object, and the expression Q points to the last element of the same array object, the expression ((Q)+1)-(P) has the same value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and has the value zero if the expression P points one past the last element of the array object, even though the expression (Q)+1 does not point to an element of the array object.106)

10 EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.

          {
                   int n = 4, m = 3;
                   int a[n][m];
                   int (*p)[m] = a;            //   p == &a[0]
                   p += 1;                     //   p == &a[1]
                   (*p)[2] = 99;               //   a[1][2] == 99
                   n = p - a;                  //   n == 1
          }

11 If array a in the above example were declared to be an array of known constant size, and pointer p were declared to be a pointer to an array of the same known constant size (pointing to a), the results would be the same.

Forward references: array declarators (6.7.6.2), common definitions <stddef.h> (7.19).

Footnotes

106) Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In this scheme the integer expression added to or subtracted from the converted pointer is first multiplied by the size of the object originally pointed to, and the resulting pointer is converted back to the original type. For pointer subtraction, the result of the difference between the character pointers is similarly divided by the size of the object originally pointed to. When viewed in this way, an implementation need only provide one extra byte (which may overlap another object in the program) just after the end of the object in order to satisfy the ''one past the last element'' requirements.

Contents

6.5.7 Bitwise shift operators

Syntax

1

          shift-expression:
                  additive-expression
                  shift-expression << additive-expression
                  shift-expression >> additive-expression

Constraints

2 Each of the operands shall have integer type.

Semantics

3 The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.

4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 x 2E2 , reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 x 2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

5 The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the resulting value is implementation-defined.

Contents

6.5.8 Relational operators

Syntax

1

          relational-expression:
                  shift-expression
                  relational-expression   <    shift-expression
                  relational-expression   >    shift-expression
                  relational-expression   <=   shift-expression
                  relational-expression   >=   shift-expression

Constraints

2 One of the following shall hold:

Semantics

3 If both of the operands have arithmetic type, the usual arithmetic conversions are performed.

4 For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

5 When two pointers are compared, the result depends on the relative locations in the address space of the objects pointed to. If two pointers to object types both point to the same object, or both point one past the last element of the same array object, they compare equal. If the objects pointed to are members of the same aggregate object, pointers to structure members declared later compare greater than pointers to members declared earlier in the structure, and pointers to array elements with larger subscript values compare greater than pointers to elements of the same array with lower subscript values. All pointers to members of the same union object compare equal. If the expression P points to an element of an array object and the expression Q points to the last element of the same array object, the pointer expression Q+1 compares greater than P. In all other cases, the behavior is undefined.

6 Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.107) The result has type int.

Footnotes

107) The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c; in other words, ''if a is less than b, compare 1 to c; otherwise, compare 0 to c''.

Contents

6.5.9 Equality operators

Syntax

1

          equality-expression:
                 relational-expression
                 equality-expression == relational-expression
                 equality-expression != relational-expression

Constraints

2 One of the following shall hold:

Semantics

3 The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence.108) Each of the operators yields 1 if the specified relation is true and 0 if it is false. The result has type int. For any pair of operands, exactly one of the relations is true.

4 If both of the operands have arithmetic type, the usual arithmetic conversions are performed. Values of complex types are equal if and only if both their real parts are equal and also their imaginary parts are equal. Any two values of arithmetic types from different type domains are equal if and only if the results of their conversions to the (complex) result type determined by the usual arithmetic conversions are equal.

5 Otherwise, at least one operand is a pointer. If one operand is a pointer and the other is a null pointer constant, the null pointer constant is converted to the type of the pointer. If one operand is a pointer to an object type and the other is a pointer to a qualified or unqualified version of void, the former is converted to the type of the latter.

6 Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.109)

7 For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

Footnotes

108) Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.

109) Two objects may be adjacent in memory because they are adjacent elements of a larger array or adjacent members of a structure with no padding between them, or because the implementation chose to place them so, even though they are unrelated. If prior invalid pointer operations (such as accesses outside array bounds) produced undefined behavior, subsequent comparisons also produce undefined behavior.

Contents

6.5.10 Bitwise AND operator

Syntax

1

          AND-expression:
                equality-expression
                AND-expression & equality-expression

Constraints

2 Each of the operands shall have integer type.

Semantics

3 The usual arithmetic conversions are performed on the operands.

4 The result of the binary & operator is the bitwise AND of the operands (that is, each bit in the result is set if and only if each of the corresponding bits in the converted operands is set).

Contents

6.5.11 Bitwise exclusive OR operator

Syntax

1

          exclusive-OR-expression:
                  AND-expression
                  exclusive-OR-expression ^ AND-expression

Constraints

2 Each of the operands shall have integer type.

Semantics

3 The usual arithmetic conversions are performed on the operands.

4 The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit in the result is set if and only if exactly one of the corresponding bits in the converted operands is set).

Contents

6.5.12 Bitwise inclusive OR operator

Syntax

1

          inclusive-OR-expression:
                  exclusive-OR-expression
                  inclusive-OR-expression | exclusive-OR-expression

Constraints

2 Each of the operands shall have integer type.

Semantics

3 The usual arithmetic conversions are performed on the operands.

4 The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in the result is set if and only if at least one of the corresponding bits in the converted operands is set).

Contents

6.5.13 Logical AND operator

Syntax

1

           logical-AND-expression:
                   inclusive-OR-expression
                   logical-AND-expression && inclusive-OR-expression

Constraints

2 Each of the operands shall have scalar type.

Semantics

3 The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

4 Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares equal to 0, the second operand is not evaluated.

Contents

6.5.14 Logical OR operator

Syntax

1

           logical-OR-expression:
                   logical-AND-expression
                   logical-OR-expression || logical-AND-expression

Constraints

2 Each of the operands shall have scalar type.

Semantics

3 The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

4 Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares unequal to 0, the second operand is not evaluated.

Contents

6.5.15 Conditional operator

Syntax

1

          conditional-expression:
                 logical-OR-expression
                 logical-OR-expression ? expression : conditional-expression

Constraints

2 The first operand shall have scalar type.

3 One of the following shall hold for the second and third operands:

Semantics

4 The first operand is evaluated; there is a sequence point between its evaluation and the evaluation of the second or third operand (whichever is evaluated). The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the result is the value of the second or third operand (whichever is evaluated), converted to the type described below.110)

5 If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result. If both the operands have structure or union type, the result has that type. If both operands have void type, the result has void type.

6 If both the second and third operands are pointers or one is a null pointer constant and the other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers of the types referenced by both operands. Furthermore, if both operands are pointers to compatible types or to differently qualified versions of compatible types, the result type is a pointer to an appropriately qualified version of the composite type; if one operand is a null pointer constant, the result has the type of the other operand; otherwise, one operand is a pointer to void or a qualified version of void, in which case the result type is a pointer to an appropriately qualified version of void.

7 EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent stages. The appropriate qualifiers, for example, do not depend on whether the two pointers have compatible types.

8 Given the declarations

           const void *c_vp;
           void *vp;
           const int *c_ip;
           volatile int *v_ip;
           int *ip;
           const char *c_cp;
the third column in the following table is the common type that is the result of a conditional expression in which the first two columns are the second and third operands (in either order):
           c_vp    c_ip      const void *
           v_ip    0         volatile int *
           c_ip    v_ip      const volatile int *
           vp      c_cp      const void *
           ip      c_ip      const int *
           vp      ip        void *

Footnotes

110) A conditional expression does not yield an lvalue.

Contents

6.5.16 Assignment operators

Syntax

1

          assignment-expression:
                 conditional-expression
                 unary-expression assignment-operator assignment-expression
          assignment-operator: one of
                 =   *=   /=   %=   +=   -=   <<=   >>=   &=   ^=   |=

Constraints

2 An assignment operator shall have a modifiable lvalue as its left operand.

Semantics

3 An assignment operator stores a value in the object designated by the left operand. An assignment expression has the value of the left operand after the assignment,111) but is not an lvalue. The type of an assignment expression is the type the left operand would have after lvalue conversion. The side effect of updating the stored value of the left operand is sequenced after the value computations of the left and right operands. The evaluations of the operands are unsequenced.

Footnotes

111) The implementation is permitted to read the object to determine the value but is not required to, even when the object has volatile-qualified type.

Contents

6.5.16.1 Simple assignment

Constraints

1 One of the following shall hold:112)

Semantics

2 In simple assignment (=), the value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand.

3 If the value being stored in an object is read from another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have qualified or unqualified versions of a compatible type; otherwise, the behavior is undefined.

4 EXAMPLE 1 In the program fragment

         int f(void);
         char c;
         /* ... */
         if ((c = f()) == -1)
                 /* ... */
the int value returned by the function may be truncated when stored in the char, and then converted back to int width prior to the comparison. In an implementation in which ''plain'' char has the same range of values as unsigned char (and char is narrower than int), the result of the conversion cannot be negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the variable c should be declared as int.

5 EXAMPLE 2 In the fragment:

         char c;
         int i;
         long l;
         l = (c = i);
the value of i is converted to the type of the assignment expression c = i, that is, char type. The value of the expression enclosed in parentheses is then converted to the type of the outer assignment expression, that is, long int type.

6 EXAMPLE 3 Consider the fragment:

         const char **cpp;
         char *p;
         const char c = 'A';
         cpp = &p;                  // constraint violation
         *cpp = &c;                 // valid
         *p = 0;                    // valid
The first assignment is unsafe because it would allow the following valid code to attempt to change the value of the const object c.

Footnotes

112) The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified in 6.3.2.1) that changes lvalues to ''the value of the expression'' and thus removes any type qualifiers that were applied to the type category of the expression (for example, it removes const but not volatile from the type int volatile * const).

Contents

6.5.16.2 Compound assignment

Constraints

1 For the operators += and -= only, either the left operand shall be an atomic, qualified, or unqualified pointer to a complete object type, and the right shall have integer type; or the left operand shall have atomic, qualified, or unqualified arithmetic type, and the right shall have arithmetic type.

2 For the other operators, the left operand shall have atomic, qualified, or unqualified arithmetic type, and (considering the type the left operand would have after lvalue conversion) each operand shall have arithmetic type consistent with those allowed by the corresponding binary operator.

Semantics

3 A compound assignment of the form E1 op = E2 is equivalent to the simple assignment expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation. If E1 has an atomic type, compound assignment is a read-modify-write operation with memory_order_seq_cst memory order semantics.113)

Footnotes

113) Where a pointer to an atomic object can be formed and E1 and E2 have integer type, this is equivalent to the following code sequence where T1 is the type of E1 and T2 is the type of E2:

           T1 *addr = &E1;
           T2 val = (E2);
           T1 old = *addr;
           T1 new;
           do {
                 new = old op val;
           } while (!atomic_compare_exchange_strong(addr, &old, new));
with new being the result of the operation. If E1 or E2 has floating type, then exceptional conditions or floating-point exceptions encountered during discarded evaluations of new should also be discarded in order to satisfy the equivalence of E1 op = E2 and E1 = E1 op (E2). For example, if annex F is in effect, the floating types involved have IEC 60559 formats, and FLT_EVAL_METHOD is 0, the equivalent code would be:
           #include <fenv.h>
           #pragma STDC FENV_ACCESS ON
           /* ... */
                   fenv_t fenv;
                   T1 *addr = &E1;
                   T2 val = E2;
                   T1 old = *addr;
                   T1 new;
                   feholdexcept(&fenv);
                   for (;;) {
                         new = old op val;
                         if (atomic_compare_exchange_strong(addr, &old, new))
                                     break;
                         feclearexcept(FE_ALL_EXCEPT);
                   }
                   feupdateenv(&fenv);
If FLT_EVAL_METHOD is not 0, then T2 must be a type with the range and precision to which E2 is evaluated in order to satisfy the equivalence.

Contents

6.5.17 Comma operator

Syntax

1

          expression:
                 assignment-expression
                 expression , assignment-expression

Semantics

2 The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.114)

3 EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists of initializers). On the other hand, it can be used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the function call

           f(a, (t=3, t+2), c)
the function has three arguments, the second of which has the value 5.

Forward references: initialization (6.7.9).

Footnotes

114) A comma operator does not yield an lvalue.

Contents

6.6 Constant expressions

Syntax

1

          constant-expression:
                 conditional-expression

Description

2 A constant expression can be evaluated during translation rather than runtime, and accordingly may be used in any place that a constant may be.

Constraints

3 Constant expressions shall not contain assignment, increment, decrement, function-call, or comma operators, except when they are contained within a subexpression that is not evaluated.115)

4 Each constant expression shall evaluate to a constant that is in the range of representable values for its type.

Semantics

5 An expression that evaluates to a constant is required in several contexts. If a floating expression is evaluated in the translation environment, the arithmetic range and precision shall be at least as great as if the expression were being evaluated in the execution environment.116)

6 An integer constant expression117) shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof or _Alignof operator.

7 More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:

8 An arithmetic constant expression shall have arithmetic type and shall only have operands that are integer constants, floating constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, and _Alignof expressions. Cast operators in an arithmetic constant expression shall only convert arithmetic types to arithmetic types, except as part of an operand to a sizeof or _Alignof operator.

9 An address constant is a null pointer, a pointer to an lvalue designating an object of static storage duration, or a pointer to a function designator; it shall be created explicitly using the unary & operator or an integer constant cast to pointer type, or implicitly by the use of an expression of array or function type. The array-subscript [] and member-access . and -> operators, the address & and indirection * unary operators, and pointer casts may be used in the creation of an address constant, but the value of an object shall not be accessed by use of these operators.

10 An implementation may accept other forms of constant expressions.

11 The semantic rules for the evaluation of a constant expression are the same as for nonconstant expressions.118)

Forward references: array declarators (6.7.6.2), initialization (6.7.9).

Footnotes

115) The operand of a sizeof or _Alignof operator is usually not evaluated (6.5.3.4).

116) The use of evaluation formats as characterized by FLT_EVAL_METHOD also applies to evaluation in the translation environment.

117) An integer constant expression is required in a number of contexts such as the size of a bit-field member of a structure, the value of an enumeration constant, and the size of a non-variable length array. Further constraints that apply to the integer constant expressions used in conditional-inclusion preprocessing directives are discussed in 6.10.1.

118) Thus, in the following initialization,

            static int i = 2 || 1 / 0;
the expression is a valid integer constant expression with value one.

Contents

6.7 Declarations

Syntax

1

          declaration:
                 declaration-specifiers init-declarator-listopt ;
                 static_assert-declaration
          declaration-specifiers:
                 storage-class-specifier declaration-specifiersopt
                 type-specifier declaration-specifiersopt
                 type-qualifier declaration-specifiersopt
                 function-specifier declaration-specifiersopt
                 alignment-specifier declaration-specifiersopt
          init-declarator-list:
                  init-declarator
                  init-declarator-list , init-declarator
          init-declarator:
                  declarator
                  declarator = initializer

Constraints

2 A declaration other than a static_assert declaration shall declare at least a declarator (other than the parameters of a function or the members of a structure or union), a tag, or the members of an enumeration.

3 If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a declarator or type specifier) with the same scope and in the same name space, except that:

4 All declarations in the same scope that refer to the same object or function shall specify compatible types.

Semantics

5 A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

6 The declaration specifiers consist of a sequence of specifiers that indicate the linkage, storage duration, and part of the type of the entities that the declarators denote. The init- declarator-list is a comma-separated sequence of declarators, each of which may have additional type information, or an initializer, or both. The declarators contain the identifiers (if any) being declared.

7 If an identifier for an object is declared with no linkage, the type for the object shall be complete by the end of its declarator, or by the end of its init-declarator if it has an initializer; in the case of function parameters (including in prototypes), it is the adjusted type (see 6.7.6.3) that is required to be complete.

Forward references: declarators (6.7.6), enumeration specifiers (6.7.2.2), initialization (6.7.9), type names (6.7.7), type qualifiers (6.7.3).

Footnotes

119) Function definitions have a different syntax, described in 6.9.1.

Contents

6.7.1 Storage-class specifiers

Syntax

1

          storage-class-specifier:
                 typedef
                 extern
                 static
                 _Thread_local
                 auto
                 register

Constraints

2 At most, one storage-class specifier may be given in the declaration specifiers in a declaration, except that _Thread_local may appear with static or extern.120)

3 In the declaration of an object with block scope, if the declaration specifiers include _Thread_local, they shall also include either static or extern. If _Thread_local appears in any declaration of an object, it shall be present in every declaration of that object.

4 _Thread_local shall not appear in the declaration specifiers of a function declaration.

Semantics

5 The typedef specifier is called a ''storage-class specifier'' for syntactic convenience only; it is discussed in 6.7.8. The meanings of the various linkages and storage durations were discussed in 6.2.2 and 6.2.4.

6 A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible. The extent to which such suggestions are effective is implementation-defined.121)

7 The declaration of an identifier for a function that has block scope shall have no explicit storage-class specifier other than extern.

8 If an aggregate or union object is declared with a storage-class specifier other than typedef, the properties resulting from the storage-class specifier, except with respect to linkage, also apply to the members of the object, and so on recursively for any aggregate or union member objects.

Forward references: type definitions (6.7.8).

Footnotes

120) See ''future language directions'' (6.11.5).

121) The implementation may treat any register declaration simply as an auto declaration. However, whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus, the only operators that can be applied to an array declared with storage-class specifier register are sizeof and _Alignof.

Contents

6.7.2 Type specifiers

Syntax

1

          type-specifier:
                 void
                 char
                 short
                 int
                 long
                 float
                 double
                 signed
                 unsigned
                 _Bool
                 _Complex
                 atomic-type-specifier
                 struct-or-union-specifier
                 enum-specifier
                 typedef-name

Constraints

2 At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name. Each list of type specifiers shall be one of the following multisets (delimited by commas, when there is more than one multiset per item); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers.

3 The type specifier _Complex shall not be used if the implementation does not support complex types (see 6.10.8.3).

Semantics

4 Specifiers for structures, unions, enumerations, and atomic types are discussed in 6.7.2.1 through 6.7.2.4. Declarations of typedef names are discussed in 6.7.8. The characteristics of the other types are discussed in 6.2.5.

5 Each of the comma-separated multisets designates the same type, except that for bit- fields, it is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int.

Forward references: atomic type specifiers (6.7.2.4), enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.8).

Contents

6.7.2.1 Structure and union specifiers

Syntax

1

          struct-or-union-specifier:
                  struct-or-union identifieropt { struct-declaration-list }
                  struct-or-union identifier
         struct-or-union:
                 struct
                 union
         struct-declaration-list:
                 struct-declaration
                 struct-declaration-list struct-declaration
         struct-declaration:
                 specifier-qualifier-list struct-declarator-listopt ;
                 static_assert-declaration
         specifier-qualifier-list:
                type-specifier specifier-qualifier-listopt
                type-qualifier specifier-qualifier-listopt
         struct-declarator-list:
                 struct-declarator
                 struct-declarator-list , struct-declarator
         struct-declarator:
                 declarator
                 declaratoropt : constant-expression

Constraints

2 A struct-declaration that does not declare an anonymous structure or anonymous union shall contain a struct-declarator-list.

3 A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type; such a structure (and any union containing, possibly recursively, a member that is such a structure) shall not be a member of a structure or an element of an array.

4 The expression that specifies the width of a bit-field shall be an integer constant expression with a nonnegative value that does not exceed the width of an object of the type that would be specified were the colon and expression omitted.122) If the value is zero, the declaration shall have no declarator.

5 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type. It is implementation-defined whether atomic types are permitted.

Semantics

6 As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose storage is allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose storage overlap.

7 Structure and union specifiers have the same form. The keywords struct and union indicate that the type being specified is, respectively, a structure type or a union type.

8 The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type, within a translation unit. The struct-declaration-list is a sequence of declarations for the members of the structure or union. If the struct-declaration-list does not contain any named members, either directly or via an anonymous structure or anonymous union, the behavior is undefined. The type is incomplete until immediately after the } that terminates the list, and complete thereafter.

9 A member of a structure or union may have any complete object type other than a variably modified type.123) In addition, a member may be declared to consist of a specified number of bits (including a sign bit, if any). Such a member is called a bit-field;124) its width is preceded by a colon.

10 A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified number of bits.125) If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool, the value of the bit-field shall compare equal to the value stored; a _Bool bit-field has the semantics of a _Bool.

11 An implementation may allocate any addressable storage unit large enough to hold a bit- field. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified.

12 A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed bit-field.126) As a special case, a bit-field structure member with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit- field, if any, was placed.

13 An unnamed member whose type specifier is a structure specifier with no tag is called an anonymous structure; an unnamed member whose type specifier is a union specifier with no tag is called an anonymous union. The members of an anonymous structure or union are considered to be members of the containing structure or union. This applies recursively if the containing structure or union is also anonymous.

14 Each non-bit-field member of a structure or union object is aligned in an implementation- defined manner appropriate to its type.

15 Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

16 The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit- field, then to the unit in which it resides), and vice versa.

17 There may be unnamed padding at the end of a structure or union.

18 As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. However, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.

19 EXAMPLE 1 The following illustrates anonymous structures and unions:

         struct v {
               union {      // anonymous union
                      struct { int i, j; };    // anonymous structure
                      struct { long k, l; } w;
               };
               int m;
         } v1;
          v1.i = 2;   // valid
          v1.k = 3;   // invalid: inner structure is not anonymous
          v1.w.k = 5; // valid

20 EXAMPLE 2 After the declaration:

          struct s { int n; double d[]; };
the structure struct s has a flexible array member d. A typical way to use this is:
          int m = /* some value */;
          struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been declared as:
          struct { int n; double d[m]; } *p;
(there are circumstances in which this equivalence is broken; in particular, the offsets of member d might not be the same).

21 Following the above declaration:

          struct s t1 = { 0 };                       //   valid
          struct s t2 = { 1, { 4.2 }};               //   invalid
          t1.n = 4;                                  //   valid
          t1.d[0] = 4.2;                             //   might be undefined behavior
The initialization of t2 is invalid (and violates a constraint) because struct s is treated as if it did not contain member d. The assignment to t1.d[0] is probably undefined behavior, but it is possible that
          sizeof (struct s) >= offsetof(struct s, d) + sizeof (double)
in which case the assignment would be legitimate. Nevertheless, it cannot appear in strictly conforming code.

22 After the further declaration:

          struct ss { int n; };
the expressions:
          sizeof (struct s) >= sizeof (struct ss)
          sizeof (struct s) >= offsetof(struct s, d)
are always equal to 1.

23 If sizeof (double) is 8, then after the following code is executed:

          struct s *s1;
          struct s *s2;
          s1 = malloc(sizeof (struct s) + 64);
          s2 = malloc(sizeof (struct s) + 46);
and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave, for most purposes, as if the identifiers had been declared as:
          struct { int n; double d[8]; } *s1;
          struct { int n; double d[5]; } *s2;

24 Following the further successful assignments:

          s1 = malloc(sizeof (struct s) + 10);
          s2 = malloc(sizeof (struct s) + 6);
they then behave as if the declarations were:
          struct { int n; double d[1]; } *s1, *s2;
and:
          double *dp;
          dp = &(s1->d[0]);          //   valid
          *dp = 42;                  //   valid
          dp = &(s2->d[0]);          //   valid
          *dp = 42;                  //   undefined behavior

25 The assignment:

          *s1 = *s2;
only copies the member n; if any of the array elements are within the first sizeof (struct s) bytes of the structure, they might be copied or simply overwritten with indeterminate values.

26 EXAMPLE 3 Because members of anonymous structures and unions are considered to be members of the containing structure or union, struct s in the following example has more than one named member and thus the use of a flexible array member is valid:

          struct s {
                struct { int i; };
                int a[];
          };

Forward references: declarators (6.7.6), tags (6.7.2.3).

Footnotes

122) While the number of bits in a _Bool object is at least CHAR_BIT, the width (number of sign and value bits) of a _Bool may be just 1 bit.

123) A structure or union cannot contain a member with a variably modified type because member names are not ordinary identifiers as defined in 6.2.3.

124) The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field objects.

125) As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int, then it is implementation-defined whether the bit-field is signed or unsigned.

126) An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts.

Contents

6.7.2.2 Enumeration specifiers

Syntax

1

          enum-specifier:
                enum identifieropt { enumerator-list }
                enum identifieropt { enumerator-list , }
                enum identifier
          enumerator-list:
                enumerator
                enumerator-list , enumerator
          enumerator:
                enumeration-constant
                enumeration-constant = constant-expression

Constraints

2 The expression that defines the value of an enumeration constant shall be an integer constant expression that has a value representable as an int.

Semantics

3 The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted.127) An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =, the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant. (The use of enumerators with = may produce enumeration constants with values that duplicate other values in the same enumeration.) The enumerators of an enumeration are also known as its members.

4 Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined,128) but shall be capable of representing the values of all the members of the enumeration. The enumerated type is incomplete until immediately after the } that terminates the list of enumerator declarations, and complete thereafter.

5 EXAMPLE The following fragment:

         enum hue { chartreuse, burgundy, claret=20, winedark };
         enum hue col, *cp;
         col = claret;
         cp = &col;
         if (*cp != burgundy)
               /* ... */
makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a pointer to an object that has that type. The enumerated values are in the set { 0, 1, 20, 21 }.

Forward references: tags (6.7.2.3).

Footnotes

127) Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from each other and from other identifiers declared in ordinary declarators.

128) An implementation may delay the choice of which integer type until all enumeration constants have been seen.

Contents

6.7.2.3 Tags

Constraints

1 A specific type shall have its content defined at most once.

2 Where two declarations that use the same tag declare the same type, they shall both use the same choice of struct, union, or enum.

3 A type specifier of the form

         enum identifier
without an enumerator list shall only appear after the type it specifies is complete.

Semantics

4 All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type. Irrespective of whether there is a tag or what other declarations of the type are in the same translation unit, the type is incomplete129) until immediately after the closing brace of the list defining the content, and complete thereafter.

5 Two declarations of structure, union, or enumerated types which are in different scopes or use different tags declare distinct types. Each declaration of a structure, union, or enumerated type which does not include a tag declares a distinct type.

6 A type specifier of the form

          struct-or-union identifieropt { struct-declaration-list }
or
          enum identifieropt { enumerator-list }
or
          enum identifieropt { enumerator-list , }
declares a structure, union, or enumerated type. The list defines the structure content, union content, or enumeration content. If an identifier is provided,130) the type specifier also declares the identifier to be the tag of that type.

7 A declaration of the form

          struct-or-union identifier ;
specifies a structure or union type and declares the identifier as a tag of that type.131)

8 If a type specifier of the form

          struct-or-union identifier
occurs other than as part of one of the above forms, and no other declaration of the identifier as a tag is visible, then it declares an incomplete structure or union type, and declares the identifier as the tag of that type.131)

9 If a type specifier of the form

          struct-or-union identifier
or
          enum identifier
occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is visible, then it specifies the same type as that other declaration, and does not redeclare the tag.

10 EXAMPLE 1 This mechanism allows declaration of a self-referential structure.

          struct tnode {
                int count;
                struct tnode *left, *right;
          };
specifies a structure that contains an integer and two pointers to objects of the same type. Once this declaration has been given, the declaration
          struct tnode s, *sp;
declares s to be an object of the given type and sp to be a pointer to an object of the given type. With these declarations, the expression sp->left refers to the left struct tnode pointer of the object to which sp points; the expression s.right->count designates the count member of the right struct tnode pointed to from s.

11 The following alternative formulation uses the typedef mechanism:

          typedef struct tnode TNODE;
          struct tnode {
                int count;
                TNODE *left, *right;
          };
          TNODE s, *sp;

12 EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the declarations

          struct s1 { struct s2 *s2p; /* ... */ }; // D1
          struct s2 { struct s1 *s1p; /* ... */ }; // D2
specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2. To eliminate this context sensitivity, the declaration
         struct s2;
may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification of the new type.

Forward references: declarators (6.7.6), type definitions (6.7.8).

Footnotes

129) An incomplete type may only by used when the size of an object of that type is not needed. It is not needed, for example, when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a structure or union is being declared. (See incomplete types in 6.2.5.) The specification has to be complete before such a function is called or defined.

130) If there is no identifier, the type can, within the translation unit, only be referred to by the declaration of which it is a part. Of course, when the declaration is of a typedef name, subsequent declarations can make use of that typedef name to declare objects having the specified structure, union, or enumerated type.

131) A similar construction with enum does not exist.

Contents

6.7.2.4 Atomic type specifiers

Syntax

1

          atomic-type-specifier:
                 _Atomic ( type-name )

Constraints

2 Atomic type specifiers shall not be used if the implementation does not support atomic types (see 6.10.8.3).

3 The type name in an atomic type specifier shall not refer to an array type, a function type, an atomic type, or a qualified type.

Semantics

4 The properties associated with atomic types are meaningful only for expressions that are lvalues. If the _Atomic keyword is immediately followed by a left parenthesis, it is interpreted as a type specifier (with a type name), not as a type qualifier.

Contents

6.7.3 Type qualifiers

Syntax

1

          type-qualifier:
                 const
                 restrict
                 volatile
                 _Atomic

Constraints

2 Types other than pointer types whose referenced type is an object type shall not be restrict-qualified.

3 The type modified by the _Atomic qualifier shall not be an array type or a function type.

Semantics

4 The properties associated with qualified types are meaningful only for expressions that are lvalues.132)

5 If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier in a specifier-qualifier- list, the resulting type is the so-qualified atomic type.

6 If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.133)

7 An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, except as modified by the unknown factors mentioned previously.134) What constitutes an access to an object that has volatile-qualified type is implementation-defined.

8 An object that is accessed through a restrict-qualified pointer has a special association with that pointer. This association, defined in 6.7.3.1 below, requires that all accesses to that object use, directly or indirectly, the value of that particular pointer.135) The intended use of the restrict qualifier (like the register storage class) is to promote optimization, and deleting all instances of the qualifier from all preprocessing translation units composing a conforming program does not change its meaning (i.e., observable behavior).

9 If the specification of an array type includes any type qualifiers, the element type is so- qualified, not the array type. If the specification of a function type includes any type qualifiers, the behavior is undefined.136)

10 For two qualified types to be compatible, both shall have the identically qualified version of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the specified type.

11 EXAMPLE 1 An object declared

         extern const volatile int real_time_clock;
may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.

12 EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type:

          const struct s { int mem; } cs = { 1 };
          struct s ncs; // the object ncs is modifiable
          typedef int A[2][3];
          const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
          int *pi;
          const int *pci;
          ncs = cs;               //   valid
          cs = ncs;               //   violates modifiable lvalue constraint for =
          pi = &ncs.mem;          //   valid
          pi = &cs.mem;           //   violates type constraints for =
          pci = &cs.mem;          //   valid
          pi = a[0];              //   invalid: a[0] has type ''const int *''

13 EXAMPLE 3 The declaration

          _Atomic volatile int *p;
specifies that p has the type ''pointer to volatile atomic int'', a pointer to a volatile-qualified atomic type.

Footnotes

132) The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

133) This applies to those objects that behave as if they were defined with qualified types, even if they are never actually defined as objects in the program (such as an object at a memory-mapped input/output address).

134) A volatile declaration may be used to describe an object corresponding to a memory-mapped input/output port or an object accessed by an asynchronously interrupting function. Actions on objects so declared shall not be ''optimized out'' by an implementation or reordered except as permitted by the rules for evaluating expressions.

135) For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between the allocated object and the pointer.

136) Both of these can occur through the use of typedefs.

Contents

6.7.3.1 Formal definition of restrict

1 Let D be a declaration of an ordinary identifier that provides a means of designating an object P as a restrict-qualified pointer to type T.

2 If D appears inside a block and does not have storage class extern, let B denote the block. If D appears in the list of parameter declarations of a function definition, let B denote the associated block. Otherwise, let B denote the block of main (or the block of whatever function is called at program startup in a freestanding environment).

3 In what follows, a pointer expression E is said to be based on object P if (at some sequence point in the execution of B prior to the evaluation of E) modifying P to point to a copy of the array object into which it formerly pointed would change the value of E.137) Note that ''based'' is defined only for expressions with pointer types.

4 During each execution of B, let L be any lvalue that has &L based on P. If L is used to access the value of the object X that it designates, and X is also modified (by any means), then the following requirements apply: T shall not be const-qualified. Every other lvalue used to access the value of X shall also have its address based on P. Every access that modifies X shall be considered also to modify P, for the purposes of this subclause. If P is assigned the value of a pointer expression E that is based on another restricted pointer object P2, associated with block B2, then either the execution of B2 shall begin before the execution of B, or the execution of B2 shall end prior to the assignment. If these requirements are not met, then the behavior is undefined.

5 Here an execution of B means that portion of the execution of the program that would correspond to the lifetime of an object with scalar type and automatic storage duration associated with B.

6 A translator is free to ignore any or all aliasing implications of uses of restrict.

7 EXAMPLE 1 The file scope declarations

         int * restrict a;
         int * restrict b;
         extern int c[];
assert that if an object is accessed using one of a, b, or c, and that object is modified anywhere in the program, then it is never accessed using either of the other two.

8 EXAMPLE 2 The function parameter declarations in the following example

         void f(int n, int * restrict p, int * restrict q)
         {
               while (n-- > 0)
                     *p++ = *q++;
         }
assert that, during each execution of the function, if an object is accessed through one of the pointer parameters, then it is not also accessed through the other.

9 The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f without examining any of the calls of f in the program. The cost is that the programmer has to examine all of those calls to ensure that none give undefined behavior. For example, the second call of f in g has undefined behavior because each of d[1] through d[49] is accessed through both p and q.

          void g(void)
          {
                extern int d[100];
                f(50, d + 50, d); // valid
                f(50, d + 1, d); // undefined behavior
          }

10 EXAMPLE 3 The function parameter declarations

         void h(int n, int * restrict p, int * restrict q, int * restrict r)
         {
               int i;
               for (i = 0; i < n; i++)
                      p[i] = q[i] + r[i];
         }
illustrate how an unmodified object can be aliased through two restricted pointers. In particular, if a and b are disjoint arrays, a call of the form h(100, a, b, b) has defined behavior, because array b is not modified within function h.

11 EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and an equivalent nested block. With one exception, only ''outer-to-inner'' assignments between restricted pointers declared in nested blocks have defined behavior.

          {
                   int * restrict p1;
                   int * restrict q1;
                   p1 = q1; // undefined behavior
                   {
                         int * restrict p2 = p1; // valid
                         int * restrict q2 = q1; // valid
                         p1 = q2;                // undefined behavior
                         p2 = q2;                // undefined behavior
                   }
          }

12 The one exception allows the value of a restricted pointer to be carried out of the block in which it (or, more precisely, the ordinary identifier used to designate it) is declared when that block finishes execution. For example, this permits new_vector to return a vector.

          typedef struct { int n; float * restrict v; } vector;
          vector new_vector(int n)
          {
                vector t;
                t.n = n;
                t.v = malloc(n * sizeof (float));
                return t;
          }

Footnotes

137) In other words, E depends on the value of P itself rather than on the value of an object referenced indirectly through P. For example, if identifier p has type (int **restrict), then the pointer expressions p and p+1 are based on the restricted pointer object designated by p, but the pointer expressions *p and p[1] are not.

Contents

6.7.4 Function specifiers

Syntax

1

          function-specifier:
                 inline
                 _Noreturn

Constraints

2 Function specifiers shall be used only in the declaration of an identifier for a function.

3 An inline definition of a function with external linkage shall not contain a definition of a modifiable object with static or thread storage duration, and shall not contain a reference to an identifier with internal linkage.

4 In a hosted environment, no function specifier(s) shall appear in a declaration of main.

Semantics

5 A function specifier may appear more than once; the behavior is the same as if it appeared only once.

6 A function declared with an inline function specifier is an inline function. Making a function an inline function suggests that calls to the function be as fast as possible.138) The extent to which such suggestions are effective is implementation-defined.139)

7 Any function with internal linkage can be an inline function. For a function with external linkage, the following restrictions apply: If a function is declared with an inline function specifier, then it shall also be defined in the same translation unit. If all of the file scope declarations for a function in a translation unit include the inline function specifier without extern, then the definition in that translation unit is an inline definition. An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit. An inline definition provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition.140)

8 A function declared with a _Noreturn function specifier shall not return to its caller.

Recommended practice

9 The implementation should produce a diagnostic message for a function declared with a _Noreturn function specifier that appears to be capable of returning to its caller.

10 EXAMPLE 1 The declaration of an inline function with external linkage can result in either an external definition, or a definition available for use only within the translation unit. A file scope declaration with extern creates an external definition. The following example shows an entire translation unit.

          inline double fahr(double t)
          {
                return (9.0 * t) / 5.0 + 32.0;
          }
          inline double cels(double t)
          {
                return (5.0 * (t - 32.0)) / 9.0;
          }
          extern double fahr(double);                  // creates an external definition
          double convert(int is_fahr, double temp)
          {
                /* A translator may perform inline substitutions */
                return is_fahr ? cels(temp) : fahr(temp);
          }

11 Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels is an inline definition. Because cels has external linkage and is referenced, an external definition has to appear in another translation unit (see 6.9); the inline definition and the external definition are distinct and either may be used for the call.

12 EXAMPLE 2

          _Noreturn void f () {
                abort(); // ok
          }
          _Noreturn void g (int i) { // causes undefined behavior if i <= 0
                if (i > 0) abort();
          }

Forward references: function definitions (6.9.1).

Footnotes

138) By using, for example, an alternative to the usual function call mechanism, such as ''inline substitution''. Inline substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro used within the body of the function uses the definition it had at the point the function body appears, and not where the function is called; and identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a single address, regardless of the number of inline definitions that occur in addition to the external definition.

139) For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration.

140) Since an inline definition is distinct from the corresponding external definition and from any other corresponding inline definitions in other translation units, all corresponding objects with static storage duration are also distinct in each of the definitions.

Contents

6.7.5 Alignment specifier

Syntax

1

          alignment-specifier:
                _Alignas ( type-name )
                _Alignas ( constant-expression )

Constraints

2 An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or a function, or a parameter, or an object declared with the register storage-class specifier.

3 The constant expression shall be an integer constant expression. It shall evaluate to a valid fundamental alignment, or to a valid extended alignment supported by the implementation in the context in which it appears, or to zero.

4 The combined effect of all alignment attributes in a declaration shall not specify an alignment that is less strict than the alignment that would otherwise be required for the type of the object or member being declared.

Semantics

5 The first form is equivalent to _Alignas (_Alignof (type-name)).

6 The alignment requirement of the declared object or member is taken to be the specified alignment. An alignment specification of zero has no effect.141) When multiple alignment specifiers occur in a declaration, the effective alignment requirement is the strictest specified alignment.

7 If the definition of an object has an alignment specifier, any other declaration of that object shall either specify equivalent alignment or have no alignment specifier. If the definition of an object does not have an alignment specifier, any other declaration of that object shall also have no alignment specifier. If declarations of an object in different translation units have different alignment specifiers, the behavior is undefined.

Footnotes

141) An alignment specification of zero also does not affect other alignment specifications in the same declaration.

Contents

6.7.6 Declarators

Syntax

1

          declarator:
                 pointeropt direct-declarator
          direct-declarator:
                  identifier
                  ( declarator )
                  direct-declarator [ type-qualifier-listopt assignment-expressionopt ]
                  direct-declarator [ static type-qualifier-listopt assignment-expression ]
                  direct-declarator [ type-qualifier-list static assignment-expression ]
                  direct-declarator [ type-qualifier-listopt * ]
                  direct-declarator ( parameter-type-list )
                  direct-declarator ( identifier-listopt )
          pointer:
                 * type-qualifier-listopt
                 * type-qualifier-listopt pointer
          type-qualifier-list:
                 type-qualifier
                 type-qualifier-list type-qualifier
          parameter-type-list:
                parameter-list
                parameter-list , ...
          parameter-list:
                parameter-declaration
                parameter-list , parameter-declaration
          parameter-declaration:
                declaration-specifiers declarator
                declaration-specifiers abstract-declaratoropt
          identifier-list:
                 identifier
                 identifier-list , identifier

Semantics

2 Each declarator declares one identifier, and asserts that when an operand of the same form as the declarator appears in an expression, it designates a function or object with the scope, storage duration, and type indicated by the declaration specifiers.

3 A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. If, in the nested sequence of declarators in a full declarator, there is a declarator specifying a variable length array type, the type specified by the full declarator is said to be variably modified. Furthermore, any type derived by declarator type derivation from a variably modified type is itself variably modified.

4 In the following subclauses, consider a declaration

          T D1
where T contains the declaration specifiers that specify a type T (such as int) and D1 is a declarator that contains an identifier ident. The type specified for the identifier ident in the various forms of declarator is described inductively using this notation.

5 If, in the declaration ''T D1'', D1 has the form

          identifier
then the type specified for ident is T .

6 If, in the declaration ''T D1'', D1 has the form

          ( D )
then ident has the type specified by the declaration ''T D''. Thus, a declarator in parentheses is identical to the unparenthesized declarator, but the binding of complicated declarators may be altered by parentheses.

Implementation limits

7 As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and function declarators that modify an arithmetic, structure, union, or void type, either directly or via one or more typedefs.

Forward references: array declarators (6.7.6.2), type definitions (6.7.8).

Contents

6.7.6.1 Pointer declarators

Semantics

1 If, in the declaration ''T D1'', D1 has the form

         * type-qualifier-listopt D
and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list T '', then the type specified for ident is ''derived-declarator-type-list type-qualifier-list pointer to T ''. For each type qualifier in the list, ident is a so-qualified pointer.

2 For two pointer types to be compatible, both shall be identically qualified and both shall be pointers to compatible types.

3 EXAMPLE The following pair of declarations demonstrates the difference between a ''variable pointer to a constant value'' and a ''constant pointer to a variable value''.

         const int *ptr_to_constant;
         int *const constant_ptr;
The contents of any object pointed to by ptr_to_constant shall not be modified through that pointer, but ptr_to_constant itself may be changed to point to another object. Similarly, the contents of the int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the same location.

4 The declaration of the constant pointer constant_ptr may be clarified by including a definition for the type ''pointer to int''.

         typedef int *int_ptr;
         const int_ptr constant_ptr;
declares constant_ptr as an object that has type ''const-qualified pointer to int''.

Contents

6.7.6.2 Array declarators

Constraints

1 In addition to optional type qualifiers and the keyword static, the [ and ] may delimit an expression or *. If they delimit an expression (which specifies the size of an array), the expression shall have an integer type. If the expression is a constant expression, it shall have a value greater than zero. The element type shall not be an incomplete or function type. The optional type qualifiers and the keyword static shall appear only in a declaration of a function parameter with an array type, and then only in the outermost array type derivation.

2 If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an identifier is declared to be an object with static or thread storage duration, it shall not have a variable length array type.

Semantics

3 If, in the declaration ''T D1'', D1 has one of the forms:

          D[ type-qualifier-listopt assignment-expressionopt ]
          D[ static type-qualifier-listopt assignment-expression ]
          D[ type-qualifier-list static assignment-expression ]
          D[ type-qualifier-listopt * ]
and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list T '', then the type specified for ident is ''derived-declarator-type-list array of T ''.142) (See 6.7.6.3 for the meaning of the optional type qualifiers and the keyword static.)

4 If the size is not present, the array type is an incomplete type. If the size is * instead of being an expression, the array type is a variable length array type of unspecified size, which can only be used in declarations or type names with function prototype scope;143) such arrays are nonetheless complete types. If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type. (Variable length arrays are a conditional feature that implementations need not support; see 6.10.8.3.)

5 If the size is an expression that is not an integer constant expression: if it occurs in a declaration at function prototype scope, it is treated as if it were replaced by *; otherwise, each time it is evaluated it shall have a value greater than zero. The size of each instance of a variable length array type does not change during its lifetime. Where a size expression is part of the operand of a sizeof operator and changing the value of the size expression would not affect the result of the operator, it is unspecified whether or not the size expression is evaluated.

6 For two array types to be compatible, both shall have compatible element types, and if both size specifiers are present, and are integer constant expressions, then both size specifiers shall have the same constant value. If the two array types are used in a context which requires them to be compatible, it is undefined behavior if the two size specifiers evaluate to unequal values.

7 EXAMPLE 1

          float fa[11], *afp[17];
declares an array of float numbers and an array of pointers to float numbers.

8 EXAMPLE 2 Note the distinction between the declarations

         extern int *x;
         extern int y[];
The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size (an incomplete type), the storage for which is defined elsewhere.

9 EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.

         extern int n;
         extern int m;
         void fcompat(void)
         {
               int a[n][6][m];
               int (*p)[4][n+1];
               int c[n][n][6][m];
               int (*r)[n][n][n+1];
               p = a;       // invalid: not compatible because 4 != 6
               r = c;       // compatible, but defined behavior only if
                            // n == 6 and m == n+1
         }

10 EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope. Array objects declared with the _Thread_local, static, or extern storage-class specifier cannot have a variable length array (VLA) type. However, an object declared with the static storage-class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members of structures or unions.

         extern int n;
         int A[n];                                            // invalid: file scope VLA
         extern int (*p2)[n];                                 // invalid: file scope VM
         int B[100];                                          // valid: file scope but not VM
         void fvla(int m, int C[m][m]);                       // valid: VLA with prototype scope
         void fvla(int m, int C[m][m])                        // valid: adjusted to auto pointer to VLA
         {
               typedef int VLA[m][m];                         // valid: block scope typedef VLA
                  struct tag {
                        int (*y)[n];                          // invalid: y not ordinary identifier
                        int z[n];                             // invalid: z not ordinary identifier
                  };
                  int D[m];                                   //   valid: auto VLA
                  static int E[m];                            //   invalid: static block scope VLA
                  extern int F[m];                            //   invalid: F has linkage and is VLA
                  int (*s)[m];                                //   valid: auto pointer to VLA
                  extern int (*r)[m];                         //   invalid: r has linkage and points to VLA
                  static int (*q)[m] = &B;                    //   valid: q is a static block pointer to VLA
         }

Forward references: function declarators (6.7.6.3), function definitions (6.9.1), initialization (6.7.9).

Footnotes

142) When several ''array of'' specifications are adjacent, a multidimensional array is declared.

143) Thus, * can be used only in function declarations that are not definitions (see 6.7.6.3).

Contents

6.7.6.3 Function declarators (including prototypes)

Constraints

1 A function declarator shall not specify a return type that is a function type or an array type.

2 The only storage-class specifier that shall occur in a parameter declaration is register.

3 An identifier list in a function declarator that is not part of a definition of that function shall be empty.

4 After adjustment, the parameters in a parameter type list in a function declarator that is part of a definition of that function shall not have incomplete type.

Semantics

5 If, in the declaration ''T D1'', D1 has the form

         D( parameter-type-list )
or
         D( identifier-listopt )
and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list T '', then the type specified for ident is ''derived-declarator-type-list function returning T ''.

6 A parameter type list specifies the types of, and may declare identifiers for, the parameters of the function.

7 A declaration of a parameter as ''array of type'' shall be adjusted to ''qualified pointer to type'', where the type qualifiers (if any) are those specified within the [ and ] of the array type derivation. If the keyword static also appears within the [ and ] of the array type derivation, then for each call to the function, the value of the corresponding actual argument shall provide access to the first element of an array with at least as many elements as specified by the size expression.

8 A declaration of a parameter as ''function returning type'' shall be adjusted to ''pointer to function returning type'', as in 6.3.2.1.

9 If the list terminates with an ellipsis (, ...), no information about the number or types of the parameters after the comma is supplied.144)

10 The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.

11 If, in a parameter declaration, an identifier can be treated either as a typedef name or as a parameter name, it shall be taken as a typedef name.

12 If the function declarator is not part of a definition of that function, parameters may have incomplete type and may use the [*] notation in their sequences of declarator specifiers to specify variable length array types.

13 The storage-class specifier in the declaration specifiers for a parameter declaration, if present, is ignored unless the declared parameter is one of the members of the parameter type list for a function definition.

14 An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.145)

15 For two function types to be compatible, both shall specify compatible return types.146) Moreover, the parameter type lists, if both are present, shall agree in the number of parameters and in use of the ellipsis terminator; corresponding parameters shall have compatible types. If one type has a parameter type list and the other type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, the parameter list shall not have an ellipsis terminator and the type of each parameter shall be compatible with the type that results from the application of the default argument promotions. If one type has a parameter type list and the other type is specified by a function definition that contains a (possibly empty) identifier list, both shall agree in the number of parameters, and the type of each prototype parameter shall be compatible with the type that results from the application of the default argument promotions to the type of the corresponding identifier. (In the determination of type compatibility and of a composite type, each parameter declared with function or array type is taken as having the adjusted type and each parameter declared with qualified type is taken as having the unqualified version of its declared type.)

16 EXAMPLE 1 The declaration

          int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an int, a function fip with no parameter specification returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an int. It is especially useful to compare the last two. The binding of *fip() is *(fip()), so that the declaration suggests, and the same construction in an expression requires, the calling of a function fip, and then using indirection through the pointer result to yield an int. In the declarator (*pfi)(), the extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function designator, which is then used to call the function; it returns an int.

17 If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the declaration occurs inside a function, the identifiers of the functions f and fip have block scope and either internal or external linkage (depending on what file scope declarations for these identifiers are visible), and the identifier of the pointer pfi has block scope and no linkage.

18 EXAMPLE 2 The declaration

           int (*apfi[3])(int *x, int *y);
declares an array apfi of three pointers to functions returning int. Each of these functions has two parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and go out of scope at the end of the declaration of apfi.

19 EXAMPLE 3 The declaration

           int (*fpfi(int (*)(long), int))(int, ...);
declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two parameters: a pointer to a function returning an int (with one parameter of type long int), and an int. The pointer returned by fpfi points to a function that has one int parameter and accepts zero or more additional arguments of any type.

20 EXAMPLE 4 The following prototype has a variably modified parameter.

           void addscalar(int n, int m,
                 double a[n][n*m+300], double x);
           int main()
           {
                 double b[4][308];
                 addscalar(4, 2, b, 2.17);
                 return 0;
           }
           void addscalar(int n, int m,
                 double a[n][n*m+300], double x)
           {
                 for (int i = 0; i < n; i++)
                       for (int j = 0, k = n*m+300; j < k; j++)
                             // a is a pointer to a VLA with n*m+300 elements
                             a[i][j] += x;
           }

21 EXAMPLE 5 The following are all compatible function prototype declarators.

           double    maximum(int       n,   int   m,   double     a[n][m]);
           double    maximum(int       n,   int   m,   double     a[*][*]);
           double    maximum(int       n,   int   m,   double     a[ ][*]);
           double    maximum(int       n,   int   m,   double     a[ ][m]);
as are:
           void   f(double      (* restrict a)[5]);
           void   f(double      a[restrict][5]);
           void   f(double      a[restrict 3][5]);
           void   f(double      a[restrict static 3][5]);
(Note that the last declaration also specifies that the argument corresponding to a in any call to f must be a non-null pointer to the first of at least three arrays of 5 doubles, which the others do not.)

Forward references: function definitions (6.9.1), type names (6.7.7).

Footnotes

144) The macros defined in the <stdarg.h> header (7.16) may be used to access arguments that correspond to the ellipsis.

145) See ''future language directions'' (6.11.6).

146) If both function types are ''old style'', parameter types are not compared.

Contents

6.7.7 Type names

Syntax

1

          type-name:
                 specifier-qualifier-list abstract-declaratoropt
          abstract-declarator:
                 pointer
                 pointeropt direct-abstract-declarator
          direct-abstract-declarator:
                  ( abstract-declarator )
                  direct-abstract-declaratoropt [ type-qualifier-listopt
                                 assignment-expressionopt ]
                  direct-abstract-declaratoropt [ static type-qualifier-listopt
                                 assignment-expression ]
                  direct-abstract-declaratoropt [ type-qualifier-list static
                                 assignment-expression ]
                  direct-abstract-declaratoropt [ * ]
                  direct-abstract-declaratoropt ( parameter-type-listopt )

Semantics

2 In several contexts, it is necessary to specify a type. This is accomplished using a type name, which is syntactically a declaration for a function or an object of that type that omits the identifier.147)

3 EXAMPLE The constructions

          (a)      int
          (b)      int   *
          (c)      int   *[3]
          (d)      int   (*)[3]
          (e)      int   (*)[*]
          (f)      int   *()
          (g)      int   (*)(void)
          (h)      int   (*const [])(unsigned int, ...)
name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an array of three ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function with no parameter specification returning a pointer to int, (g) pointer to function with no parameters returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one parameter that has type unsigned int and an unspecified number of other parameters, returning an int.

Footnotes

147) As indicated by the syntax, empty parentheses in a type name are interpreted as ''function with no parameter specification'', rather than redundant parentheses around the omitted identifier.

Contents

6.7.8 Type definitions

Syntax

1

          typedef-name:
                 identifier

Constraints

2 If a typedef name specifies a variably modified type then it shall have block scope.

Semantics

3 In a declaration whose storage-class specifier is typedef, each declarator defines an identifier to be a typedef name that denotes the type specified for the identifier in the way described in 6.7.6. Any array size expressions associated with variable length array declarators are evaluated each time the declaration of the typedef name is reached in the order of execution. A typedef declaration does not introduce a new type, only a synonym for the type so specified. That is, in the following declarations:

          typedef T type_ident;
          type_ident D;
type_ident is defined as a typedef name with the type specified by the declaration specifiers in T (known as T ), and the identifier in D has the type ''derived-declarator- type-list T '' where the derived-declarator-type-list is specified by the declarators of D. A typedef name shares the same name space as other identifiers declared in ordinary declarators.

4 EXAMPLE 1 After

          typedef int MILES, KLICKSP();
          typedef struct { double hi, lo; } range;
the constructions
          MILES distance;
          extern KLICKSP *metricp;
          range x;
          range z, *zp;
are all valid declarations. The type of distance is int, that of metricp is ''pointer to function with no parameter specification returning int'', and that of x and z is the specified structure; zp is a pointer to such a structure. The object distance has a type compatible with any other int object.

5 EXAMPLE 2 After the declarations

          typedef struct s1 { int x; } t1, *tp1;
          typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct s1, but not compatible with the types struct s2, t2, the type pointed to by tp2, or int.

6 EXAMPLE 3 The following obscure constructions

          typedef signed int t;
          typedef int plain;
          struct tag {
                unsigned t:4;
                const t:5;
                plain r:5;
          };
declare a typedef name t with type signed int, a typedef name plain with type int, and a structure with three bit-field members, one named t that contains values in the range [0, 15], an unnamed const- qualified bit-field which (if it could be accessed) would contain values in either the range [-15, +15] or [-16, +15], and one named r that contains values in one of the ranges [0, 31], [-15, +15], or [-16, +15]. (The choice of range is implementation-defined.) The first two bit-field declarations differ in that unsigned is a type specifier (which forces t to be the name of a structure member), while const is a type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed in an inner scope by
          t f(t (t));
          long t;
then a function f is declared with type ''function returning signed int with one unnamed parameter with type pointer to function returning signed int with one unnamed parameter with type signed int'', and an identifier t with type long int.

7 EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the following declarations of the signal function specify exactly the same type, the first without making use of any typedef names.

          typedef void fv(int), (*pfv)(int);
          void (*signal(int, void (*)(int)))(int);
          fv *signal(int, fv *);
          pfv signal(int, pfv);

8 EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef name is defined, not each time it is used:

          void copyt(int n)
          {
                typedef int B[n];   //  B is n ints, n evaluated now
                n += 1;
                B a;                //  a is n ints, n without += 1
                int b[n];           //  a and b are different sizes
                for (int i = 1; i < n; i++)
                      a[i-1] = b[i];
          }

Contents

6.7.9 Initialization

Syntax

1

          initializer:
                   assignment-expression
                   { initializer-list }
                   { initializer-list , }
          initializer-list:
                   designationopt initializer
                   initializer-list , designationopt initializer
          designation:
                 designator-list =
          designator-list:
                 designator
                 designator-list designator
          designator:
                 [ constant-expression ]
                 . identifier

Constraints

2 No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

3 The type of the entity to be initialized shall be an array of unknown size or a complete object type that is not a variable length array type.

4 All the expressions in an initializer for an object that has static or thread storage duration shall be constant expressions or string literals.

5 If the declaration of an identifier has block scope, and the identifier has external or internal linkage, the declaration shall have no initializer for the identifier.

6 If a designator has the form

          [ constant-expression ]
then the current object (defined below) shall have array type and the expression shall be an integer constant expression. If the array is of unknown size, any nonnegative value is valid.

7 If a designator has the form

          . identifier
then the current object (defined below) shall have structure or union type and the identifier shall be the name of a member of that type.

Semantics

8 An initializer specifies the initial value stored in an object.

9 Except where explicitly stated otherwise, for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization. Unnamed members of structure objects have indeterminate value even after initialization.

10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

11 The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression (after conversion); the same type constraints and conversions as for simple assignment apply, taking the type of the scalar to be the unqualified version of its declared type.

12 The rest of this subclause deals with initializers for objects that have aggregate or union type.

13 The initializer for a structure or union object that has automatic storage duration shall be either an initializer list as described below, or a single expression that has compatible structure or union type. In the latter case, the initial value of the object, including unnamed members, is that of the expression.

14 An array of character type may be initialized by a character string literal or UTF-8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

15 An array with element type compatible with a qualified or unqualified version of wchar_t, char16_t, or char32_t may be initialized by a wide string literal with the corresponding encoding prefix (L, u, or U, respectively), optionally enclosed in braces. Successive wide characters of the wide string literal (including the terminating null wide character if there is room or if the array is of unknown size) initialize the elements of the array.

16 Otherwise, the initializer for an object that has aggregate or union type shall be a brace- enclosed list of initializers for the elements or named members.

17 Each brace-enclosed initializer list has an associated current object. When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order, and the first named member of a union.148) In contrast, a designation causes the following initializer to begin initialization of the subobject described by the designator. Initialization then continues forward in order, beginning with the next subobject after that described by the designator.149)

18 Each designator list begins its description with the current object associated with the closest surrounding brace pair. Each item in the designator list (in order) specifies a particular member of its current object and changes the current object for the next designator (if any) to be that member.150) The current object that results at the end of the designator list is the subobject to be initialized by the following initializer.

19 The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;151) all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

20 If the aggregate or union contains elements or members that are aggregates or unions, these rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only enough initializers from the list are taken to account for the elements or members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next element or member of the aggregate of which the current subaggregate or contained union is a part.

21 If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

22 If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. The array type is completed at the end of its initializer list.

23 The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified.152)

24 EXAMPLE 1 Provided that <complex.h> has been #included, the declarations

          int i = 3.5;
          double complex c = 5 + 3 * I;
define and initialize i with the value 3 and c with the value 5.0 + i3.0.

25 EXAMPLE 2 The declaration

          int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array object that has three elements, as no size was specified and there are three initializers.

26 EXAMPLE 3 The declaration

          int y[4][3] = {
                { 1, 3, 5 },
                { 2, 4, 6 },
                { 3, 5, 7 },
          };
is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object y[0]), namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, so y[3] is initialized with zeros. Precisely the same effect could have been achieved by
          int y[4][3] = {
                1, 3, 5, 2, 4, 6, 3, 5, 7
          };
The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the next three are taken successively for y[1] and y[2].

27 EXAMPLE 4 The declaration

          int z[4][3] = {
                { 1 }, { 2 }, { 3 }, { 4 }
          };
initializes the first column of z as specified and initializes the rest with zeros.

28 EXAMPLE 5 The declaration

          struct { int a[3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed initialization. It defines an array with two element structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero.

29 EXAMPLE 6 The declaration

           short q[4][3][2] = {
                 { 1 },
                 { 2, 3 },
                 { 4, 5, 6 }
           };
contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize q[2][0][0], q[2][0][1], and q[2][1][0], respectively; all the rest are zero. The initializer for q[0][0] does not begin with a left brace, so up to six items from the current list may be used. There is only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers for q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their respective two-dimensional subaggregates. If there had been more than six items in any of the lists, a diagnostic message would have been issued. The same initialization result could have been achieved by:
           short q[4][3][2] = {
                 1, 0, 0, 0, 0, 0,
                 2, 3, 0, 0, 0, 0,
                 4, 5, 6
           };
or by:
           short q[4][3][2] = {
                 {
                       { 1 },
                 },
                 {
                       { 2, 3 },
                 },
                 {
                       { 4, 5 },
                       { 6 },
                 }
           };
in a fully bracketed form.

30 Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.

31 EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration

           typedef int A[];            // OK - declared with block scope
the declaration
           A a = { 1, 2 }, b = { 3, 4, 5 };
is identical to
           int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.

32 EXAMPLE 8 The declaration

          char s[] = "abc", t[3] = "abc";
defines ''plain'' char array objects s and t whose elements are initialized with character string literals. This declaration is identical to
          char s[] = { 'a', 'b', 'c', '\0' },
               t[] = { 'a', 'b', 'c' };
The contents of the arrays are modifiable. On the other hand, the declaration
          char *p = "abc";
defines p with type ''pointer to char'' and initializes it to point to an object with type ''array of char'' with length 4 whose elements are initialized with a character string literal. If an attempt is made to use p to modify the contents of the array, the behavior is undefined.

33 EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:

          enum { member_one, member_two };
          const char *nm[] = {
                [member_two] = "member two",
                [member_one] = "member one",
          };

34 EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:

          div_t answer = { .quot = 2, .rem = -1 };

35 EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunderstood:

          struct { int a[3], b; } w[] =
                { [0].a = {1}, [1].a[0] = 2 };

36 EXAMPLE 12 Space can be ''allocated'' from both ends of an array by using a single designator:

          int a[MAX] = {
                1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
          };

37 In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of the values provided by the first five initializers will be overridden by the second five.

38 EXAMPLE 13 Any member of a union can be initialized:

          union { /* ... */ } u = { .any_member = 42 };

Forward references: common definitions <stddef.h> (7.19).

Footnotes

148) If the initializer list for a subaggregate or contained union does not begin with a left brace, its subobjects are initialized as usual, but the subaggregate or contained union does not become the current object: current objects are associated only with brace-enclosed initializer lists.

149) After a union member is initialized, the next object is not the next member of the union; instead, it is the next subobject of an object containing the union.

150) Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with the surrounding brace pair. Note, too, that each separate designator list is independent.

151) Any initializer for the subobject which is overridden and so not used to initialize that subobject might not be evaluated at all.

152) In particular, the evaluation order need not be the same as the order of subobject initialization.

Contents

6.7.10 Static assertions

Syntax

1

          static_assert-declaration:
                  _Static_assert ( constant-expression , string-literal ) ;

Constraints

2 The constant expression shall compare unequal to 0.

Semantics

3 The constant expression shall be an integer constant expression. If the value of the constant expression compares unequal to 0, the declaration has no effect. Otherwise, the constraint is violated and the implementation shall produce a diagnostic message that includes the text of the string literal, except that characters not in the basic source character set are not required to appear in the message.

Forward references: diagnostics (7.2).

Contents

6.8 Statements and blocks

Syntax

1

          statement:
                 labeled-statement
                 compound-statement
                 expression-statement
                 selection-statement
                 iteration-statement
                 jump-statement

Semantics

2 A statement specifies an action to be performed. Except as indicated, statements are executed in sequence.

3 A block allows a set of declarations and statements to be grouped into one syntactic unit. The initializers of objects that have automatic storage duration, and the variable length array declarators of ordinary identifiers with block scope, are evaluated and the values are stored in the objects (including storing an indeterminate value in objects without an initializer) each time the declaration is reached in the order of execution, as if it were a statement, and within each declaration in the order that declarators appear.

4 A full expression is an expression that is not part of another expression or of a declarator. Each of the following is a full expression: an initializer that is not part of a compound literal; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the (optional) expressions of a for statement; the (optional) expression in a return statement. There is a sequence point between the evaluation of a full expression and the evaluation of the next full expression to be evaluated.

Forward references: expression and null statements (6.8.3), selection statements (6.8.4), iteration statements (6.8.5), the return statement (6.8.6.4).

Contents

6.8.1 Labeled statements

Syntax

1

          labeled-statement:
                 identifier : statement
                 case constant-expression : statement
                 default : statement

Constraints

2 A case or default label shall appear only in a switch statement. Further constraints on such labels are discussed under the switch statement.

3 Label names shall be unique within a function.

Semantics

4 Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them.

Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2).

Contents

6.8.2 Compound statement

Syntax

1

          compound-statement:
                { block-item-listopt }
          block-item-list:
                  block-item
                  block-item-list block-item
          block-item:
                  declaration
                  statement

Semantics

2 A compound statement is a block.

Contents

6.8.3 Expression and null statements

Syntax

1

          expression-statement:
                 expressionopt ;

Semantics

2 The expression in an expression statement is evaluated as a void expression for its side effects.153)

3 A null statement (consisting of just a semicolon) performs no operations.

4 EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value may be made explicit by converting the expression to a void expression by means of a cast:

          int p(int);
          /* ... */
          (void)p(0);

5 EXAMPLE 2 In the program fragment

          char *s;
          /* ... */
          while (*s++ != '\0')
                  ;
a null statement is used to supply an empty loop body to the iteration statement.

6 EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement.

          while (loop1) {
                /* ... */
                while (loop2) {
                        /* ... */
                        if (want_out)
                                goto end_loop1;
                        /* ... */
                }
                /* ... */
          end_loop1: ;
          }

Forward references: iteration statements (6.8.5).

Footnotes

153) Such as assignments, and function calls which have side effects.

Contents

6.8.4 Selection statements

Syntax

1

          selection-statement:
                  if ( expression ) statement
                  if ( expression ) statement else statement
                  switch ( expression ) statement

Semantics

2 A selection statement selects among a set of statements depending on the value of a controlling expression.

3 A selection statement is a block whose scope is a strict subset of the scope of its enclosing block. Each associated substatement is also a block whose scope is a strict subset of the scope of the selection statement.

Contents

6.8.4.1 The if statement

Constraints

1 The controlling expression of an if statement shall have scalar type.

Semantics

2 In both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed.

3 An else is associated with the lexically nearest preceding if that is allowed by the syntax.

Contents

6.8.4.2 The switch statement

Constraints

1 The controlling expression of a switch statement shall have integer type.

2 If a switch statement has an associated case or default label within the scope of an identifier with a variably modified type, the entire switch statement shall be within the scope of that identifier.154)

3 The expression of each case label shall be an integer constant expression and no two of the case constant expressions in the same switch statement shall have the same value after conversion. There may be at most one default label in a switch statement. (Any enclosed switch statement may have a default label or case constant expressions with values that duplicate case constant expressions in the enclosing switch statement.)

Semantics

4 A switch statement causes control to jump to, into, or past the statement that is the switch body, depending on the value of a controlling expression, and on the presence of a default label and the values of any case labels on or in the switch body. A case or default label is accessible only within the closest enclosing switch statement.

5 The integer promotions are performed on the controlling expression. The constant expression in each case label is converted to the promoted type of the controlling expression. If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label. Otherwise, if there is a default label, control jumps to the labeled statement. If no converted case constant expression matches and there is no default label, no part of the switch body is executed.

Implementation limits

6 As discussed in 5.2.4.1, the implementation may limit the number of case values in a switch statement.

7 EXAMPLE In the artificial program fragment

          switch (expr)
          {
                int i = 4;
                f(i);
          case 0:
                i = 17;
                /* falls through into default code */
          default:
                printf("%d\n", i);
          }
the object whose identifier is i exists with automatic storage duration (within the block) but is never initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will access an indeterminate value. Similarly, the call to the function f cannot be reached.

Footnotes

154) That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with the switch that is in the block containing the declaration.

Contents

6.8.5 Iteration statements

Syntax

1

          iteration-statement:
                  while ( expression ) statement
                  do statement while ( expression ) ;
                  for ( expressionopt ; expressionopt ; expressionopt ) statement
                  for ( declaration expressionopt ; expressionopt ) statement

Constraints

2 The controlling expression of an iteration statement shall have scalar type.

3 The declaration part of a for statement shall only declare identifiers for objects having storage class auto or register.

Semantics

4 An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0. The repetition occurs regardless of whether the loop body is entered from the iteration statement or by a jump.155)

5 An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block. The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.

6 An iteration statement whose controlling expression is not a constant expression,156) that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.157)

Footnotes

155) Code jumped over is not executed. In particular, the controlling expression of a for or while statement is not evaluated before entering the loop body, nor is clause-1 of a for statement.

156) An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.

157) This is intended to allow compiler transformations such as removal of empty loops even when termination cannot be proven.

Contents

6.8.5.1 The while statement

1 The evaluation of the controlling expression takes place before each execution of the loop body.

Contents

6.8.5.2 The do statement

1 The evaluation of the controlling expression takes place after each execution of the loop body.

Contents

6.8.5.3 The for statement

1 The statement

          for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any identifiers it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.158)

2 Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

Footnotes

158) Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration, such that execution of the loop continues until the expression compares equal to 0; and expression-3 specifies an operation (such as incrementing) that is performed after each iteration.

Contents

6.8.6 Jump statements

Syntax

1

          jump-statement:
                 goto identifier ;
                 continue ;
                 break ;
                 return expressionopt ;

Semantics

2 A jump statement causes an unconditional jump to another place.

Contents

6.8.6.1 The goto statement

Constraints

1 The identifier in a goto statement shall name a label located somewhere in the enclosing function. A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier.

Semantics

2 A goto statement causes an unconditional jump to the statement prefixed by the named label in the enclosing function.

3 EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline presents one possible approach to a problem based on these three assumptions:

  1. The general initialization code accesses objects only visible to the current function.
  2. The general initialization code is too large to warrant duplication.
  3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements, for example.)
        /* ... */
        goto first_time;
        for (;;) {
                // determine next operation
                /* ... */
                if (need to reinitialize) {
                        // reinitialize-only code
                        /* ... */
                first_time:
                        // general initialization code
                        /* ... */
                        continue;
                }
                // handle other operations
                /* ... */
        }
    

4 EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump within the scope, however, is permitted.

         goto lab3;                         // invalid: going INTO scope of VLA.
         {
               double a[n];
               a[j] = 4.4;
         lab3:
               a[j] = 3.3;
               goto lab4;                   // valid: going WITHIN scope of VLA.
               a[j] = 5.5;
         lab4:
               a[j] = 6.6;
         }
         goto lab4;                         // invalid: going INTO scope of VLA.

Contents

6.8.6.2 The continue statement

Constraints

1 A continue statement shall appear only in or as a loop body.

Semantics

2 A continue statement causes a jump to the loop-continuation portion of the smallest enclosing iteration statement; that is, to the end of the loop body. More precisely, in each of the statements

 while (/* ... */) {                  do {                                 for (/* ... */) {
    /* ... */                            /* ... */                            /* ... */
    continue;                            continue;                            continue;
    /* ... */                            /* ... */                            /* ... */
 contin: ;                            contin: ;                            contin: ;
 }                                    } while (/* ... */);                 }
unless the continue statement shown is in an enclosed iteration statement (in which case it is interpreted within that statement), it is equivalent to goto contin;.159)

Footnotes

159) Following the contin: label is a null statement.

Contents

6.8.6.3 The break statement

Constraints

1 A break statement shall appear only in or as a switch body or loop body.

Semantics

2 A break statement terminates execution of the smallest enclosing switch or iteration statement.

Contents

6.8.6.4 The return statement

Constraints

1 A return statement with an expression shall not appear in a function whose return type is void. A return statement without an expression shall only appear in a function whose return type is void.

Semantics

2 A return statement terminates execution of the current function and returns control to its caller. A function may have any number of return statements.

3 If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.160)

4 EXAMPLE In:

         struct s { double i; } f(void);
         union {
               struct {
                     int f1;
                     struct s f2;
               } u1;
               struct {
                     struct s f3;
                     int f4;
               } u2;
         } g;
         struct s f(void)
         {
               return g.u1.f2;
         }
         /* ... */
         g.u2.f3 = f();
there is no undefined behavior, although there would be if the assignment were done directly (without using a function call to fetch the value).

Footnotes

160) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. The representation of floating-point values may have wider range or precision than implied by the type; a cast may be used to remove this extra range and precision.

Contents

6.9 External definitions

Syntax

1

          translation-unit:
                  external-declaration
                  translation-unit external-declaration
          external-declaration:
                 function-definition
                 declaration

Constraints

2 The storage-class specifiers auto and register shall not appear in the declaration specifiers in an external declaration.

3 There shall be no more than one external definition for each identifier declared with internal linkage in a translation unit. Moreover, if an identifier declared with internal linkage is used in an expression (other than as a part of the operand of a sizeof or _Alignof operator whose result is an integer constant), there shall be exactly one external definition for the identifier in the translation unit.

Semantics

4 As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a sequence of external declarations. These are described as ''external'' because they appear outside any function (and hence have file scope). As discussed in 6.7, a declaration that also causes storage to be reserved for an object or a function named by the identifier is a definition.

5 An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof or _Alignof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.161)

Footnotes

161) Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for it.

Contents

6.9.1 Function definitions

Syntax

1

          function-definition:
                 declaration-specifiers declarator declaration-listopt compound-statement
          declaration-list:
                 declaration
                 declaration-list declaration

Constraints

2 The identifier declared in a function definition (which is the name of the function) shall have a function type, as specified by the declarator portion of the function definition.162)

3 The return type of a function shall be void or a complete object type other than array type.

4 The storage-class specifier, if any, in the declaration specifiers shall be either extern or static.

5 If the declarator includes a parameter type list, the declaration of each parameter shall include an identifier, except for the special case of a parameter list consisting of a single parameter of type void, in which case there shall not be an identifier. No declaration list shall follow.

6 If the declarator includes an identifier list, each declaration in the declaration list shall have at least one declarator, those declarators shall declare only identifiers from the identifier list, and every identifier in the identifier list shall be declared. An identifier declared as a typedef name shall not be redeclared as a parameter. The declarations in the declaration list shall contain no storage-class specifier other than register and no initializations.

Semantics

7 The declarator in a function definition specifies the name of the function being defined and the identifiers of its parameters. If the declarator includes a parameter type list, the list also specifies the types of all the parameters; such a declarator also serves as a function prototype for later calls to the same function in the same translation unit. If the declarator includes an identifier list,163) the types of the parameters shall be declared in a following declaration list. In either case, the type of each parameter is adjusted as described in 6.7.6.3 for a parameter type list; the resulting type shall be a complete object type.

8 If a function that accepts a variable number of arguments is defined without a parameter type list that ends with the ellipsis notation, the behavior is undefined.

9 Each parameter has automatic storage duration; its identifier is an lvalue.164) The layout of the storage for parameters is unspecified.

10 On entry to the function, the size expressions of each variably modified parameter are evaluated and the value of each argument expression is converted to the type of the corresponding parameter as if by assignment. (Array expressions and function designators as arguments were converted to pointers before the call.)

11 After all parameters have been assigned, the compound statement that constitutes the body of the function definition is executed.

12 If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

13 EXAMPLE 1 In the following:

          extern int max(int a, int b)
          {
                return a > b ? a : b;
          }
extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and
          { return a > b ? a : b; }
is the function body. The following similar definition uses the identifier-list form for the parameter declarations:
          extern int max(a, b)
          int a, b;
          {
                return a > b ? a : b;
          }
Here int a, b; is the declaration list for the parameters. The difference between these two definitions is that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls to the function, whereas the second form does not.

14 EXAMPLE 2 To pass one function to another, one might say

                      int f(void);
                      /* ... */
                      g(f);
Then the definition of g might read
          void g(int (*funcp)(void))
          {
                /* ... */
                (*funcp)(); /* or funcp(); ...                    */
          }
or, equivalently,
          void g(int func(void))
          {
                /* ... */
                func(); /* or (*func)(); ...                   */
          }

Footnotes

162) The intent is that the type category in a function definition cannot be inherited from a typedef:

          typedef int F(void);                          //   type F is ''function with no parameters
                                                        //                  returning int''
          F f, g;                                       //   f and g both have type compatible with F
          F f { /* ... */ }                             //   WRONG: syntax/constraint error
          F g() { /* ... */ }                           //   WRONG: declares that g returns a function
          int f(void) { /* ... */ }                     //   RIGHT: f has type compatible with F
          int g() { /* ... */ }                         //   RIGHT: g has type compatible with F
          F *e(void) { /* ... */ }                      //   e returns a pointer to a function
          F *((e))(void) { /* ... */ }                  //   same: parentheses irrelevant
          int (*fp)(void);                              //   fp points to a function that has type F
          F *Fp;                                        //   Fp points to a function that has type F

163) See ''future language directions'' (6.11.7).

164) A parameter identifier cannot be redeclared in the function body except in an enclosed block.

Contents

6.9.2 External object definitions

Semantics

1 If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier.

2 A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

3 If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.

4 EXAMPLE 1

          int i1 = 1;                    // definition, external linkage
          static int i2 = 2;             // definition, internal linkage
          extern int i3 = 3;             // definition, external linkage
          int i4;                        // tentative definition, external linkage
          static int i5;                 // tentative definition, internal linkage
          int   i1;                      // valid tentative definition, refers to previous
          int   i2;                      // 6.2.2 renders undefined, linkage disagreement
          int   i3;                      // valid tentative definition, refers to previous
          int   i4;                      // valid tentative definition, refers to previous
          int   i5;                      // 6.2.2 renders undefined, linkage disagreement
          extern    int   i1;            // refers to previous, whose linkage is external
          extern    int   i2;            // refers to previous, whose linkage is internal
          extern    int   i3;            // refers to previous, whose linkage is external
          extern    int   i4;            // refers to previous, whose linkage is external
          extern    int   i5;            // refers to previous, whose linkage is internal

5 EXAMPLE 2 If at the end of the translation unit containing

          int i[];
the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to zero on program startup.

Contents

6.10 Preprocessing directives

Syntax

1

          preprocessing-file:
                 groupopt
          group:
                   group-part
                   group group-part
          group-part:
                 if-section
                 control-line
                 text-line
                 # non-directive
          if-section:
                   if-group elif-groupsopt else-groupopt endif-line
          if-group:
                  # if     constant-expression new-line groupopt
                  # ifdef identifier new-line groupopt
                  # ifndef identifier new-line groupopt
          elif-groups:
                  elif-group
                  elif-groups elif-group
          elif-group:
                  # elif        constant-expression new-line groupopt
          else-group:
                  # else        new-line groupopt
          endif-line:
                  # endif       new-line
          control-line:
                 # include pp-tokens new-line
                 # define identifier replacement-list new-line
                 # define identifier lparen identifier-listopt )
                                                 replacement-list new-line
                 # define identifier lparen ... ) replacement-list new-line
                 # define identifier lparen identifier-list , ... )
                                                 replacement-list new-line
                 # undef   identifier new-line
                 # line    pp-tokens new-line
                 # error   pp-tokensopt new-line
                 # pragma pp-tokensopt new-line
                 #         new-line
          text-line:
                  pp-tokensopt new-line
          non-directive:
                 pp-tokens new-line
          lparen:
                    a ( character not immediately preceded by white-space
          replacement-list:
                 pp-tokensopt
          pp-tokens:
                 preprocessing-token
                 pp-tokens preprocessing-token
          new-line:
                 the new-line character

Description

2 A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints: The first token in the sequence is a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character. The last token in the sequence is the first new- line character that follows the first token in the sequence.165) A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.

3 A text line shall not begin with a # preprocessing token. A non-directive shall not begin with any of the directive names appearing in the syntax.

4 When in a group that is skipped (6.10.1), the directive syntax is relaxed to allow any sequence of preprocessing tokens to occur between the directive name and the following new-line character.

Constraints

5 The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing # preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other white-space characters in translation phase 3).

Semantics

6 The implementation can process and skip sections of source files conditionally, include other source files, and replace macros. These capabilities are called preprocessing, because conceptually they occur before translation of the resulting translation unit.

7 The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

8 EXAMPLE In:

          #define EMPTY
          EMPTY # include <file.h>
the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTY has been replaced.

Footnotes

165) Thus, preprocessing directives are commonly called ''lines''. These ''lines'' have no other syntactic significance, as all white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in 6.10.3.2, for example).

Contents

6.10.1 Conditional inclusion

Constraints

1 The expression that controls conditional inclusion shall be an integer constant expression except that: identifiers (including those lexically identical to keywords) are interpreted as described below;166) and it may contain unary operator expressions of the form

      defined identifier
or
      defined ( identifier )
which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.

2 Each preprocessing token that remains (in the list of preprocessing tokens that will become the controlling expression) after all macro replacements have occurred shall be in the lexical form of a token (6.4).

Semantics

3 Preprocessing directives of the forms

    # if   constant-expression new-line groupopt
    # elif constant-expression new-line groupopt
check whether the controlling constant expression evaluates to nonzero.

4 Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined. After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers (including those lexically identical to keywords) are replaced with the pp-number 0, and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6. For the purposes of this token conversion and evaluation, all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>.167) This includes interpreting character constants, which may involve converting escape sequences into execution character set members. Whether the numeric value for these character constants matches the value obtained when an identical character constant occurs in an expression (other than within a #if or #elif directive) is implementation-defined.168) Also, whether a single-character character constant may have a negative value is implementation-defined.

5 Preprocessing directives of the forms

    # ifdef identifier new-line groupopt
    # ifndef identifier new-line groupopt
check whether the identifier is or is not currently defined as a macro name. Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively.

6 Each directive's condition is checked in order. If it evaluates to false (zero), the group that it controls is skipped: directives are processed only through the name that determines the directive in order to keep track of the level of nested conditionals; the rest of the directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose control condition evaluates to true (nonzero) is processed. If none of the conditions evaluates to true, and there is a #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped.169)

Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest integer types (7.20.1.5).

Footnotes

166) Because the controlling constant expression is evaluated during translation phase 4, all identifiers either are or are not macro names -- there simply are no keywords, enumeration constants, etc.

167) Thus, on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant 0x8000 is signed and positive within a #if expression even though it would be unsigned in translation phase 7.

168) Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same value in these two contexts.

    #if 'z' - 'a' == 25
    if ('z' - 'a' == 25)

169) As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive before the terminating new-line character. However, comments may appear anywhere in a source file, including within a preprocessing directive.

Contents

6.10.2 Source file inclusion

Constraints

1 A #include directive shall identify a header or source file that can be processed by the implementation.

Semantics

2 A preprocessing directive of the form

    # include <h-char-sequence> new-line
searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.

3 A preprocessing directive of the form

    # include "q-char-sequence" new-line
causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read
    # include <h-char-sequence> new-line
with the identical contained sequence (including > characters, if any) from the original directive.

4 A preprocessing directive of the form

    # include pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing tokens after include in the directive are processed just as in normal text. (Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms.170) The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.

5 The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to eight significant characters before the period.

6 A #include preprocessing directive may appear in a source file that has been read because of a #include directive in another file, up to an implementation-defined nesting limit (see 5.2.4.1).

7 EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:

           #include <stdio.h>
           #include "myprog.h"

8 EXAMPLE 2 This illustrates macro-replaced #include directives:

        #if VERSION == 1
              #define INCFILE            "vers1.h"
        #elif VERSION == 2
              #define INCFILE            "vers2.h"        // and so on
        #else
               #define INCFILE           "versN.h"
        #endif
        #include INCFILE

Forward references: macro replacement (6.10.3).

Footnotes

170) Note that adjacent string literals are not concatenated into a single string literal (see the translation phases in 5.1.1.2); thus, an expansion that results in two string literals is an invalid directive.

Contents

6.10.3 Macro replacement

Constraints

1 Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.

2 An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical.

3 There shall be white-space between the identifier and the replacement list in the definition of an object-like macro.

4 If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of parameters in the macro definition. Otherwise, there shall be more arguments in the invocation than there are parameters in the macro definition (excluding the ...). There shall exist a ) preprocessing token that terminates the invocation.

5 The identifier __VA_ARGS__ shall occur only in the replacement-list of a function-like macro that uses the ellipsis notation in the parameters.

6 A parameter identifier in a function-like macro shall be uniquely declared within its scope.

Semantics

7 The identifier immediately following the define is called the macro name. There is one name space for macro names. Any white-space characters preceding or following the replacement list of preprocessing tokens are not considered part of the replacement list for either form of macro.

8 If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a preprocessing directive could begin, the identifier is not subject to macro replacement.

9 A preprocessing directive of the form

    # define identifier replacement-list new-line
defines an object-like macro that causes each subsequent instance of the macro name171) to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive. The replacement list is then rescanned for more macro names as specified below.

10 A preprocessing directive of the form

    # define identifier lparen identifier-listopt ) replacement-list new-line
    # define identifier lparen ... ) replacement-list new-line
    # define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with parameters, whose use is similar syntactically to a function call. The parameters are specified by the optional list of identifiers, whose scope extends from their declaration in the identifier list until the new-line character that terminates the #define preprocessing directive. Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing token introduces the sequence of preprocessing tokens that is replaced by the replacement list in the definition (an invocation of the macro). The replaced sequence of preprocessing tokens is terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an invocation of a function-like macro, new-line is considered a normal white-space character.

11 The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro. The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives,172) the behavior is undefined.

12 If there is a ... in the identifier-list in the macro definition, then the trailing arguments, including any separating comma preprocessing tokens, are merged to form a single item: the variable arguments. The number of arguments so combined is such that, following merger, the number of arguments is one more than the number of parameters in the macro definition (excluding the ...).

Footnotes

171) Since, by macro-replacement time, all character constants and string literals are preprocessing tokens, not sequences possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they are never scanned for macro names or parameters.

172) Despite the name, a non-directive is a preprocessing directive.

Contents

6.10.3.1 Argument substitution

1 After the arguments for the invocation of a function-like macro have been identified, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein have been expanded. Before being substituted, each argument's preprocessing tokens are completely macro replaced as if they formed the rest of the preprocessing file; no other preprocessing tokens are available.

2 An identifier __VA_ARGS__ that occurs in the replacement list shall be treated as if it were a parameter, and the variable arguments shall form the preprocessing tokens used to replace it.

Contents

6.10.3.2 The # operator

Constraints

1 Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.

Semantics

2 If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument. Each occurrence of white space between the argument's preprocessing tokens becomes a single space character in the character string literal. White space before the first preprocessing token and after the last preprocessing token composing the argument is deleted. Otherwise, the original spelling of each preprocessing token in the argument is retained in the character string literal, except for special handling for producing the spelling of string literals and character constants: a \ character is inserted before each " and \ character of a character constant or string literal (including the delimiting " characters), except that it is implementation-defined whether a \ character is inserted before the \ character beginning a universal character name. If the replacement that results is not a valid character string literal, the behavior is undefined. The character string literal corresponding to an empty argument is "". The order of evaluation of # and ## operators is unspecified.

Contents

6.10.3.3 The ## operator

Constraints

1 A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

Semantics

2 If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument's preprocessing token sequence; however, if an argument consists of no preprocessing tokens, the parameter is replaced by a placemarker preprocessing token instead.173)

3 For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. Placemarker preprocessing tokens are handled specially: concatenation of two placemarkers results in a single placemarker preprocessing token, and concatenation of a placemarker with a non-placemarker preprocessing token results in the non-placemarker preprocessing token. If the result is not a valid preprocessing token, the behavior is undefined. The resulting token is available for further macro replacement. The order of evaluation of ## operators is unspecified.

4 EXAMPLE In the following fragment:

         #define     hash_hash # ## #
         #define     mkstr(a) # a
         #define     in_between(a) mkstr(a)
         #define     join(c, d) in_between(c hash_hash d)
         char p[] = join(x, y); // equivalent to
                                // char p[] = "x ## y";
The expansion produces, at various stages:
         join(x, y)
         in_between(x hash_hash y)
         in_between(x ## y)
         mkstr(x ## y)
         "x ## y"
In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the ## operator.

Footnotes

173) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that exist only within translation phase 4.

Contents

6.10.3.4 Rescanning and further replacement

1 After all parameters in the replacement list have been substituted and # and ## processing has taken place, all placemarker preprocessing tokens are removed. The resulting preprocessing token sequence is then rescanned, along with all subsequent preprocessing tokens of the source file, for more macro names to replace.

2 If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Furthermore, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

3 The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, but all pragma unary operator expressions within it are then processed as specified in 6.10.9 below.

4 EXAMPLE There are cases where it is not clear whether a replacement is nested or not. For example, given the following macro definitions:

         #define f(a) a*g
         #define g(a) f(a)
the invocation
         f(2)(9)
may expand to either
         2*f(9)
or
         2*9*g
Strictly conforming programs are not permitted to depend on such unspecified behavior.

Contents

6.10.3.5 Scope of macro definitions

1 A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the preprocessing translation unit. Macro definitions have no significance after translation phase 4.

2 A preprocessing directive of the form

      # undef identifier new-line
causes the specified identifier no longer to be defined as a macro name. It is ignored if the specified identifier is not currently defined as a macro name.

3 EXAMPLE 1 The simplest use of this facility is to define a ''manifest constant'', as in

         #define TABSIZE 100
          int table[TABSIZE];

4 EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments. It has the advantages of working for any compatible types of the arguments and of generating in-line code without the overhead of function calling. It has the disadvantages of evaluating one or the other of its arguments a second time (including side effects) and generating more code than a function if invoked several times. It also cannot have its address taken, as it has none.

          #define max(a, b) ((a) > (b) ? (a) : (b))
The parentheses ensure that the arguments and the resulting expression are bound properly.

5 EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence

          #define    x          3
          #define    f(a)       f(x * (a))
          #undef     x
          #define    x          2
          #define    g          f
          #define    z          z[0]
          #define    h          g(~
          #define    m(a)       a(w)
          #define    w          0,1
          #define    t(a)       a
          #define    p()        int
          #define    q(x)       x
          #define    r(x,y)     x ## y
          #define    str(x)     # x
          f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
          g(x+(3,4)-w) | h 5) & m
                (f)^m(m);
          p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
          char c[2][6] = { str(hello), str() };
results in
          f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
          f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
          int i[] = { 1, 23, 4, 5, };
          char c[2][6] = { "hello", "" };

6 EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the sequence

          #define str(s)      # s
          #define xstr(s)     str(s)
          #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
                                  x ## s, x ## t)
          #define INCFILE(n) vers ## n
          #define glue(a, b) a ## b
          #define xglue(a, b) glue(a, b)
          #define HIGHLOW     "hello"
          #define LOW         LOW ", world"
          debug(1, 2);
          fputs(str(strncmp("abc\0d", "abc", '\4') // this goes away
                == 0) str(: @\n), s);
          #include xstr(INCFILE(2).h)
          glue(HIGH, LOW);
          xglue(HIGH, LOW)
results in
          printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
          fputs(
            "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n",
            s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello" ", world"
or, after concatenation of the character string literals,
          printf("x1= %d, x2= %s", x1, x2);
          fputs(
            "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n",
            s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello, world"
Space around the # and ## tokens in the macro definition is optional.

7 EXAMPLE 5 To illustrate the rules for placemarker preprocessing tokens, the sequence

          #define t(x,y,z) x ## y ## z
          int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
                     t(10,,), t(,11,), t(,,12), t(,,) };
results in
          int j[] = { 123, 45, 67, 89,
                      10, 11, 12, };

8 EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.

          #define      OBJ_LIKE      (1-1)
          #define      OBJ_LIKE      /* white space */ (1-1) /* other */
          #define      FUNC_LIKE(a)   ( a )
          #define      FUNC_LIKE( a )( /* note the white space */ \
                                       a /* other stuff on this line
                                           */ )
But the following redefinitions are invalid:
          #define      OBJ_LIKE    (0)     // different token sequence
          #define      OBJ_LIKE    (1 - 1) // different white space
          #define      FUNC_LIKE(b) ( a ) // different parameter usage
          #define      FUNC_LIKE(b) ( b ) // different parameter spelling

9 EXAMPLE 7 Finally, to show the variable argument list macro facilities:

          #define debug(...)       fprintf(stderr, __VA_ARGS__)
          #define showlist(...)    puts(#__VA_ARGS__)
          #define report(test, ...) ((test)?puts(#test):\
                      printf(__VA_ARGS__))
          debug("Flag");
          debug("X = %d\n", x);
          showlist(The first, second, and third items.);
          report(x>y, "x is %d but y is %d", x, y);
results in
          fprintf(stderr, "Flag" );
          fprintf(stderr, "X = %d\n", x );
          puts( "The first, second, and third items." );
          ((x>y)?puts("x>y"):
                      printf("x is %d but y is %d", x, y));

Contents

6.10.4 Line control

Constraints

1 The string literal of a #line directive, if present, shall be a character string literal.

Semantics

2 The line number of the current source line is one greater than the number of new-line characters read or introduced in translation phase 1 (5.1.1.2) while processing the source file to the current token.

3 A preprocessing directive of the form

    # line digit-sequence new-line
causes the implementation to behave as if the following sequence of source lines begins with a source line that has a line number as specified by the digit sequence (interpreted as a decimal integer). The digit sequence shall not specify zero, nor a number greater than 2147483647.

4 A preprocessing directive of the form

    # line digit-sequence "s-char-sequenceopt" new-line
sets the presumed line number similarly and changes the presumed name of the source file to be the contents of the character string literal.

5 A preprocessing directive of the form

    # line pp-tokens new-line
(that does not match one of the two previous forms) is permitted. The preprocessing tokens after line on the directive are processed just as in normal text (each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens). The directive resulting after all replacements shall match one of the two previous forms and is then processed as appropriate.

Contents

6.10.5 Error directive

Semantics

1 A preprocessing directive of the form

    # error pp-tokensopt new-line
causes the implementation to produce a diagnostic message that includes the specified sequence of preprocessing tokens.

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6.10.6 Pragma directive

Semantics

1 A preprocessing directive of the form

    # pragma pp-tokensopt new-line
where the preprocessing token STDC does not immediately follow pragma in the directive (prior to any macro replacement)174) causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any such pragma that is not recognized by the implementation is ignored.

2 If the preprocessing token STDC does immediately follow pragma in the directive (prior to any macro replacement), then no macro replacement is performed on the directive, and the directive shall have one of the following forms175) whose meanings are described elsewhere:

    #pragma STDC FP_CONTRACT on-off-switch
    #pragma STDC FENV_ACCESS on-off-switch
    #pragma STDC CX_LIMITED_RANGE on-off-switch
    on-off-switch: one of
                ON     OFF           DEFAULT

Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma (7.6.1), the CX_LIMITED_RANGE pragma (7.3.4).

Footnotes

174) An implementation is not required to perform macro replacement in pragmas, but it is permitted except for in standard pragmas (where STDC immediately follows pragma). If the result of macro replacement in a non-standard pragma has the same form as a standard pragma, the behavior is still implementation-defined; an implementation is permitted to behave as if it were the standard pragma, but is not required to.

175) See ''future language directions'' (6.11.8).

Contents

6.10.7 Null directive

Semantics

1 A preprocessing directive of the form

    # new-line
has no effect.

Contents

6.10.8 Predefined macro names

1 The values of the predefined macros listed in the following subclauses176) (except for __FILE__ and __LINE__) remain constant throughout the translation unit.

2 None of these macro names, nor the identifier defined, shall be the subject of a #define or a #undef preprocessing directive. Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore.

3 The implementation shall not predefine the macro __cplusplus, nor shall it define it in any standard header.

Forward references: standard headers (7.1.2).

Footnotes

176) See ''future language directions'' (6.11.9).

Contents

6.10.8.1 Mandatory macros

1 The following macro names shall be defined by the implementation:

__DATE__
The date of translation of the preprocessing translation unit: a character string literal of the form "Mmm dd yyyy", where the names of the months are the same as those generated by the asctime function, and the first character of dd is a space character if the value is less than 10. If the date of translation is not available, an implementation-defined valid date shall be supplied.
__FILE__
The presumed name of the current source file (a character string literal).177)
__LINE__
The presumed line number (within the current source file) of the current source line (an integer constant).177)
__STDC__
The integer constant 1, intended to indicate a conforming implementation.
__STDC_HOSTED__
The integer constant 1 if the implementation is a hosted implementation or the integer constant 0 if it is not.
__STDC_VERSION__
The integer constant 201ymmL.178)
__TIME__
The time of translation of the preprocessing translation unit: a character string literal of the form "hh:mm:ss" as in the time generated by the asctime function. If the time of translation is not available, an implementation-defined valid time shall be supplied.

Forward references: the asctime function (7.27.3.1).

Footnotes

177) The presumed source file name and line number can be changed by the #line directive.

178) This macro was not specified in ISO/IEC 9899:1990 and was specified as 199409L in ISO/IEC 9899/AMD1:1995 and as 199901L in ISO/IEC 9899:1999. The intention is that this will remain an integer constant of type long int that is increased with each revision of this International Standard.

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6.10.8.2 Environment macros

1 The following macro names are conditionally defined by the implementation:

__STDC_ISO_10646__
An integer constant of the form yyyymmL (for example, 199712L). If this symbol is defined, then every character in the Unicode required set, when stored in an object of type wchar_t, has the same value as the short identifier of that character. The Unicode required set consists of all the characters that are defined by ISO/IEC 10646, along with all amendments and technical corrigenda, as of the specified year and month. If some other encoding is used, the macro shall not be defined and the actual encoding used is implementation-defined.
__STDC_MB_MIGHT_NEQ_WC__
The integer constant 1, intended to indicate that, in the encoding for wchar_t, a member of the basic character set need not have a code value equal to its value when used as the lone character in an integer character constant.
__STDC_UTF_16__
The integer constant 1, intended to indicate that values of type char16_t are UTF-16 encoded. If some other encoding is used, the macro shall not be defined and the actual encoding used is implementation- defined.
__STDC_UTF_32__
The integer constant 1, intended to indicate that values of type char32_t are UTF-32 encoded. If some other encoding is used, the macro shall not be defined and the actual encoding used is implementation- defined.

Forward references: common definitions (7.19), unicode utilities (7.28).

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6.10.8.3 Conditional feature macros

1 The following macro names are conditionally defined by the implementation:

__STDC_ANALYZABLE__
The integer constant 1, intended to indicate conformance to the specifications in annex L (Analyzability).
__STDC_IEC_559__
The integer constant 1, intended to indicate conformance to the specifications in annex F (IEC 60559 floating-point arithmetic).
__STDC_IEC_559_COMPLEX__
The integer constant 1, intended to indicate adherence to the specifications in annex G (IEC 60559 compatible complex arithmetic).
__STDC_LIB_EXT1__
The integer constant 201ymmL, intended to indicate support for the extensions defined in annex K (Bounds-checking interfaces).179)
__STDC_NO_ATOMICS__
The integer constant 1, intended to indicate that the implementation does not support atomic types (including the _Atomic type qualifier) and the <stdatomic.h> header.
__STDC_NO_COMPLEX__
The integer constant 1, intended to indicate that the implementation does not support complex types or the <complex.h> header.
__STDC_NO_THREADS__
The integer constant 1, intended to indicate that the implementation does not support the <threads.h> header.
__STDC_NO_VLA__
The integer constant 1, intended to indicate that the implementation does not support variable length arrays or variably modified types.

2 An implementation that defines __STDC_NO_COMPLEX__ shall not define __STDC_IEC_559_COMPLEX__.

Footnotes

179) The intention is that this will remain an integer constant of type long int that is increased with each revision of this International Standard.

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6.10.9 Pragma operator

Semantics

1 A unary operator expression of the form:

    _Pragma ( string-literal )
is processed as follows: The string literal is destringized by deleting any encoding prefix, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash. The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary operator expression are removed.

2 EXAMPLE A directive of the form:

          #pragma listing on "..\listing.dir"
can also be expressed as:
          _Pragma ( "listing on \"..\\listing.dir\"" )
The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in:
          #define LISTING(x) PRAGMA(listing on #x)
          #define PRAGMA(x) _Pragma(#x)
          LISTING ( ..\listing.dir )

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6.11 Future language directions

Contents

6.11.1 Floating types

1 Future standardization may include additional floating-point types, including those with greater range, precision, or both than long double.

Contents

6.11.2 Linkages of identifiers

1 Declaring an identifier with internal linkage at file scope without the static storage- class specifier is an obsolescent feature.

Contents

6.11.3 External names

1 Restriction of the significance of an external name to fewer than 255 characters (considering each universal character name or extended source character as a single character) is an obsolescent feature that is a concession to existing implementations.

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6.11.4 Character escape sequences

1 Lowercase letters as escape sequences are reserved for future standardization. Other characters may be used in extensions.

Contents

6.11.5 Storage-class specifiers

1 The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature.

Contents

6.11.6 Function declarators

1 The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

Contents

6.11.7 Function definitions

1 The use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature.

Contents

6.11.8 Pragma directives

1 Pragmas whose first preprocessing token is STDC are reserved for future standardization.

Contents

6.11.9 Predefined macro names

1 Macro names beginning with __STDC_ are reserved for future standardization.

Contents

7. Library

Contents

7.1 Introduction

Contents

7.1.1 Definitions of terms

1 A string is a contiguous sequence of characters terminated by and including the first null character. The term multibyte string is sometimes used instead to emphasize special processing given to multibyte characters contained in the string or to avoid confusion with a wide string. A pointer to a string is a pointer to its initial (lowest addressed) character. The length of a string is the number of bytes preceding the null character and the value of a string is the sequence of the values of the contained characters, in order.

2 The decimal-point character is the character used by functions that convert floating-point numbers to or from character sequences to denote the beginning of the fractional part of such character sequences.180) It is represented in the text and examples by a period, but may be changed by the setlocale function.

3 A null wide character is a wide character with code value zero.

4 A wide string is a contiguous sequence of wide characters terminated by and including the first null wide character. A pointer to a wide string is a pointer to its initial (lowest addressed) wide character. The length of a wide string is the number of wide characters preceding the null wide character and the value of a wide string is the sequence of code values of the contained wide characters, in order.

5 A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes a change in shift state (see 5.2.1.2). A shift sequence shall not have a corresponding wide character; it is instead taken to be an adjunct to an adjacent multibyte character.181)

Forward references: character handling (7.4), the setlocale function (7.11.1.1).

Footnotes

180) The functions that make use of the decimal-point character are the numeric conversion functions (7.22.1, 7.29.4.1) and the formatted input/output functions (7.21.6, 7.29.2).

181) For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX shall thus be large enough to count all the bytes in any complete multibyte character plus at least one adjacent shift sequence of maximum length. Whether these counts provide for more than one shift sequence is the implementation's choice.

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7.1.2 Standard headers

1 Each library function is declared, with a type that includes a prototype, in a header,182) whose contents are made available by the #include preprocessing directive. The header declares a set of related functions, plus any necessary types and additional macros needed to facilitate their use. Declarations of types described in this clause shall not include type qualifiers, unless explicitly stated otherwise.

2 The standard headers are183)

        <assert.h>                     <math.h>                        <stdlib.h>
        <complex.h>                    <setjmp.h>                      <stdnoreturn.h>
        <ctype.h>                      <signal.h>                      <string.h>
        <errno.h>                      <stdalign.h>                    <tgmath.h>
        <fenv.h>                       <stdarg.h>                      <threads.h>
        <float.h>                      <stdatomic.h>                   <time.h>
        <inttypes.h>                   <stdbool.h>                     <uchar.h>
        <iso646.h>                     <stddef.h>                      <wchar.h>
        <limits.h>                     <stdint.h>                      <wctype.h>
        <locale.h>                     <stdio.h>

3 If a file with the same name as one of the above < and > delimited sequences, not provided as part of the implementation, is placed in any of the standard places that are searched for included source files, the behavior is undefined.

4 Standard headers may be included in any order; each may be included more than once in a given scope, with no effect different from being included only once, except that the effect of including <assert.h> depends on the definition of NDEBUG (see 7.2). If used, a header shall be included outside of any external declaration or definition, and it shall first be included before the first reference to any of the functions or objects it declares, or to any of the types or macros it defines. However, if an identifier is declared or defined in more than one header, the second and subsequent associated headers may be included after the initial reference to the identifier. The program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion of the header or when any macro defined in the header is expanded.

5 Any definition of an object-like macro described in this clause shall expand to code that is fully protected by parentheses where necessary, so that it groups in an arbitrary expression as if it were a single identifier.

6 Any declaration of a library function shall have external linkage.

7 A summary of the contents of the standard headers is given in annex B.

Forward references: diagnostics (7.2).

Footnotes

182) A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source file names.

183) The headers <complex.h>, <stdatomic.h>, and <threads.h> are conditional features that implementations need not support; see 6.10.8.3.

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7.1.3 Reserved identifiers

1 Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.

2 No other identifiers are reserved. If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.

3 If the program removes (with #undef) any macro definition of an identifier in the first group listed above, the behavior is undefined.

Footnotes

184) The list of reserved identifiers with external linkage includes math_errhandling, setjmp, va_copy, and va_end.

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7.1.4 Use of library functions

1 Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined. If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid. Any function declared in a header may be additionally implemented as a function-like macro defined in the header, so if a library function is declared explicitly when its header is included, one of the techniques shown below can be used to ensure the declaration is not affected by such a macro. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro.185) The use of #undef to remove any macro definition will also ensure that an actual function is referred to. Any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once, fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as arguments.186) Likewise, those function-like macros described in the following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called.187) All object-like macros listed as expanding to integer constant expressions shall additionally be suitable for use in #if preprocessing directives.

2 Provided that a library function can be declared without reference to any type defined in a header, it is also permissible to declare the function and use it without including its associated header.

3 There is a sequence point immediately before a library function returns.

4 The functions in the standard library are not guaranteed to be reentrant and may modify objects with static or thread storage duration.188)

5 Unless explicitly stated otherwise in the detailed descriptions that follow, library functions shall prevent data races as follows: A library function shall not directly or indirectly access objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's arguments. A library function shall not directly or indirectly modify objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments.189) Implementations may share their own internal objects between threads if the objects are not visible to users and are protected against data races.

6 Unless otherwise specified, library functions shall perform all operations solely within the current thread if those operations have effects that are visible to users.190)

7 EXAMPLE The function atoi may be used in any of several ways:

Footnotes

185) This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function.

186) Such macros might not contain the sequence points that the corresponding function calls do.

187) Because external identifiers and some macro names beginning with an underscore are reserved, implementations may provide special semantics for such names. For example, the identifier _BUILTIN_abs could be used to indicate generation of in-line code for the abs function. Thus, the appropriate header could specify

          #define abs(x) _BUILTIN_abs(x)
for a compiler whose code generator will accept it. In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine function may write
          #undef abs
whether the implementation's header provides a macro implementation of abs or a built-in implementation. The prototype for the function, which precedes and is hidden by any macro definition, is thereby revealed also.

188) Thus, a signal handler cannot, in general, call standard library functions.

189) This means, for example, that an implementation is not permitted to use a static object for internal purposes without synchronization because it could cause a data race even in programs that do not explicitly share objects between threads. Similarly, an implementation of memcpy is not permitted to copy bytes beyond the specified length of the destination object and then restore the original values because it could cause a data race if the program shared those bytes between threads.

190) This allows implementations to parallelize operations if there are no visible side effects.

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7.2 Diagnostics <assert.h>

1 The header <assert.h> defines the assert and static_assert macros and refers to another macro,

         NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the point in the source file where <assert.h> is included, the assert macro is defined simply as
         #define assert(ignore) ((void)0)
The assert macro is redefined according to the current state of NDEBUG each time that <assert.h> is included.

2 The assert macro shall be implemented as a macro, not as an actual function. If the macro definition is suppressed in order to access an actual function, the behavior is undefined.

3 The macro

         static_assert
expands to _Static_assert.

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7.2.1 Program diagnostics

Contents

7.2.1.1 The assert macro

Synopsis

1

         #include <assert.h>
         void assert(scalar expression);

Description

2 The assert macro puts diagnostic tests into programs; it expands to a void expression. When it is executed, if expression (which shall have a scalar type) is false (that is, compares equal to 0), the assert macro writes information about the particular call that failed (including the text of the argument, the name of the source file, the source line number, and the name of the enclosing function -- the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ and of the identifier __func__) on the standard error stream in an implementation-defined format.191) It then calls the abort function.

Returns

3 The assert macro returns no value.

Forward references: the abort function (7.22.4.1).

Footnotes

191) The message written might be of the form:

  Assertion failed: expression, function abc, file xyz, line nnn.

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7.3 Complex arithmetic <complex.h>

Contents

7.3.1 Introduction

1 The header <complex.h> defines macros and declares functions that support complex arithmetic.192)

2 Implementations that define the macro __STDC_NO_COMPLEX__ need not provide this header nor support any of its facilities.

3 Each synopsis specifies a family of functions consisting of a principal function with one or more double complex parameters and a double complex or double return value; and other functions with the same name but with f and l suffixes which are corresponding functions with float and long double parameters and return values.

4 The macro

          complex
expands to _Complex; the macro
          _Complex_I
expands to a constant expression of type const float _Complex, with the value of the imaginary unit.193)

5 The macros

          imaginary
and
          _Imaginary_I
are defined if and only if the implementation supports imaginary types;194) if defined, they expand to _Imaginary and a constant expression of type const float _Imaginary with the value of the imaginary unit.

6 The macro

          I
expands to either _Imaginary_I or _Complex_I. If _Imaginary_I is not defined, I shall expand to _Complex_I.

7 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the macros complex, imaginary, and I.

Forward references: IEC 60559-compatible complex arithmetic (annex G).

Footnotes

192) See ''future library directions'' (7.31.1).

193) The imaginary unit is a number i such that i2 = -1.

194) A specification for imaginary types is in informative annex G.

Contents

7.3.2 Conventions

1 Values are interpreted as radians, not degrees. An implementation may set errno but is not required to.

Contents

7.3.3 Branch cuts

1 Some of the functions below have branch cuts, across which the function is discontinuous. For implementations with a signed zero (including all IEC 60559 implementations) that follow the specifications of annex G, the sign of zero distinguishes one side of a cut from another so the function is continuous (except for format limitations) as the cut is approached from either side. For example, for the square root function, which has a branch cut along the negative real axis, the top of the cut, with imaginary part +0, maps to the positive imaginary axis, and the bottom of the cut, with imaginary part -0, maps to the negative imaginary axis.

2 Implementations that do not support a signed zero (see annex F) cannot distinguish the sides of branch cuts. These implementations shall map a cut so the function is continuous as the cut is approached coming around the finite endpoint of the cut in a counter clockwise direction. (Branch cuts for the functions specified here have just one finite endpoint.) For example, for the square root function, coming counter clockwise around the finite endpoint of the cut along the negative real axis approaches the cut from above, so the cut maps to the positive imaginary axis.

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7.3.4 The CX_LIMITED_RANGE pragma

Synopsis

1

          #include <complex.h>
          #pragma STDC CX_LIMITED_RANGE on-off-switch

Description

2 The usual mathematical formulas for complex multiply, divide, and absolute value are problematic because of their treatment of infinities and because of undue overflow and underflow. The CX_LIMITED_RANGE pragma can be used to inform the implementation that (where the state is ''on'') the usual mathematical formulas are acceptable.195) The pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. The default state for the pragma is ''off''.

Footnotes

195) The purpose of the pragma is to allow the implementation to use the formulas:

     (x + iy) x (u + iv) = (xu - yv) + i(yu + xv)
     (x + iy) / (u + iv) = [(xu + yv) + i(yu - xv)]/(u2 + v2 )
     | x + iy | = (sqrt)(x2 + y2)
                  -----
where the programmer can determine they are safe.

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7.3.5 Trigonometric functions

Contents

7.3.5.1 The cacos functions

Synopsis

1

          #include <complex.h>
          double complex cacos(double complex z);
          float complex cacosf(float complex z);
          long double complex cacosl(long double complex z);

Description

2 The cacos functions compute the complex arc cosine of z, with branch cuts outside the interval [-1, +1] along the real axis.

Returns

3 The cacos functions return the complex arc cosine value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [0, pi ] along the real axis.

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7.3.5.2 The casin functions

Synopsis

1

          #include <complex.h>
          double complex casin(double complex z);
          float complex casinf(float complex z);
          long double complex casinl(long double complex z);

Description

2 The casin functions compute the complex arc sine of z, with branch cuts outside the interval [-1, +1] along the real axis.

Returns

3 The casin functions return the complex arc sine value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [-pi /2, +pi /2] along the real axis.

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7.3.5.3 The catan functions

Synopsis

1

         #include <complex.h>
         double complex catan(double complex z);
         float complex catanf(float complex z);
         long double complex catanl(long double complex z);

Description

2 The catan functions compute the complex arc tangent of z, with branch cuts outside the interval [-i, +i] along the imaginary axis.

Returns

3 The catan functions return the complex arc tangent value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [-pi /2, +pi /2] along the real axis.

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7.3.5.4 The ccos functions

Synopsis

1

         #include <complex.h>
         double complex ccos(double complex z);
         float complex ccosf(float complex z);
         long double complex ccosl(long double complex z);

Description

2 The ccos functions compute the complex cosine of z.

Returns

3 The ccos functions return the complex cosine value.

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7.3.5.5 The csin functions

Synopsis

1

         #include <complex.h>
         double complex csin(double complex z);
         float complex csinf(float complex z);
         long double complex csinl(long double complex z);

Description

2 The csin functions compute the complex sine of z.

Returns

3 The csin functions return the complex sine value.

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7.3.5.6 The ctan functions

Synopsis

1

        #include <complex.h>
        double complex ctan(double complex z);
        float complex ctanf(float complex z);
        long double complex ctanl(long double complex z);

Description

2 The ctan functions compute the complex tangent of z.

Returns

3 The ctan functions return the complex tangent value.

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7.3.6 Hyperbolic functions

Contents

7.3.6.1 The cacosh functions

Synopsis

1

        #include <complex.h>
        double complex cacosh(double complex z);
        float complex cacoshf(float complex z);
        long double complex cacoshl(long double complex z);

Description

2 The cacosh functions compute the complex arc hyperbolic cosine of z, with a branch cut at values less than 1 along the real axis.

Returns

3 The cacosh functions return the complex arc hyperbolic cosine value, in the range of a half-strip of nonnegative values along the real axis and in the interval [-ipi , +ipi ] along the imaginary axis.

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7.3.6.2 The casinh functions

Synopsis

1

        #include <complex.h>
        double complex casinh(double complex z);
        float complex casinhf(float complex z);
        long double complex casinhl(long double complex z);

Description

2 The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts outside the interval [-i, +i] along the imaginary axis.

Returns

3 The casinh functions return the complex arc hyperbolic sine value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi /2, +ipi /2] along the imaginary axis.

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7.3.6.3 The catanh functions

Synopsis

1

         #include <complex.h>
         double complex catanh(double complex z);
         float complex catanhf(float complex z);
         long double complex catanhl(long double complex z);

Description

2 The catanh functions compute the complex arc hyperbolic tangent of z, with branch cuts outside the interval [-1, +1] along the real axis.

Returns

3 The catanh functions return the complex arc hyperbolic tangent value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi /2, +ipi /2] along the imaginary axis.

Contents

7.3.6.4 The ccosh functions

Synopsis

1

         #include <complex.h>
         double complex ccosh(double complex z);
         float complex ccoshf(float complex z);
         long double complex ccoshl(long double complex z);

Description

2 The ccosh functions compute the complex hyperbolic cosine of z.

Returns

3 The ccosh functions return the complex hyperbolic cosine value.

Contents

7.3.6.5 The csinh functions

Synopsis

1

        #include <complex.h>
        double complex csinh(double complex z);
        float complex csinhf(float complex z);
        long double complex csinhl(long double complex z);

Description

2 The csinh functions compute the complex hyperbolic sine of z.

Returns

3 The csinh functions return the complex hyperbolic sine value.

Contents

7.3.6.6 The ctanh functions

Synopsis

1

        #include <complex.h>
        double complex ctanh(double complex z);
        float complex ctanhf(float complex z);
        long double complex ctanhl(long double complex z);

Description

2 The ctanh functions compute the complex hyperbolic tangent of z.

Returns

3 The ctanh functions return the complex hyperbolic tangent value.

Contents

7.3.7 Exponential and logarithmic functions

Contents

7.3.7.1 The cexp functions

Synopsis

1

        #include <complex.h>
        double complex cexp(double complex z);
        float complex cexpf(float complex z);
        long double complex cexpl(long double complex z);

Description

2 The cexp functions compute the complex base-e exponential of z.

Returns

3 The cexp functions return the complex base-e exponential value.

Contents

7.3.7.2 The clog functions

Synopsis

1

         #include <complex.h>
         double complex clog(double complex z);
         float complex clogf(float complex z);
         long double complex clogl(long double complex z);

Description

2 The clog functions compute the complex natural (base-e) logarithm of z, with a branch cut along the negative real axis.

Returns

3 The clog functions return the complex natural logarithm value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi , +ipi ] along the imaginary axis.

Contents

7.3.8 Power and absolute-value functions

Contents

7.3.8.1 The cabs functions

Synopsis

1

         #include <complex.h>
         double cabs(double complex z);
         float cabsf(float complex z);
         long double cabsl(long double complex z);

Description

2 The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z.

Returns

3 The cabs functions return the complex absolute value.

Contents

7.3.8.2 The cpow functions

Synopsis

1

         #include <complex.h>
         double complex cpow(double complex x, double complex y);
         float complex cpowf(float complex x, float complex y);
         long double complex cpowl(long double complex x,
              long double complex y);

Description

2 The cpow functions compute the complex power function xy , with a branch cut for the first parameter along the negative real axis.

Returns

3 The cpow functions return the complex power function value.

Contents

7.3.8.3 The csqrt functions

Synopsis

1

        #include <complex.h>
        double complex csqrt(double complex z);
        float complex csqrtf(float complex z);
        long double complex csqrtl(long double complex z);

Description

2 The csqrt functions compute the complex square root of z, with a branch cut along the negative real axis.

Returns

3 The csqrt functions return the complex square root value, in the range of the right half- plane (including the imaginary axis).

Contents

7.3.9 Manipulation functions

Contents

7.3.9.1 The carg functions

Synopsis

1

        #include <complex.h>
        double carg(double complex z);
        float cargf(float complex z);
        long double cargl(long double complex z);

Description

2 The carg functions compute the argument (also called phase angle) of z, with a branch cut along the negative real axis.

Returns

3 The carg functions return the value of the argument in the interval [-pi , +pi ].

Contents

7.3.9.2 The cimag functions

Synopsis

1

         #include <complex.h>
         double cimag(double complex z);
         float cimagf(float complex z);
         long double cimagl(long double complex z);

Description

2 The cimag functions compute the imaginary part of z.196)

Returns

3 The cimag functions return the imaginary part value (as a real).

Footnotes

196) For a variable z of complex type, z == creal(z) + cimag(z)*I.

Contents

7.3.9.3 The CMPLX macros

Synopsis

1

         #include <complex.h>
         double complex CMPLX(double x, double y);
         float complex CMPLXF(float x, float y);
         long double complex CMPLXL(long double x, long double y);

Description

2 The CMPLX macros expand to an expression of the specified complex type, with the real part having the (converted) value of x and the imaginary part having the (converted) value of y. The resulting expression shall be suitable for use as an initializer for an object with static or thread storage duration, provided both arguments are likewise suitable.

Returns

3 The CMPLX macros return the complex value x + i y.

4 NOTE These macros act as if the implementation supported imaginary types and the definitions were:

      #define CMPLX(x, y)  ((double complex)((double)(x) + \
                                    _Imaginary_I * (double)(y)))
      #define CMPLXF(x, y) ((float complex)((float)(x) + \
                                    _Imaginary_I * (float)(y)))
      #define CMPLXL(x, y) ((long double complex)((long double)(x) + \
                                    _Imaginary_I * (long double)(y)))

Contents

7.3.9.4 The conj functions

Synopsis

1

        #include <complex.h>
        double complex conj(double complex z);
        float complex conjf(float complex z);
        long double complex conjl(long double complex z);

Description

2 The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part.

Returns

3 The conj functions return the complex conjugate value.

Contents

7.3.9.5 The cproj functions

Synopsis

1

        #include <complex.h>
        double complex cproj(double complex z);
        float complex cprojf(float complex z);
        long double complex cprojl(long double complex z);

Description

2 The cproj functions compute a projection of z onto the Riemann sphere: z projects to z except that all complex infinities (even those with one infinite part and one NaN part) project to positive infinity on the real axis. If z has an infinite part, then cproj(z) is equivalent to

        INFINITY + I * copysign(0.0, cimag(z))

Returns

3 The cproj functions return the value of the projection onto the Riemann sphere.

Contents

7.3.9.6 The creal functions

Synopsis

1

        #include <complex.h>
        double creal(double complex z);
        float crealf(float complex z);
        long double creall(long double complex z);

Description

2 The creal functions compute the real part of z.197)

Returns

3 The creal functions return the real part value.

Footnotes

197) For a variable z of complex type, z == creal(z) + cimag(z)*I.

Contents

7.4 Character handling <ctype.h>

1 The header <ctype.h> declares several functions useful for classifying and mapping characters.198) In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

2 The behavior of these functions is affected by the current locale. Those functions that have locale-specific aspects only when not in the "C" locale are noted below.

3 The term printing character refers to a member of a locale-specific set of characters, each of which occupies one printing position on a display device; the term control character refers to a member of a locale-specific set of characters that are not printing characters.199) All letters and digits are printing characters.

Forward references: EOF (7.21.1), localization (7.11).

Footnotes

198) See ''future library directions'' (7.31.2).

199) In an implementation that uses the seven-bit US ASCII character set, the printing characters are those whose values lie from 0x20 (space) through 0x7E (tilde); the control characters are those whose values lie from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL).

Contents

7.4.1 Character classification functions

1 The functions in this subclause return nonzero (true) if and only if the value of the argument c conforms to that in the description of the function.

Contents

7.4.1.1 The isalnum function

Synopsis

1

          #include <ctype.h>
          int isalnum(int c);

Description

2 The isalnum function tests for any character for which isalpha or isdigit is true.

Contents

7.4.1.2 The isalpha function

Synopsis

1

          #include <ctype.h>
          int isalpha(int c);

Description

2 The isalpha function tests for any character for which isupper or islower is true, or any character that is one of a locale-specific set of alphabetic characters for which none of iscntrl, isdigit, ispunct, or isspace is true.200) In the "C" locale, isalpha returns true only for the characters for which isupper or islower is true.

Footnotes

200) The functions islower and isupper test true or false separately for each of these additional characters; all four combinations are possible.

Contents

7.4.1.3 The isblank function

Synopsis

1

         #include <ctype.h>
         int isblank(int c);

Description

2 The isblank function tests for any character that is a standard blank character or is one of a locale-specific set of characters for which isspace is true and that is used to separate words within a line of text. The standard blank characters are the following: space (' '), and horizontal tab ('\t'). In the "C" locale, isblank returns true only for the standard blank characters.

Contents

7.4.1.4 The iscntrl function

Synopsis

1

         #include <ctype.h>
         int iscntrl(int c);

Description

2 The iscntrl function tests for any control character.

Contents

7.4.1.5 The isdigit function

Synopsis

1

         #include <ctype.h>
         int isdigit(int c);

Description

2 The isdigit function tests for any decimal-digit character (as defined in 5.2.1).

Contents

7.4.1.6 The isgraph function

Synopsis

1

         #include <ctype.h>
         int isgraph(int c);

Description

2 The isgraph function tests for any printing character except space (' ').

Contents

7.4.1.7 The islower function

Synopsis

1

        #include <ctype.h>
        int islower(int c);

Description

2 The islower function tests for any character that is a lowercase letter or is one of a locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale, islower returns true only for the lowercase letters (as defined in 5.2.1).

Contents

7.4.1.8 The isprint function

Synopsis

1

        #include <ctype.h>
        int isprint(int c);

Description

2 The isprint function tests for any printing character including space (' ').

Contents

7.4.1.9 The ispunct function

Synopsis

1

        #include <ctype.h>
        int ispunct(int c);

Description

2 The ispunct function tests for any printing character that is one of a locale-specific set of punctuation characters for which neither isspace nor isalnum is true. In the "C" locale, ispunct returns true for every printing character for which neither isspace nor isalnum is true.

Contents

7.4.1.10 The isspace function

Synopsis

1

        #include <ctype.h>
        int isspace(int c);

Description

2 The isspace function tests for any character that is a standard white-space character or is one of a locale-specific set of characters for which isalnum is false. The standard white-space characters are the following: space (' '), form feed ('\f'), new-line ('\n'), carriage return ('\r'), horizontal tab ('\t'), and vertical tab ('\v'). In the "C" locale, isspace returns true only for the standard white-space characters.

Contents

7.4.1.11 The isupper function

Synopsis

1

         #include <ctype.h>
         int isupper(int c);

Description

2 The isupper function tests for any character that is an uppercase letter or is one of a locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale, isupper returns true only for the uppercase letters (as defined in 5.2.1).

Contents

7.4.1.12 The isxdigit function

Synopsis

1

         #include <ctype.h>
         int isxdigit(int c);

Description

2 The isxdigit function tests for any hexadecimal-digit character (as defined in 6.4.4.1).

Contents

7.4.2 Character case mapping functions

Contents

7.4.2.1 The tolower function

Synopsis

1

         #include <ctype.h>
         int tolower(int c);

Description

2 The tolower function converts an uppercase letter to a corresponding lowercase letter.

Returns

3 If the argument is a character for which isupper is true and there are one or more corresponding characters, as specified by the current locale, for which islower is true, the tolower function returns one of the corresponding characters (always the same one for any given locale); otherwise, the argument is returned unchanged.

Contents

7.4.2.2 The toupper function

Synopsis

1

        #include <ctype.h>
        int toupper(int c);

Description

2 The toupper function converts a lowercase letter to a corresponding uppercase letter.

Returns

3 If the argument is a character for which islower is true and there are one or more corresponding characters, as specified by the current locale, for which isupper is true, the toupper function returns one of the corresponding characters (always the same one for any given locale); otherwise, the argument is returned unchanged.

Contents

7.5 Errors <errno.h>

1 The header <errno.h> defines several macros, all relating to the reporting of error conditions.

2 The macros are

          EDOM
          EILSEQ
          ERANGE
which expand to integer constant expressions with type int, distinct positive values, and which are suitable for use in #if preprocessing directives; and
          errno
which expands to a modifiable lvalue201) that has type int and thread local storage duration, the value of which is set to a positive error number by several library functions. If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name errno, the behavior is undefined.

3 The value of errno in the initial thread is zero at program startup (the initial value of errno in other threads is an indeterminate value), but is never set to zero by any library function.202) The value of errno may be set to nonzero by a library function call whether or not there is an error, provided the use of errno is not documented in the description of the function in this International Standard.

4 Additional macro definitions, beginning with E and a digit or E and an uppercase letter,203) may also be specified by the implementation.

Footnotes

201) The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue resulting from a function call (for example, *errno()).

202) Thus, a program that uses errno for error checking should set it to zero before a library function call, then inspect it before a subsequent library function call. Of course, a library function can save the value of errno on entry and then set it to zero, as long as the original value is restored if errno's value is still zero just before the return.

203) See ''future library directions'' (7.31.3).

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7.6 Floating-point environment <fenv.h>

1 The header <fenv.h> defines several macros, and declares types and functions that provide access to the floating-point environment. The floating-point environment refers collectively to any floating-point status flags and control modes supported by the implementation.204) A floating-point status flag is a system variable whose value is set (but never cleared) when a floating-point exception is raised, which occurs as a side effect of exceptional floating-point arithmetic to provide auxiliary information.205) A floating- point control mode is a system variable whose value may be set by the user to affect the subsequent behavior of floating-point arithmetic.

2 The floating-point environment has thread storage duration. The initial state for a thread's floating-point environment is the current state of the floating-point environment of the thread that creates it at the time of creation.

3 Certain programming conventions support the intended model of use for the floating- point environment:206)

4 The type

         fenv_t
represents the entire floating-point environment.

5 The type

         fexcept_t
represents the floating-point status flags collectively, including any status the implementation associates with the flags.

6 Each of the macros

          FE_DIVBYZERO
          FE_INEXACT
          FE_INVALID
          FE_OVERFLOW
          FE_UNDERFLOW
is defined if and only if the implementation supports the floating-point exception by means of the functions in 7.6.2.207) Additional implementation-defined floating-point exceptions, with macro definitions beginning with FE_ and an uppercase letter,208) may also be specified by the implementation. The defined macros expand to integer constant expressions with values such that bitwise ORs of all combinations of the macros result in distinct values, and furthermore, bitwise ANDs of all combinations of the macros result in zero.209)

7 The macro

          FE_ALL_EXCEPT
is simply the bitwise OR of all floating-point exception macros defined by the implementation. If no such macros are defined, FE_ALL_EXCEPT shall be defined as 0.

8 Each of the macros

          FE_DOWNWARD
          FE_TONEAREST
          FE_TOWARDZERO
          FE_UPWARD
is defined if and only if the implementation supports getting and setting the represented rounding direction by means of the fegetround and fesetround functions. Additional implementation-defined rounding directions, with macro definitions beginning with FE_ and an uppercase letter,210) may also be specified by the implementation. The defined macros expand to integer constant expressions whose values are distinct nonnegative values.211)

9 The macro

          FE_DFL_ENV
represents the default floating-point environment -- the one installed at program startup -- and has type ''pointer to const-qualified fenv_t''. It can be used as an argument to <fenv.h> functions that manage the floating-point environment.

10 Additional implementation-defined environments, with macro definitions beginning with FE_ and an uppercase letter,212) and having type ''pointer to const-qualified fenv_t'', may also be specified by the implementation.

Footnotes

204) This header is designed to support the floating-point exception status flags and directed-rounding control modes required by IEC 60559, and other similar floating-point state information. It is also designed to facilitate code portability among all systems.

205) A floating-point status flag is not an object and can be set more than once within an expression.

206) With these conventions, a programmer can safely assume default floating-point control modes (or be unaware of them). The responsibilities associated with accessing the floating-point environment fall on the programmer or program that does so explicitly.

207) The implementation supports a floating-point exception if there are circumstances where a call to at least one of the functions in 7.6.2, using the macro as the appropriate argument, will succeed. It is not necessary for all the functions to succeed all the time.

208) See ''future library directions'' (7.31.4).

209) The macros should be distinct powers of two.

210) See ''future library directions'' (7.31.4).

211) Even though the rounding direction macros may expand to constants corresponding to the values of FLT_ROUNDS, they are not required to do so.

212) See ''future library directions'' (7.31.4).

Contents

7.6.1 The FENV_ACCESS pragma

Synopsis

1

          #include <fenv.h>
          #pragma STDC FENV_ACCESS on-off-switch

Description

2 The FENV_ACCESS pragma provides a means to inform the implementation when a program might access the floating-point environment to test floating-point status flags or run under non-default floating-point control modes.213) The pragma shall occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. If part of a program tests floating-point status flags, sets floating-point control modes, or runs under non-default mode settings, but was translated with the state for the FENV_ACCESS pragma ''off'', the behavior is undefined. The default state (''on'' or ''off'') for the pragma is implementation-defined. (When execution passes from a part of the program translated with FENV_ACCESS ''off'' to a part translated with FENV_ACCESS ''on'', the state of the floating-point status flags is unspecified and the floating-point control modes have their default settings.)

3 EXAMPLE

         #include <fenv.h>
         void f(double x)
         {
               #pragma STDC FENV_ACCESS ON
               void g(double);
               void h(double);
               /* ... */
               g(x + 1);
               h(x + 1);
               /* ... */
         }

4 If the function g might depend on status flags set as a side effect of the first x + 1, or if the second x + 1 might depend on control modes set as a side effect of the call to function g, then the program shall contain an appropriately placed invocation of #pragma STDC FENV_ACCESS ON.214)

Footnotes

213) The purpose of the FENV_ACCESS pragma is to allow certain optimizations that could subvert flag tests and mode changes (e.g., global common subexpression elimination, code motion, and constant folding). In general, if the state of FENV_ACCESS is ''off'', the translator can assume that default modes are in effect and the flags are not tested.

214) The side effects impose a temporal ordering that requires two evaluations of x + 1. On the other hand, without the #pragma STDC FENV_ACCESS ON pragma, and assuming the default state is ''off'', just one evaluation of x + 1 would suffice.

Contents

7.6.2 Floating-point exceptions

1 The following functions provide access to the floating-point status flags.215) The int input argument for the functions represents a subset of floating-point exceptions, and can be zero or the bitwise OR of one or more floating-point exception macros, for example FE_OVERFLOW | FE_INEXACT. For other argument values the behavior of these functions is undefined.

Footnotes

215) The functions fetestexcept, feraiseexcept, and feclearexcept support the basic abstraction of flags that are either set or clear. An implementation may endow floating-point status flags with more information -- for example, the address of the code which first raised the floating- point exception; the functions fegetexceptflag and fesetexceptflag deal with the full content of flags.

Contents

7.6.2.1 The feclearexcept function

Synopsis

1

         #include <fenv.h>
         int feclearexcept(int excepts);

Description

2 The feclearexcept function attempts to clear the supported floating-point exceptions represented by its argument.

Returns

3 The feclearexcept function returns zero if the excepts argument is zero or if all the specified exceptions were successfully cleared. Otherwise, it returns a nonzero value.

Contents

7.6.2.2 The fegetexceptflag function

Synopsis

1

          #include <fenv.h>
          int fegetexceptflag(fexcept_t *flagp,
               int excepts);

Description

2 The fegetexceptflag function attempts to store an implementation-defined representation of the states of the floating-point status flags indicated by the argument excepts in the object pointed to by the argument flagp.

Returns

3 The fegetexceptflag function returns zero if the representation was successfully stored. Otherwise, it returns a nonzero value.

Contents

7.6.2.3 The feraiseexcept function

Synopsis

1

          #include <fenv.h>
          int feraiseexcept(int excepts);

Description

2 The feraiseexcept function attempts to raise the supported floating-point exceptions represented by its argument.216) The order in which these floating-point exceptions are raised is unspecified, except as stated in F.8.6. Whether the feraiseexcept function additionally raises the ''inexact'' floating-point exception whenever it raises the ''overflow'' or ''underflow'' floating-point exception is implementation-defined.

Returns

3 The feraiseexcept function returns zero if the excepts argument is zero or if all the specified exceptions were successfully raised. Otherwise, it returns a nonzero value.

Footnotes

216) The effect is intended to be similar to that of floating-point exceptions raised by arithmetic operations. Hence, enabled traps for floating-point exceptions raised by this function are taken. The specification in F.8.6 is in the same spirit.

Contents

7.6.2.4 The fesetexceptflag function

Synopsis

1

         #include <fenv.h>
         int fesetexceptflag(const fexcept_t *flagp,
              int excepts);

Description

2 The fesetexceptflag function attempts to set the floating-point status flags indicated by the argument excepts to the states stored in the object pointed to by flagp. The value of *flagp shall have been set by a previous call to fegetexceptflag whose second argument represented at least those floating-point exceptions represented by the argument excepts. This function does not raise floating- point exceptions, but only sets the state of the flags.

Returns

3 The fesetexceptflag function returns zero if the excepts argument is zero or if all the specified flags were successfully set to the appropriate state. Otherwise, it returns a nonzero value.

Contents

7.6.2.5 The fetestexcept function

Synopsis

1

         #include <fenv.h>
         int fetestexcept(int excepts);

Description

2 The fetestexcept function determines which of a specified subset of the floating- point exception flags are currently set. The excepts argument specifies the floating- point status flags to be queried.217)

Returns

3 The fetestexcept function returns the value of the bitwise OR of the floating-point exception macros corresponding to the currently set floating-point exceptions included in excepts.

4 EXAMPLE Call f if ''invalid'' is set, then g if ''overflow'' is set:

        #include <fenv.h>
        /* ... */
        {
                #pragma STDC FENV_ACCESS ON
                int set_excepts;
                feclearexcept(FE_INVALID | FE_OVERFLOW);
                // maybe raise exceptions
                set_excepts = fetestexcept(FE_INVALID | FE_OVERFLOW);
                if (set_excepts & FE_INVALID) f();
                if (set_excepts & FE_OVERFLOW) g();
                /* ... */
        }

Footnotes

217) This mechanism allows testing several floating-point exceptions with just one function call.

Contents

7.6.3 Rounding

1 The fegetround and fesetround functions provide control of rounding direction modes.

Contents

7.6.3.1 The fegetround function

Synopsis

1

        #include <fenv.h>
        int fegetround(void);

Description

2 The fegetround function gets the current rounding direction.

Returns

3 The fegetround function returns the value of the rounding direction macro representing the current rounding direction or a negative value if there is no such rounding direction macro or the current rounding direction is not determinable.

Contents

7.6.3.2 The fesetround function

Synopsis

1

        #include <fenv.h>
        int fesetround(int round);

Description

2 The fesetround function establishes the rounding direction represented by its argument round. If the argument is not equal to the value of a rounding direction macro, the rounding direction is not changed.

Returns

3 The fesetround function returns zero if and only if the requested rounding direction was established.

4 EXAMPLE Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails.

         #include <fenv.h>
         #include <assert.h>
         void f(int round_dir)
         {
               #pragma STDC FENV_ACCESS ON
               int save_round;
               int setround_ok;
               save_round = fegetround();
               setround_ok = fesetround(round_dir);
               assert(setround_ok == 0);
               /* ... */
               fesetround(save_round);
               /* ... */
         }

Contents

7.6.4 Environment

1 The functions in this section manage the floating-point environment -- status flags and control modes -- as one entity.

Contents

7.6.4.1 The fegetenv function

Synopsis

1

         #include <fenv.h>
         int fegetenv(fenv_t *envp);

Description

2 The fegetenv function attempts to store the current floating-point environment in the object pointed to by envp.

Returns

3 The fegetenv function returns zero if the environment was successfully stored. Otherwise, it returns a nonzero value.

Contents

7.6.4.2 The feholdexcept function

Synopsis

1

         #include <fenv.h>
         int feholdexcept(fenv_t *envp);

Description

2 The feholdexcept function saves the current floating-point environment in the object pointed to by envp, clears the floating-point status flags, and then installs a non-stop (continue on floating-point exceptions) mode, if available, for all floating-point exceptions.218)

Returns

3 The feholdexcept function returns zero if and only if non-stop floating-point exception handling was successfully installed.

Footnotes

218) IEC 60559 systems have a default non-stop mode, and typically at least one other mode for trap handling or aborting; if the system provides only the non-stop mode then installing it is trivial. For such systems, the feholdexcept function can be used in conjunction with the feupdateenv function to write routines that hide spurious floating-point exceptions from their callers.

Contents

7.6.4.3 The fesetenv function

Synopsis

1

         #include <fenv.h>
         int fesetenv(const fenv_t *envp);

Description

2 The fesetenv function attempts to establish the floating-point environment represented by the object pointed to by envp. The argument envp shall point to an object set by a call to fegetenv or feholdexcept, or equal a floating-point environment macro. Note that fesetenv merely installs the state of the floating-point status flags represented through its argument, and does not raise these floating-point exceptions.

Returns

3 The fesetenv function returns zero if the environment was successfully established. Otherwise, it returns a nonzero value.

Contents

7.6.4.4 The feupdateenv function

Synopsis

1

         #include <fenv.h>
         int feupdateenv(const fenv_t *envp);

Description

2 The feupdateenv function attempts to save the currently raised floating-point exceptions in its automatic storage, install the floating-point environment represented by the object pointed to by envp, and then raise the saved floating-point exceptions. The argument envp shall point to an object set by a call to feholdexcept or fegetenv, or equal a floating-point environment macro.

Returns

3 The feupdateenv function returns zero if all the actions were successfully carried out. Otherwise, it returns a nonzero value.

4 EXAMPLE Hide spurious underflow floating-point exceptions:

         #include <fenv.h>
         double f(double x)
         {
               #pragma STDC FENV_ACCESS ON
               double result;
               fenv_t save_env;
               if (feholdexcept(&save_env))
                     return /* indication of an environmental problem */;
               // compute result
               if (/* test spurious underflow */)
                     if (feclearexcept(FE_UNDERFLOW))
                              return /* indication of an environmental problem */;
               if (feupdateenv(&save_env))
                     return /* indication of an environmental problem */;
               return result;
         }

Contents

7.7 Characteristics of floating types <float.h>

1 The header <float.h> defines several macros that expand to various limits and parameters of the standard floating-point types.

2 The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.2.

Contents

7.8 Format conversion of integer types <inttypes.h>

1 The header <inttypes.h> includes the header <stdint.h> and extends it with additional facilities provided by hosted implementations.

2 It declares functions for manipulating greatest-width integers and converting numeric character strings to greatest-width integers, and it declares the type

          imaxdiv_t
which is a structure type that is the type of the value returned by the imaxdiv function. For each type declared in <stdint.h>, it defines corresponding macros for conversion specifiers for use with the formatted input/output functions.219)

Forward references: integer types <stdint.h> (7.20), formatted input/output functions (7.21.6), formatted wide character input/output functions (7.29.2).

Footnotes

219) See ''future library directions'' (7.31.5).

Contents

7.8.1 Macros for format specifiers

1 Each of the following object-like macros expands to a character string literal containing a conversion specifier, possibly modified by a length modifier, suitable for use within the format argument of a formatted input/output function when converting the corresponding integer type. These macro names have the general form of PRI (character string literals for the fprintf and fwprintf family) or SCN (character string literals for the fscanf and fwscanf family),220) followed by the conversion specifier, followed by a name corresponding to a similar type name in 7.20.1. In these names, N represents the width of the type as described in 7.20.1. For example, PRIdFAST32 can be used in a format string to print the value of an integer of type int_fast32_t.

2 The fprintf macros for signed integers are:

        PRIdN             PRIdLEASTN                PRIdFASTN          PRIdMAX             PRIdPTR
        PRIiN             PRIiLEASTN                PRIiFASTN          PRIiMAX             PRIiPTR

3 The fprintf macros for unsigned integers are:

        PRIoN             PRIoLEASTN                PRIoFASTN          PRIoMAX             PRIoPTR
        PRIuN             PRIuLEASTN                PRIuFASTN          PRIuMAX             PRIuPTR
        PRIxN             PRIxLEASTN                PRIxFASTN          PRIxMAX             PRIxPTR
        PRIXN             PRIXLEASTN                PRIXFASTN          PRIXMAX             PRIXPTR

4 The fscanf macros for signed integers are:

        SCNdN           SCNdLEASTN               SCNdFASTN              SCNdMAX             SCNdPTR
        SCNiN           SCNiLEASTN               SCNiFASTN              SCNiMAX             SCNiPTR

5 The fscanf macros for unsigned integers are:

        SCNoN           SCNoLEASTN               SCNoFASTN              SCNoMAX             SCNoPTR
        SCNuN           SCNuLEASTN               SCNuFASTN              SCNuMAX             SCNuPTR
        SCNxN           SCNxLEASTN               SCNxFASTN              SCNxMAX             SCNxPTR

6 For each type that the implementation provides in <stdint.h>, the corresponding fprintf macros shall be defined and the corresponding fscanf macros shall be defined unless the implementation does not have a suitable fscanf length modifier for the type.

7 EXAMPLE

         #include <inttypes.h>
         #include <wchar.h>
         int main(void)
         {
               uintmax_t i = UINTMAX_MAX;    // this type always exists
               wprintf(L"The largest integer value is %020"
                     PRIxMAX "\n", i);
               return 0;
         }

Footnotes

220) Separate macros are given for use with fprintf and fscanf functions because, in the general case, different format specifiers may be required for fprintf and fscanf, even when the type is the same.

Contents

7.8.2 Functions for greatest-width integer types

Contents

7.8.2.1 The imaxabs function

Synopsis

1

         #include <inttypes.h>
         intmax_t imaxabs(intmax_t j);

Description

2 The imaxabs function computes the absolute value of an integer j. If the result cannot be represented, the behavior is undefined.221)

Returns

3 The imaxabs function returns the absolute value.

Footnotes

221) The absolute value of the most negative number cannot be represented in two's complement.

Contents

7.8.2.2 The imaxdiv function

Synopsis

1

         #include <inttypes.h>
         imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);

Description

2 The imaxdiv function computes numer / denom and numer % denom in a single operation.

Returns

3 The imaxdiv function returns a structure of type imaxdiv_t comprising both the quotient and the remainder. The structure shall contain (in either order) the members quot (the quotient) and rem (the remainder), each of which has type intmax_t. If either part of the result cannot be represented, the behavior is undefined.

Contents

7.8.2.3 The strtoimax and strtoumax functions

Synopsis

1

         #include <inttypes.h>
         intmax_t strtoimax(const char * restrict nptr,
              char ** restrict endptr, int base);
         uintmax_t strtoumax(const char * restrict nptr,
              char ** restrict endptr, int base);

Description

2 The strtoimax and strtoumax functions are equivalent to the strtol, strtoll, strtoul, and strtoull functions, except that the initial portion of the string is converted to intmax_t and uintmax_t representation, respectively.

Returns

3 The strtoimax and strtoumax functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.22.1.4).

Contents

7.8.2.4 The wcstoimax and wcstoumax functions

Synopsis

1

        #include <stddef.h>           // for wchar_t
        #include <inttypes.h>
        intmax_t wcstoimax(const wchar_t * restrict nptr,
             wchar_t ** restrict endptr, int base);
        uintmax_t wcstoumax(const wchar_t * restrict nptr,
             wchar_t ** restrict endptr, int base);

Description

2 The wcstoimax and wcstoumax functions are equivalent to the wcstol, wcstoll, wcstoul, and wcstoull functions except that the initial portion of the wide string is converted to intmax_t and uintmax_t representation, respectively.

Returns

3 The wcstoimax function returns the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

Forward references: the wcstol, wcstoll, wcstoul, and wcstoull functions (7.29.4.1.2).

Contents

7.9 Alternative spellings <iso646.h>

1 The header <iso646.h> defines the following eleven macros (on the left) that expand to the corresponding tokens (on the right):

         and        &&
         and_eq     &=
         bitand     &
         bitor      |
         compl      ~
         not        !
         not_eq     !=
         or         ||
         or_eq      |=
         xor        ^
         xor_eq     ^=

Contents

7.10 Sizes of integer types <limits.h>

1 The header <limits.h> defines several macros that expand to various limits and parameters of the standard integer types.

2 The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.1.

Contents

7.11 Localization <locale.h>

1 The header <locale.h> declares two functions, one type, and defines several macros.

2 The type is

         struct lconv
which contains members related to the formatting of numeric values. The structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges are explained in 7.11.2.1. In the "C" locale, the members shall have the values specified in the comments.
         char   *decimal_point;                //   "."
         char   *thousands_sep;                //   ""
         char   *grouping;                     //   ""
         char   *mon_decimal_point;            //   ""
         char   *mon_thousands_sep;            //   ""
         char   *mon_grouping;                 //   ""
         char   *positive_sign;                //   ""
         char   *negative_sign;                //   ""
         char   *currency_symbol;              //   ""
         char   frac_digits;                   //   CHAR_MAX
         char   p_cs_precedes;                 //   CHAR_MAX
         char   n_cs_precedes;                 //   CHAR_MAX
         char   p_sep_by_space;                //   CHAR_MAX
         char   n_sep_by_space;                //   CHAR_MAX
         char   p_sign_posn;                   //   CHAR_MAX
         char   n_sign_posn;                   //   CHAR_MAX
         char   *int_curr_symbol;              //   ""
         char   int_frac_digits;               //   CHAR_MAX
         char   int_p_cs_precedes;             //   CHAR_MAX
         char   int_n_cs_precedes;             //   CHAR_MAX
         char   int_p_sep_by_space;            //   CHAR_MAX
         char   int_n_sep_by_space;            //   CHAR_MAX
         char   int_p_sign_posn;               //   CHAR_MAX
         char   int_n_sign_posn;               //   CHAR_MAX

3 The macros defined are NULL (described in 7.19); and

          LC_ALL
          LC_COLLATE
          LC_CTYPE
          LC_MONETARY
          LC_NUMERIC
          LC_TIME
which expand to integer constant expressions with distinct values, suitable for use as the first argument to the setlocale function.222) Additional macro definitions, beginning with the characters LC_ and an uppercase letter,223) may also be specified by the implementation.

Footnotes

222) ISO/IEC 9945-2 specifies locale and charmap formats that may be used to specify locales for C.

223) See ''future library directions'' (7.31.6).

Contents

7.11.1 Locale control

Contents

7.11.1.1 The setlocale function

Synopsis

1

          #include <locale.h>
          char *setlocale(int category, const char *locale);

Description

2 The setlocale function selects the appropriate portion of the program's locale as specified by the category and locale arguments. The setlocale function may be used to change or query the program's entire current locale or portions thereof. The value LC_ALL for category names the program's entire locale; the other values for category name only a portion of the program's locale. LC_COLLATE affects the behavior of the strcoll and strxfrm functions. LC_CTYPE affects the behavior of the character handling functions224) and the multibyte and wide character functions. LC_MONETARY affects the monetary formatting information returned by the localeconv function. LC_NUMERIC affects the decimal-point character for the formatted input/output functions and the string conversion functions, as well as the nonmonetary formatting information returned by the localeconv function. LC_TIME affects the behavior of the strftime and wcsftime functions.

3 A value of "C" for locale specifies the minimal environment for C translation; a value of "" for locale specifies the locale-specific native environment. Other implementation-defined strings may be passed as the second argument to setlocale.

4 At program startup, the equivalent of

         setlocale(LC_ALL, "C");
is executed.

5 A call to the setlocale function may introduce a data race with other calls to the setlocale function or with calls to functions that are affected by the current locale. The implementation shall behave as if no library function calls the setlocale function.

Returns

6 If a pointer to a string is given for locale and the selection can be honored, the setlocale function returns a pointer to the string associated with the specified category for the new locale. If the selection cannot be honored, the setlocale function returns a null pointer and the program's locale is not changed.

7 A null pointer for locale causes the setlocale function to return a pointer to the string associated with the category for the program's current locale; the program's locale is not changed.225)

8 The pointer to string returned by the setlocale function is such that a subsequent call with that string value and its associated category will restore that part of the program's locale. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the setlocale function.

Forward references: formatted input/output functions (7.21.6), multibyte/wide character conversion functions (7.22.7), multibyte/wide string conversion functions (7.22.8), numeric conversion functions (7.22.1), the strcoll function (7.24.4.3), the strftime function (7.27.3.5), the strxfrm function (7.24.4.5).

Footnotes

224) The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and isxdigit.

225) The implementation shall arrange to encode in a string the various categories due to a heterogeneous locale when category has the value LC_ALL.

Contents

7.11.2 Numeric formatting convention inquiry

Contents

7.11.2.1 The localeconv function

Synopsis

1

         #include <locale.h>
         struct lconv *localeconv(void);

Description

2 The localeconv function sets the components of an object with type struct lconv with values appropriate for the formatting of numeric quantities (monetary and otherwise) according to the rules of the current locale.

3 The members of the structure with type char * are pointers to strings, any of which (except decimal_point) can point to "", to indicate that the value is not available in the current locale or is of zero length. Apart from grouping and mon_grouping, the strings shall start and end in the initial shift state. The members with type char are nonnegative numbers, any of which can be CHAR_MAX to indicate that the value is not available in the current locale. The members include the following:

char *decimal_point
The decimal-point character used to format nonmonetary quantities.
char *thousands_sep
The character used to separate groups of digits before the decimal-point character in formatted nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each group of digits in formatted nonmonetary quantities.
char *mon_decimal_point
The decimal-point used to format monetary quantities.
char *mon_thousands_sep
The separator for groups of digits before the decimal-point in formatted monetary quantities.
char *mon_grouping
A string whose elements indicate the size of each group of digits in formatted monetary quantities.
char *positive_sign
The string used to indicate a nonnegative-valued formatted monetary quantity.
char *negative_sign
The string used to indicate a negative-valued formatted monetary quantity.
char *currency_symbol
The local currency symbol applicable to the current locale.
char frac_digits
The number of fractional digits (those after the decimal-point) to be displayed in a locally formatted monetary quantity.
char p_cs_precedes
Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a nonnegative locally formatted monetary quantity.
char n_cs_precedes
Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a negative locally formatted monetary quantity.
char p_sep_by_space
Set to a value indicating the separation of the currency_symbol, the sign string, and the value for a nonnegative locally formatted monetary quantity.
char n_sep_by_space
Set to a value indicating the separation of the currency_symbol, the sign string, and the value for a negative locally formatted monetary quantity.
char p_sign_posn
Set to a value indicating the positioning of the positive_sign for a nonnegative locally formatted monetary quantity.
char n_sign_posn
Set to a value indicating the positioning of the negative_sign for a negative locally formatted monetary quantity.
char *int_curr_symbol
The international currency symbol applicable to the current locale. The first three characters contain the alphabetic international currency symbol in accordance with those specified in ISO 4217. The fourth character (immediately preceding the null character) is the character used to separate the international currency symbol from the monetary quantity.
char int_frac_digits
The number of fractional digits (those after the decimal-point) to be displayed in an internationally formatted monetary quantity.
char int_p_cs_precedes
Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a nonnegative internationally formatted monetary quantity.
char int_n_cs_precedes
Set to 1 or 0 if the int_curr_symbol respectively precedes or succeeds the value for a negative internationally formatted monetary quantity.
char int_p_sep_by_space
Set to a value indicating the separation of the int_curr_symbol, the sign string, and the value for a nonnegative internationally formatted monetary quantity.
char int_n_sep_by_space
Set to a value indicating the separation of the int_curr_symbol, the sign string, and the value for a negative internationally formatted monetary quantity.
char int_p_sign_posn
Set to a value indicating the positioning of the positive_sign for a nonnegative internationally formatted monetary quantity.
char int_n_sign_posn
Set to a value indicating the positioning of the negative_sign for a negative internationally formatted monetary quantity.

4 The elements of grouping and mon_grouping are interpreted according to the following:

CHAR_MAX
No further grouping is to be performed.
0
The previous element is to be repeatedly used for the remainder of the digits.
other
The integer value is the number of digits that compose the current group. The next element is examined to determine the size of the next group of digits before the current group.

5 The values of p_sep_by_space, n_sep_by_space, int_p_sep_by_space, and int_n_sep_by_space are interpreted according to the following:

0
No space separates the currency symbol and value.
1
If the currency symbol and sign string are adjacent, a space separates them from the value; otherwise, a space separates the currency symbol from the value.
2
If the currency symbol and sign string are adjacent, a space separates them; otherwise, a space separates the sign string from the value.
For int_p_sep_by_space and int_n_sep_by_space, the fourth character of int_curr_symbol is used instead of a space.

6 The values of p_sign_posn, n_sign_posn, int_p_sign_posn, and int_n_sign_posn are interpreted according to the following:

0
Parentheses surround the quantity and currency symbol.
1
The sign string precedes the quantity and currency symbol.
2
The sign string succeeds the quantity and currency symbol.
3
The sign string immediately precedes the currency symbol.
4
The sign string immediately succeeds the currency symbol.

7 The implementation shall behave as if no library function calls the localeconv function.

Returns

8 The localeconv function returns a pointer to the filled-in object. The structure pointed to by the return value shall not be modified by the program, but may be overwritten by a subsequent call to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the contents of the structure.

9 EXAMPLE 1 The following table illustrates rules which may well be used by four countries to format monetary quantities.

                          Local format                                     International format
 
 Country        Positive                  Negative                    Positive               Negative
 
 Country1     1.234,56 mk             -1.234,56 mk                  FIM   1.234,56         FIM -1.234,56
 Country2     L.1.234                 -L.1.234                      ITL   1.234            -ITL 1.234
 Country3     fl. 1.234,56            fl. -1.234,56                 NLG   1.234,56         NLG -1.234,56
 Country4     SFrs.1,234.56           SFrs.1,234.56C                CHF   1,234.56         CHF 1,234.56C

10 For these four countries, the respective values for the monetary members of the structure returned by localeconv could be:

                                   Country1              Country2             Country3            Country4
 
 mon_decimal_point                 ","                   ""                   ","                 "."
 mon_thousands_sep                 "."                   "."                  "."                 ","
 mon_grouping                      "\3"                  "\3"                 "\3"                "\3"
 positive_sign                     ""                    ""                   ""                  ""
 negative_sign                     "-"                   "-"                  "-"                 "C"
 currency_symbol                   "mk"                  "L."                 "\u0192"            "SFrs."
 frac_digits                       2                     0                    2                   2
 p_cs_precedes                     0                     1                    1                   1
 n_cs_precedes                     0                     1                    1                   1
 p_sep_by_space                    1                     0                    1                   0
 n_sep_by_space                    1                     0                    2                   0
 p_sign_posn                       1                     1                    1                   1
 n_sign_posn                       1                     1                    4                   2
 int_curr_symbol                   "FIM "                "ITL "               "NLG "              "CHF "
 int_frac_digits                   2                     0                    2                   2
 int_p_cs_precedes                 1                     1                    1                   1
 int_n_cs_precedes                 1                     1                    1                   1
 int_p_sep_by_space                1                     1                    1                   1
 int_n_sep_by_space                2                     1                    2                   1
 int_p_sign_posn                   1                     1                    1                   1
 int_n_sign_posn                   4                     1                    4                   2

11 EXAMPLE 2 The following table illustrates how the cs_precedes, sep_by_space, and sign_posn members affect the formatted value.

                                                               p_sep_by_space
 
    p_cs_precedes           p_sign_posn            0                   1                  2
 
                 0                    0         (1.25$)            (1.25 $)            (1.25$)
                                      1         +1.25$             +1.25 $             + 1.25$
                                      2         1.25$+             1.25 $+             1.25$ +
                                      3         1.25+$             1.25 +$             1.25+ $
                                      4         1.25$+             1.25 $+             1.25$ +

                 1                    0         ($1.25)            ($ 1.25)            ($1.25)
                                      1         +$1.25             +$ 1.25             + $1.25
                                      2         $1.25+             $ 1.25+             $1.25 +
                                      3         +$1.25             +$ 1.25             + $1.25
                                      4         $+1.25             $+ 1.25             $ +1.25

Contents

7.12 Mathematics <math.h>

1 The header <math.h> declares two types and many mathematical functions and defines several macros. Most synopses specify a family of functions consisting of a principal function with one or more double parameters, a double return value, or both; and other functions with the same name but with f and l suffixes, which are corresponding functions with float and long double parameters, return values, or both.226) Integer arithmetic functions and conversion functions are discussed later.

2 The types

         float_t
         double_t
are floating types at least as wide as float and double, respectively, and such that double_t is at least as wide as float_t. If FLT_EVAL_METHOD equals 0, float_t and double_t are float and double, respectively; if FLT_EVAL_METHOD equals 1, they are both double; if FLT_EVAL_METHOD equals 2, they are both long double; and for other values of FLT_EVAL_METHOD, they are otherwise implementation-defined.227)

3 The macro

         HUGE_VAL
expands to a positive double constant expression, not necessarily representable as a float. The macros
         HUGE_VALF
         HUGE_VALL
are respectively float and long double analogs of HUGE_VAL.228)

4 The macro

         INFINITY
expands to a constant expression of type float representing positive or unsigned infinity, if available; else to a positive constant of type float that overflows at translation time.229)

5 The macro

          NAN
is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a constant expression of type float representing a quiet NaN.

6 The number classification macros

          FP_INFINITE
          FP_NAN
          FP_NORMAL
          FP_SUBNORMAL
          FP_ZERO
represent the mutually exclusive kinds of floating-point values. They expand to integer constant expressions with distinct values. Additional implementation-defined floating- point classifications, with macro definitions beginning with FP_ and an uppercase letter, may also be specified by the implementation.

7 The macro

          FP_FAST_FMA
is optionally defined. If defined, it indicates that the fma function generally executes about as fast as, or faster than, a multiply and an add of double operands.230) The macros
          FP_FAST_FMAF
          FP_FAST_FMAL
are, respectively, float and long double analogs of FP_FAST_FMA. If defined, these macros expand to the integer constant 1.

8 The macros

          FP_ILOGB0
          FP_ILOGBNAN
expand to integer constant expressions whose values are returned by ilogb(x) if x is zero or NaN, respectively. The value of FP_ILOGB0 shall be either INT_MIN or -INT_MAX. The value of FP_ILOGBNAN shall be either INT_MAX or INT_MIN.

9 The macros

           MATH_ERRNO
           MATH_ERREXCEPT
expand to the integer constants 1 and 2, respectively; the macro
         math_errhandling
expands to an expression that has type int and the value MATH_ERRNO, MATH_ERREXCEPT, or the bitwise OR of both. The value of math_errhandling is constant for the duration of the program. It is unspecified whether math_errhandling is a macro or an identifier with external linkage. If a macro definition is suppressed or a program defines an identifier with the name math_errhandling, the behavior is undefined. If the expression math_errhandling & MATH_ERREXCEPT can be nonzero, the implementation shall define the macros FE_DIVBYZERO, FE_INVALID, and FE_OVERFLOW in <fenv.h>.

Footnotes

226) Particularly on systems with wide expression evaluation, a <math.h> function might pass arguments and return values in wider format than the synopsis prototype indicates.

227) The types float_t and double_t are intended to be the implementation's most efficient types at least as wide as float and double, respectively. For FLT_EVAL_METHOD equal 0, 1, or 2, the type float_t is the narrowest type used by the implementation to evaluate floating expressions.

228) HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities.

229) In this case, using INFINITY will violate the constraint in 6.4.4 and thus require a diagnostic.

230) Typically, the FP_FAST_FMA macro is defined if and only if the fma function is implemented directly with a hardware multiply-add instruction. Software implementations are expected to be substantially slower.

Contents

7.12.1 Treatment of error conditions

1 The behavior of each of the functions in <math.h> is specified for all representable values of its input arguments, except where stated otherwise. Each function shall execute as if it were a single operation without raising SIGFPE and without generating any of the floating-point exceptions ''invalid'', ''divide-by-zero'', or ''overflow'' except to reflect the result of the function.

2 For all functions, a domain error occurs if an input argument is outside the domain over which the mathematical function is defined. The description of each function lists any required domain errors; an implementation may define additional domain errors, provided that such errors are consistent with the mathematical definition of the function.231) On a domain error, the function returns an implementation-defined value; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value EDOM; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the ''invalid'' floating-point exception is raised.

3 Similarly, a pole error (also known as a singularity or infinitary) occurs if the mathematical function has an exact infinite result as the finite input argument(s) are approached in the limit (for example, log(0.0)). The description of each function lists any required pole errors; an implementation may define additional pole errors, provided that such errors are consistent with the mathematical definition of the function. On a pole error, the function returns an implementation-defined value; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the ''divide-by-zero'' floating-point exception is raised.

4 Likewise, a range error occurs if the mathematical result of the function cannot be represented in an object of the specified type, due to extreme magnitude.

5 A floating result overflows if the magnitude of the mathematical result is finite but so large that the mathematical result cannot be represented without extraordinary roundoff error in an object of the specified type. If a floating result overflows and default rounding is in effect, then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or HUGE_VALL according to the return type, with the same sign as the correct value of the function; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the ''overflow'' floating- point exception is raised.

6 The result underflows if the magnitude of the mathematical result is so small that the mathematical result cannot be represented, without extraordinary roundoff error, in an object of the specified type.232) If the result underflows, the function returns an implementation-defined value whose magnitude is no greater than the smallest normalized positive number in the specified type; if the integer expression math_errhandling & MATH_ERRNO is nonzero, whether errno acquires the value ERANGE is implementation-defined; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, whether the ''underflow'' floating-point exception is raised is implementation-defined.

7 If a domain, pole, or range error occurs and the integer expression math_errhandling & MATH_ERRNO is zero,233) then errno shall either be set to the value corresponding to the error or left unmodified. If no such error occurs, errno shall be left unmodified regardless of the setting of math_errhandling.

Footnotes

231) In an implementation that supports infinities, this allows an infinity as an argument to be a domain error if the mathematical domain of the function does not include the infinity.

232) The term underflow here is intended to encompass both ''gradual underflow'' as in IEC 60559 and also ''flush-to-zero'' underflow.

233) Math errors are being indicated by the floating-point exception flags rather than by errno.

Contents

7.12.2 The FP_CONTRACT pragma

Synopsis

1

          #include <math.h>
          #pragma STDC FP_CONTRACT on-off-switch

Description

2 The FP_CONTRACT pragma can be used to allow (if the state is ''on'') or disallow (if the state is ''off'') the implementation to contract expressions (6.5). Each pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. The default state (''on'' or ''off'') for the pragma is implementation-defined.

Contents

7.12.3 Classification macros

1 In the synopses in this subclause, real-floating indicates that the argument shall be an expression of real floating type.

Contents

7.12.3.1 The fpclassify macro

Synopsis

1

          #include <math.h>
          int fpclassify(real-floating x);

Description

2 The fpclassify macro classifies its argument value as NaN, infinite, normal, subnormal, zero, or into another implementation-defined category. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then classification is based on the type of the argument.234)

Returns

3 The fpclassify macro returns the value of the number classification macro appropriate to the value of its argument.

Footnotes

234) Since an expression can be evaluated with more range and precision than its type has, it is important to know the type that classification is based on. For example, a normal long double value might become subnormal when converted to double, and zero when converted to float.

Contents

7.12.3.2 The isfinite macro

Synopsis

1

         #include <math.h>
         int isfinite(real-floating x);

Description

2 The isfinite macro determines whether its argument has a finite value (zero, subnormal, or normal, and not infinite or NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

3 The isfinite macro returns a nonzero value if and only if its argument has a finite value.

Contents

7.12.3.3 The isinf macro

Synopsis

1

         #include <math.h>
         int isinf(real-floating x);

Description

2 The isinf macro determines whether its argument value is an infinity (positive or negative). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

3 The isinf macro returns a nonzero value if and only if its argument has an infinite value.

Contents

7.12.3.4 The isnan macro

Synopsis

1

         #include <math.h>
         int isnan(real-floating x);

Description

2 The isnan macro determines whether its argument value is a NaN. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.235)

Returns

3 The isnan macro returns a nonzero value if and only if its argument has a NaN value.

Footnotes

235) For the isnan macro, the type for determination does not matter unless the implementation supports NaNs in the evaluation type but not in the semantic type.

Contents

7.12.3.5 The isnormal macro

Synopsis

1

         #include <math.h>
         int isnormal(real-floating x);

Description

2 The isnormal macro determines whether its argument value is normal (neither zero, subnormal, infinite, nor NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

3 The isnormal macro returns a nonzero value if and only if its argument has a normal value.

Contents

7.12.3.6 The signbit macro

Synopsis

1

         #include <math.h>
         int signbit(real-floating x);

Description

2 The signbit macro determines whether the sign of its argument value is negative.236)

Returns

3 The signbit macro returns a nonzero value if and only if the sign of its argument value is negative.

Footnotes

236) The signbit macro reports the sign of all values, including infinities, zeros, and NaNs. If zero is unsigned, it is treated as positive.

Contents

7.12.4 Trigonometric functions

Contents

7.12.4.1 The acos functions

Synopsis

1

        #include <math.h>
        double acos(double x);
        float acosf(float x);
        long double acosl(long double x);

Description

2 The acos functions compute the principal value of the arc cosine of x. A domain error occurs for arguments not in the interval [-1, +1].

Returns

3 The acos functions return arccos x in the interval [0, pi ] radians.

Contents

7.12.4.2 The asin functions

Synopsis

1

        #include <math.h>
        double asin(double x);
        float asinf(float x);
        long double asinl(long double x);

Description

2 The asin functions compute the principal value of the arc sine of x. A domain error occurs for arguments not in the interval [-1, +1].

Returns

3 The asin functions return arcsin x in the interval [-pi /2, +pi /2] radians.

Contents

7.12.4.3 The atan functions

Synopsis

1

        #include <math.h>
        double atan(double x);
        float atanf(float x);
        long double atanl(long double x);

Description

2 The atan functions compute the principal value of the arc tangent of x.

Returns

3 The atan functions return arctan x in the interval [-pi /2, +pi /2] radians.

Contents

7.12.4.4 The atan2 functions

Synopsis

1

         #include <math.h>
         double atan2(double y, double x);
         float atan2f(float y, float x);
         long double atan2l(long double y, long double x);

Description

2 The atan2 functions compute the value of the arc tangent of y/x, using the signs of both arguments to determine the quadrant of the return value. A domain error may occur if both arguments are zero.

Returns

3 The atan2 functions return arctan y/x in the interval [-pi , +pi ] radians.

Contents

7.12.4.5 The cos functions

Synopsis

1

         #include <math.h>
         double cos(double x);
         float cosf(float x);
         long double cosl(long double x);

Description

2 The cos functions compute the cosine of x (measured in radians).

Returns

3 The cos functions return cos x.

Contents

7.12.4.6 The sin functions

Synopsis

1

         #include <math.h>
         double sin(double x);
         float sinf(float x);
         long double sinl(long double x);

Description

2 The sin functions compute the sine of x (measured in radians).

Returns

3 The sin functions return sin x.

Contents

7.12.4.7 The tan functions

Synopsis

1

        #include <math.h>
        double tan(double x);
        float tanf(float x);
        long double tanl(long double x);

Description

2 The tan functions return the tangent of x (measured in radians).

Returns

3 The tan functions return tan x.

Contents

7.12.5 Hyperbolic functions

Contents

7.12.5.1 The acosh functions

Synopsis

1

        #include <math.h>
        double acosh(double x);
        float acoshf(float x);
        long double acoshl(long double x);

Description

2 The acosh functions compute the (nonnegative) arc hyperbolic cosine of x. A domain error occurs for arguments less than 1.

Returns

3 The acosh functions return arcosh x in the interval [0, +(inf)].

Contents

7.12.5.2 The asinh functions

Synopsis

1

        #include <math.h>
        double asinh(double x);
        float asinhf(float x);
        long double asinhl(long double x);

Description

2 The asinh functions compute the arc hyperbolic sine of x.

Returns

3 The asinh functions return arsinh x.

Contents

7.12.5.3 The atanh functions

Synopsis

1

         #include <math.h>
         double atanh(double x);
         float atanhf(float x);
         long double atanhl(long double x);

Description

2 The atanh functions compute the arc hyperbolic tangent of x. A domain error occurs for arguments not in the interval [-1, +1]. A pole error may occur if the argument equals -1 or +1.

Returns

3 The atanh functions return artanh x.

Contents

7.12.5.4 The cosh functions

Synopsis

1

         #include <math.h>
         double cosh(double x);
         float coshf(float x);
         long double coshl(long double x);

Description

2 The cosh functions compute the hyperbolic cosine of x. A range error occurs if the magnitude of x is too large.

Returns

3 The cosh functions return cosh x.

Contents

7.12.5.5 The sinh functions

Synopsis

1

         #include <math.h>
         double sinh(double x);
         float sinhf(float x);
         long double sinhl(long double x);

Description

2 The sinh functions compute the hyperbolic sine of x. A range error occurs if the magnitude of x is too large.

Returns

3 The sinh functions return sinh x.

Contents

7.12.5.6 The tanh functions

Synopsis

1

        #include <math.h>
        double tanh(double x);
        float tanhf(float x);
        long double tanhl(long double x);

Description

2 The tanh functions compute the hyperbolic tangent of x.

Returns

3 The tanh functions return tanh x.

Contents

7.12.6 Exponential and logarithmic functions

Contents

7.12.6.1 The exp functions

Synopsis

1

        #include <math.h>
        double exp(double x);
        float expf(float x);
        long double expl(long double x);

Description

2 The exp functions compute the base-e exponential of x. A range error occurs if the magnitude of x is too large.

Returns

3 The exp functions return ex.

Contents

7.12.6.2 The exp2 functions

Synopsis

1

        #include <math.h>
        double exp2(double x);
        float exp2f(float x);
        long double exp2l(long double x);

Description

2 The exp2 functions compute the base-2 exponential of x. A range error occurs if the magnitude of x is too large.

Returns

3 The exp2 functions return 2x.

Contents

7.12.6.3 The expm1 functions

Synopsis

1

         #include <math.h>
         double expm1(double x);
         float expm1f(float x);
         long double expm1l(long double x);

Description

2 The expm1 functions compute the base-e exponential of the argument, minus 1. A range error occurs if x is too large.237)

Returns

3 The expm1 functions return ex - 1.

Footnotes

237) For small magnitude x, expm1(x) is expected to be more accurate than exp(x) - 1.

Contents

7.12.6.4 The frexp functions

Synopsis

1

         #include <math.h>
         double frexp(double value, int *exp);
         float frexpf(float value, int *exp);
         long double frexpl(long double value, int *exp);

Description

2 The frexp functions break a floating-point number into a normalized fraction and an integral power of 2. They store the integer in the int object pointed to by exp.

Returns

3 If value is not a floating-point number or if the integral power of 2 is outside the range of int, the results are unspecified. Otherwise, the frexp functions return the value x, such that x has a magnitude in the interval [1/2, 1) or zero, and value equals x 2*exp. If value is zero, both parts of the result are zero.

Contents

7.12.6.5 The ilogb functions

Synopsis

1

        #include <math.h>
        int ilogb(double x);
        int ilogbf(float x);
        int ilogbl(long double x);

Description

2 The ilogb functions extract the exponent of x as a signed int value. If x is zero they compute the value FP_ILOGB0; if x is infinite they compute the value INT_MAX; if x is a NaN they compute the value FP_ILOGBNAN; otherwise, they are equivalent to calling the corresponding logb function and casting the returned value to type int. A domain error or range error may occur if x is zero, infinite, or NaN. If the correct value is outside the range of the return type, the numeric result is unspecified.

Returns

3 The ilogb functions return the exponent of x as a signed int value.

Forward references: the logb functions (7.12.6.11).

Contents

7.12.6.6 The ldexp functions

Synopsis

1

        #include <math.h>
        double ldexp(double x, int exp);
        float ldexpf(float x, int exp);
        long double ldexpl(long double x, int exp);

Description

2 The ldexp functions multiply a floating-point number by an integral power of 2. A range error may occur.

Returns

3 The ldexp functions return x 2exp.

Contents

7.12.6.7 The log functions

Synopsis

1

        #include <math.h>
        double log(double x);
        float logf(float x);
        long double logl(long double x);

Description

2 The log functions compute the base-e (natural) logarithm of x. A domain error occurs if the argument is negative. A pole error may occur if the argument is zero.

Returns

3 The log functions return loge x.

Contents

7.12.6.8 The log10 functions

Synopsis

1

         #include <math.h>
         double log10(double x);
         float log10f(float x);
         long double log10l(long double x);

Description

2 The log10 functions compute the base-10 (common) logarithm of x. A domain error occurs if the argument is negative. A pole error may occur if the argument is zero.

Returns

3 The log10 functions return log10 x.

Contents

7.12.6.9 The log1p functions

Synopsis

1

         #include <math.h>
         double log1p(double x);
         float log1pf(float x);
         long double log1pl(long double x);

Description

2 The log1p functions compute the base-e (natural) logarithm of 1 plus the argument.238) A domain error occurs if the argument is less than -1. A pole error may occur if the argument equals -1.

Returns

3 The log1p functions return loge (1 + x).

Footnotes

238) For small magnitude x, log1p(x) is expected to be more accurate than log(1 + x).

Contents

7.12.6.10 The log2 functions

Synopsis

1

        #include <math.h>
        double log2(double x);
        float log2f(float x);
        long double log2l(long double x);

Description

2 The log2 functions compute the base-2 logarithm of x. A domain error occurs if the argument is less than zero. A pole error may occur if the argument is zero.

Returns

3 The log2 functions return log2 x.

Contents

7.12.6.11 The logb functions

Synopsis

1

        #include <math.h>
        double logb(double x);
        float logbf(float x);
        long double logbl(long double x);

Description

2 The logb functions extract the exponent of x, as a signed integer value in floating-point format. If x is subnormal it is treated as though it were normalized; thus, for positive finite x,

       1 <= x FLT_RADIX-logb(x) < FLT_RADIX
A domain error or pole error may occur if the argument is zero.

Returns

3 The logb functions return the signed exponent of x.

Contents

7.12.6.12 The modf functions

Synopsis

1

        #include <math.h>
        double modf(double value, double *iptr);
        float modff(float value, float *iptr);
        long double modfl(long double value, long double *iptr);

Description

2 The modf functions break the argument value into integral and fractional parts, each of which has the same type and sign as the argument. They store the integral part (in floating-point format) in the object pointed to by iptr.

Returns

3 The modf functions return the signed fractional part of value.

Contents

7.12.6.13 The scalbn and scalbln functions

Synopsis

1

         #include <math.h>
         double scalbn(double x, int n);
         float scalbnf(float x, int n);
         long double scalbnl(long double x, int n);
         double scalbln(double x, long int n);
         float scalblnf(float x, long int n);
         long double scalblnl(long double x, long int n);

Description

2 The scalbn and scalbln functions compute x FLT_RADIXn efficiently, not normally by computing FLT_RADIXn explicitly. A range error may occur.

Returns

3 The scalbn and scalbln functions return x FLT_RADIXn.

Contents

7.12.7 Power and absolute-value functions

Contents

7.12.7.1 The cbrt functions

Synopsis

1

         #include <math.h>
         double cbrt(double x);
         float cbrtf(float x);
         long double cbrtl(long double x);

Description

2 The cbrt functions compute the real cube root of x.

Returns

3 The cbrt functions return x1/3.

Contents

7.12.7.2 The fabs functions

Synopsis

1

        #include <math.h>
        double fabs(double x);
        float fabsf(float x);
        long double fabsl(long double x);

Description

2 The fabs functions compute the absolute value of a floating-point number x.

Returns

3 The fabs functions return | x |.

Contents

7.12.7.3 The hypot functions

Synopsis

1

        #include <math.h>
        double hypot(double x, double y);
        float hypotf(float x, float y);
        long double hypotl(long double x, long double y);

Description

2 The hypot functions compute the square root of the sum of the squares of x and y, without undue overflow or underflow. A range error may occur.

3

Returns

4 The hypot functions return (sqrt)(x2 + y2).

Contents

7.12.7.4 The pow functions

Synopsis

1

        #include <math.h>
        double pow(double x, double y);
        float powf(float x, float y);
        long double powl(long double x, long double y);

Description

2 The pow functions compute x raised to the power y. A domain error occurs if x is finite and negative and y is finite and not an integer value. A range error may occur. A domain error may occur if x is zero and y is zero. A domain error or pole error may occur if x is zero and y is less than zero.

Returns

3 The pow functions return xy.

Contents

7.12.7.5 The sqrt functions

Synopsis

1

         #include <math.h>
         double sqrt(double x);
         float sqrtf(float x);
         long double sqrtl(long double x);

Description

2 The sqrt functions compute the nonnegative square root of x. A domain error occurs if the argument is less than zero.

Returns

3 The sqrt functions return (sqrt)(x).

Contents

7.12.8 Error and gamma functions

Contents

7.12.8.1 The erf functions

Synopsis

1

         #include <math.h>
         double erf(double x);
         float erff(float x);
         long double erfl(long double x);

Description

2 The erf functions compute the error function of x.

Returns

3 The erf functions return

              2        x
 erf x =     ---    (integral)  e-t2 dt .
          (sqrt)(pi)   0 

Contents

7.12.8.2 The erfc functions

Synopsis

1

         #include <math.h>
         double erfc(double x);
         float erfcf(float x);
         long double erfcl(long double x);

Description

2 The erfc functions compute the complementary error function of x. A range error occurs if x is too large.

Returns

3 The erfc functions return

                           2       (inf)
 erfc x = 1 - erf x =     ---    (integral)  e-t2 dt .
                       (sqrt)(pi)    x 

Contents

7.12.8.3 The lgamma functions

Synopsis

1

        #include <math.h>
        double lgamma(double x);
        float lgammaf(float x);
        long double lgammal(long double x);

Description

2 The lgamma functions compute the natural logarithm of the absolute value of gamma of x. A range error occurs if x is too large. A pole error may occur if x is a negative integer or zero.

Returns

3 The lgamma functions return loge | (Gamma)(x) |.

Contents

7.12.8.4 The tgamma functions

Synopsis

1

        #include <math.h>
        double tgamma(double x);
        float tgammaf(float x);
        long double tgammal(long double x);

Description

2 The tgamma functions compute the gamma function of x. A domain error or pole error may occur if x is a negative integer or zero. A range error occurs if the magnitude of x is too large and may occur if the magnitude of x is too small.

Returns

3 The tgamma functions return (Gamma)(x).

Contents

7.12.9 Nearest integer functions

Contents

7.12.9.1 The ceil functions

Synopsis

1

         #include <math.h>
         double ceil(double x);
         float ceilf(float x);
         long double ceill(long double x);

Description

2 The ceil functions compute the smallest integer value not less than x.

Returns

3 The ceil functions return [^x^], expressed as a floating-point number.

Contents

7.12.9.2 The floor functions

Synopsis

1

         #include <math.h>
         double floor(double x);
         float floorf(float x);
         long double floorl(long double x);

Description

2 The floor functions compute the largest integer value not greater than x.

Returns

3 The floor functions return [_x_], expressed as a floating-point number.

Contents

7.12.9.3 The nearbyint functions

Synopsis

1

         #include <math.h>
         double nearbyint(double x);
         float nearbyintf(float x);
         long double nearbyintl(long double x);

Description

2 The nearbyint functions round their argument to an integer value in floating-point format, using the current rounding direction and without raising the ''inexact'' floating- point exception.

Returns

3 The nearbyint functions return the rounded integer value.

Contents

7.12.9.4 The rint functions

Synopsis

1

        #include <math.h>
        double rint(double x);
        float rintf(float x);
        long double rintl(long double x);

Description

2 The rint functions differ from the nearbyint functions (7.12.9.3) only in that the rint functions may raise the ''inexact'' floating-point exception if the result differs in value from the argument.

Returns

3 The rint functions return the rounded integer value.

Contents

7.12.9.5 The lrint and llrint functions

Synopsis

1

        #include <math.h>
        long int lrint(double x);
        long int lrintf(float x);
        long int lrintl(long double x);
        long long int llrint(double x);
        long long int llrintf(float x);
        long long int llrintl(long double x);

Description

2 The lrint and llrint functions round their argument to the nearest integer value, rounding according to the current rounding direction. If the rounded value is outside the range of the return type, the numeric result is unspecified and a domain error or range error may occur.

Returns

3 The lrint and llrint functions return the rounded integer value.

Contents

7.12.9.6 The round functions

Synopsis

1

         #include <math.h>
         double round(double x);
         float roundf(float x);
         long double roundl(long double x);

Description

2 The round functions round their argument to the nearest integer value in floating-point format, rounding halfway cases away from zero, regardless of the current rounding direction.

Returns

3 The round functions return the rounded integer value.

Contents

7.12.9.7 The lround and llround functions

Synopsis

1

         #include <math.h>
         long int lround(double x);
         long int lroundf(float x);
         long int lroundl(long double x);
         long long int llround(double x);
         long long int llroundf(float x);
         long long int llroundl(long double x);

Description

2 The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. If the rounded value is outside the range of the return type, the numeric result is unspecified and a domain error or range error may occur.

Returns

3 The lround and llround functions return the rounded integer value.

Contents

7.12.9.8 The trunc functions

Synopsis

1

         #include <math.h>
         double trunc(double x);
         float truncf(float x);
         long double truncl(long double x);

Description

2 The trunc functions round their argument to the integer value, in floating format, nearest to but no larger in magnitude than the argument.

Returns

3 The trunc functions return the truncated integer value.

Contents

7.12.10 Remainder functions

Contents

7.12.10.1 The fmod functions

Synopsis

1

          #include <math.h>
          double fmod(double x, double y);
          float fmodf(float x, float y);
          long double fmodl(long double x, long double y);

Description

2 The fmod functions compute the floating-point remainder of x/y.

Returns

3 The fmod functions return the value x - ny, for some integer n such that, if y is nonzero, the result has the same sign as x and magnitude less than the magnitude of y. If y is zero, whether a domain error occurs or the fmod functions return zero is implementation- defined.

Contents

7.12.10.2 The remainder functions

Synopsis

1

          #include <math.h>
          double remainder(double x, double y);
          float remainderf(float x, float y);
          long double remainderl(long double x, long double y);

Description

2 The remainder functions compute the remainder x REM y required by IEC 60559.239)

Returns

3 The remainder functions return x REM y. If y is zero, whether a domain error occurs or the functions return zero is implementation defined.

Footnotes

239) ''When y != 0, the remainder r = x REM y is defined regardless of the rounding mode by the mathematical relation r = x - ny, where n is the integer nearest the exact value of x/y; whenever | n - x/y | = 1/2, then n is even. If r = 0, its sign shall be that of x.'' This definition is applicable for all implementations.

Contents

7.12.10.3 The remquo functions

Synopsis

1

         #include <math.h>
         double remquo(double x, double y, int *quo);
         float remquof(float x, float y, int *quo);
         long double remquol(long double x, long double y,
              int *quo);

Description

2 The remquo functions compute the same remainder as the remainder functions. In the object pointed to by quo they store a value whose sign is the sign of x/y and whose magnitude is congruent modulo 2n to the magnitude of the integral quotient of x/y, where n is an implementation-defined integer greater than or equal to 3.

Returns

3 The remquo functions return x REM y. If y is zero, the value stored in the object pointed to by quo is unspecified and whether a domain error occurs or the functions return zero is implementation defined.

Contents

7.12.11 Manipulation functions

Contents

7.12.11.1 The copysign functions

Synopsis

1

         #include <math.h>
         double copysign(double x, double y);
         float copysignf(float x, float y);
         long double copysignl(long double x, long double y);

Description

2 The copysign functions produce a value with the magnitude of x and the sign of y. They produce a NaN (with the sign of y) if x is a NaN. On implementations that represent a signed zero but do not treat negative zero consistently in arithmetic operations, the copysign functions regard the sign of zero as positive.

Returns

3 The copysign functions return a value with the magnitude of x and the sign of y.

Contents

7.12.11.2 The nan functions

Synopsis

1

         #include <math.h>
         double nan(const char *tagp);
         float nanf(const char *tagp);
         long double nanl(const char *tagp);

Description

2 The call nan("n-char-sequence") is equivalent to strtod("NAN(n-char- sequence)", (char**) NULL); the call nan("") is equivalent to strtod("NAN()", (char**) NULL). If tagp does not point to an n-char sequence or an empty string, the call is equivalent to strtod("NAN", (char**) NULL). Calls to nanf and nanl are equivalent to the corresponding calls to strtof and strtold.

Returns

3 The nan functions return a quiet NaN, if available, with content indicated through tagp. If the implementation does not support quiet NaNs, the functions return zero.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3).

Contents

7.12.11.3 The nextafter functions

Synopsis

1

         #include <math.h>
         double nextafter(double x, double y);
         float nextafterf(float x, float y);
         long double nextafterl(long double x, long double y);

Description

2 The nextafter functions determine the next representable value, in the type of the function, after x in the direction of y, where x and y are first converted to the type of the function.240) The nextafter functions return y if x equals y. A range error may occur if the magnitude of x is the largest finite value representable in the type and the result is infinite or not representable in the type.

Returns

3 The nextafter functions return the next representable value in the specified format after x in the direction of y.

Footnotes

240) The argument values are converted to the type of the function, even by a macro implementation of the function.

Contents

7.12.11.4 The nexttoward functions

Synopsis

1

         #include <math.h>
         double nexttoward(double x, long double y);
         float nexttowardf(float x, long double y);
         long double nexttowardl(long double x, long double y);

Description

2 The nexttoward functions are equivalent to the nextafter functions except that the second parameter has type long double and the functions return y converted to the type of the function if x equals y.241)

Footnotes

241) The result of the nexttoward functions is determined in the type of the function, without loss of range or precision in a floating second argument.

Contents

7.12.12 Maximum, minimum, and positive difference functions

Contents

7.12.12.1 The fdim functions

Synopsis

1

         #include <math.h>
         double fdim(double x, double y);
         float fdimf(float x, float y);
         long double fdiml(long double x, long double y);

Description

2 The fdim functions determine the positive difference between their arguments:

       {x - y  if x > y
       {
       {+0     if x <= y
A range error may occur.

Returns

3 The fdim functions return the positive difference value.

Contents

7.12.12.2 The fmax functions

Synopsis

1

         #include <math.h>
         double fmax(double x, double y);
         float fmaxf(float x, float y);
         long double fmaxl(long double x, long double y);

Description

2 The fmax functions determine the maximum numeric value of their arguments.242)

Returns

3 The fmax functions return the maximum numeric value of their arguments.

Footnotes

242) NaN arguments are treated as missing data: if one argument is a NaN and the other numeric, then the fmax functions choose the numeric value. See F.10.9.2.

Contents

7.12.12.3 The fmin functions

Synopsis

1

         #include <math.h>
         double fmin(double x, double y);
         float fminf(float x, float y);
         long double fminl(long double x, long double y);

Description

2 The fmin functions determine the minimum numeric value of their arguments.243)

Returns

3 The fmin functions return the minimum numeric value of their arguments.

Footnotes

243) The fmin functions are analogous to the fmax functions in their treatment of NaNs.

Contents

7.12.13 Floating multiply-add

Contents

7.12.13.1 The fma functions

Synopsis

1

         #include <math.h>
         double fma(double x, double y, double z);
         float fmaf(float x, float y, float z);
         long double fmal(long double x, long double y,
              long double z);

Description

2 The fma functions compute (x y) + z, rounded as one ternary operation: they compute the value (as if) to infinite precision and round once to the result format, according to the current rounding mode. A range error may occur.

Returns

3 The fma functions return (x y) + z, rounded as one ternary operation.

Contents

7.12.14 Comparison macros

1 The relational and equality operators support the usual mathematical relationships between numeric values. For any ordered pair of numeric values exactly one of the relationships -- less, greater, and equal -- is true. Relational operators may raise the ''invalid'' floating-point exception when argument values are NaNs. For a NaN and a numeric value, or for two NaNs, just the unordered relationship is true.244) The following subclauses provide macros that are quiet (non floating-point exception raising) versions of the relational operators, and other comparison macros that facilitate writing efficient code that accounts for NaNs without suffering the ''invalid'' floating-point exception. In the synopses in this subclause, real-floating indicates that the argument shall be an expression of real floating type245) (both arguments need not have the same type).246)

Footnotes

244) IEC 60559 requires that the built-in relational operators raise the ''invalid'' floating-point exception if the operands compare unordered, as an error indicator for programs written without consideration of NaNs; the result in these cases is false.

245) If any argument is of integer type, or any other type that is not a real floating type, the behavior is undefined.

246) Whether an argument represented in a format wider than its semantic type is converted to the semantic type is unspecified.

Contents

7.12.14.1 The isgreater macro

Synopsis

1

          #include <math.h>
          int isgreater(real-floating x, real-floating y);

Description

2 The isgreater macro determines whether its first argument is greater than its second argument. The value of isgreater(x, y) is always equal to (x) > (y); however, unlike (x) > (y), isgreater(x, y) does not raise the ''invalid'' floating-point exception when x and y are unordered.

Returns

3 The isgreater macro returns the value of (x) > (y).

Contents

7.12.14.2 The isgreaterequal macro

Synopsis

1

          #include <math.h>
          int isgreaterequal(real-floating x, real-floating y);

Description

2 The isgreaterequal macro determines whether its first argument is greater than or equal to its second argument. The value of isgreaterequal(x, y) is always equal to (x) >= (y); however, unlike (x) >= (y), isgreaterequal(x, y) does not raise the ''invalid'' floating-point exception when x and y are unordered.

Returns

3 The isgreaterequal macro returns the value of (x) >= (y).

Contents

7.12.14.3 The isless macro

Synopsis

1

       #include <math.h>
       int isless(real-floating x, real-floating y);

Description

2 The isless macro determines whether its first argument is less than its second argument. The value of isless(x, y) is always equal to (x) < (y); however, unlike (x) < (y), isless(x, y) does not raise the ''invalid'' floating-point exception when x and y are unordered.

Returns

3 The isless macro returns the value of (x) < (y).

Contents

7.12.14.4 The islessequal macro

Synopsis

1

       #include <math.h>
       int islessequal(real-floating x, real-floating y);

Description

2 The islessequal macro determines whether its first argument is less than or equal to its second argument. The value of islessequal(x, y) is always equal to (x) <= (y); however, unlike (x) <= (y), islessequal(x, y) does not raise the ''invalid'' floating-point exception when x and y are unordered.

Returns

3 The islessequal macro returns the value of (x) <= (y).

Contents

7.12.14.5 The islessgreater macro

Synopsis

1

         #include <math.h>
         int islessgreater(real-floating x, real-floating y);

Description

2 The islessgreater macro determines whether its first argument is less than or greater than its second argument. The islessgreater(x, y) macro is similar to (x) < (y) || (x) > (y); however, islessgreater(x, y) does not raise the ''invalid'' floating-point exception when x and y are unordered (nor does it evaluate x and y twice).

Returns

3 The islessgreater macro returns the value of (x) < (y) || (x) > (y).

Contents

7.12.14.6 The isunordered macro

Synopsis

1

         #include <math.h>
         int isunordered(real-floating x, real-floating y);

Description

2 The isunordered macro determines whether its arguments are unordered.

Returns

3 The isunordered macro returns 1 if its arguments are unordered and 0 otherwise.

Contents

7.13 Nonlocal jumps <setjmp.h>

1 The header <setjmp.h> defines the macro setjmp, and declares one function and one type, for bypassing the normal function call and return discipline.247)

2 The type declared is

         jmp_buf
which is an array type suitable for holding the information needed to restore a calling environment. The environment of a call to the setjmp macro consists of information sufficient for a call to the longjmp function to return execution to the correct block and invocation of that block, were it called recursively. It does not include the state of the floating-point status flags, of open files, or of any other component of the abstract machine.

3 It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name setjmp, the behavior is undefined.

Footnotes

247) These functions are useful for dealing with unusual conditions encountered in a low-level function of a program.

Contents

7.13.1 Save calling environment

Contents

7.13.1.1 The setjmp macro

Synopsis

1

         #include <setjmp.h>
         int setjmp(jmp_buf env);

Description

2 The setjmp macro saves its calling environment in its jmp_buf argument for later use by the longjmp function.

Returns

3 If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is from a call to the longjmp function, the setjmp macro returns a nonzero value.

Environmental limits

4 An invocation of the setjmp macro shall appear only in one of the following contexts:

5 If the invocation appears in any other context, the behavior is undefined.

Contents

7.13.2 Restore calling environment

Contents

7.13.2.1 The longjmp function

Synopsis

1

          #include <setjmp.h>
          _Noreturn void longjmp(jmp_buf env, int val);

Description

2 The longjmp function restores the environment saved by the most recent invocation of the setjmp macro in the same invocation of the program with the corresponding jmp_buf argument. If there has been no such invocation, or if the invocation was from another thread of execution, or if the function containing the invocation of the setjmp macro has terminated execution248) in the interim, or if the invocation of the setjmp macro was within the scope of an identifier with variably modified type and execution has left that scope in the interim, the behavior is undefined.

3 All accessible objects have values, and all other components of the abstract machine249) have state, as of the time the longjmp function was called, except that the values of objects of automatic storage duration that are local to the function containing the invocation of the corresponding setjmp macro that do not have volatile-qualified type and have been changed between the setjmp invocation and longjmp call are indeterminate.

Returns

4 After longjmp is completed, thread execution continues as if the corresponding invocation of the setjmp macro had just returned the value specified by val. The longjmp function cannot cause the setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1.

5 EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation might cause memory associated with a variable length array object to be squandered.

       #include <setjmp.h>
       jmp_buf buf;
       void g(int n);
       void h(int n);
       int n = 6;
       void f(void)
       {
             int x[n];             // valid: f is not terminated
             setjmp(buf);
             g(n);
       }
       void g(int n)
       {
             int a[n];             // a may remain allocated
             h(n);
       }
       void h(int n)
       {
             int b[n];             // b may remain allocated
             longjmp(buf, 2);      // might cause memory loss
       }

Footnotes

248) For example, by executing a return statement or because another longjmp call has caused a transfer to a setjmp invocation in a function earlier in the set of nested calls.

249) This includes, but is not limited to, the floating-point status flags and the state of open files.

Contents

7.14 Signal handling <signal.h>

1 The header <signal.h> declares a type and two functions and defines several macros, for handling various signals (conditions that may be reported during program execution).

2 The type defined is

          sig_atomic_t
which is the (possibly volatile-qualified) integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts.

3 The macros defined are

          SIG_DFL
          SIG_ERR
          SIG_IGN
which expand to constant expressions with distinct values that have type compatible with the second argument to, and the return value of, the signal function, and whose values compare unequal to the address of any declarable function; and the following, which expand to positive integer constant expressions with type int and distinct values that are the signal numbers, each corresponding to the specified condition:
          SIGABRT  abnormal termination, such as is initiated by the abort function
          SIGFPE   an erroneous arithmetic operation, such as zero divide or an operation
                   resulting in overflow
          SIGILL   detection of an invalid function image, such as an invalid instruction
          SIGINT   receipt of an interactive attention signal
          SIGSEGV  an invalid access to storage
          SIGTERM  a termination request sent to the program

4 An implementation need not generate any of these signals, except as a result of explicit calls to the raise function. Additional signals and pointers to undeclarable functions, with macro definitions beginning, respectively, with the letters SIG and an uppercase letter or with SIG_ and an uppercase letter,250) may also be specified by the implementation. The complete set of signals, their semantics, and their default handling is implementation-defined; all signal numbers shall be positive.

Footnotes

250) See ''future library directions'' (7.31.7). The names of the signal numbers reflect the following terms (respectively): abort, floating-point exception, illegal instruction, interrupt, segmentation violation, and termination.

Contents

7.14.1 Specify signal handling

Contents

7.14.1.1 The signal function

Synopsis

1

         #include <signal.h>
         void (*signal(int sig, void (*func)(int)))(int);

Description

2 The signal function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled. If the value of func is SIG_DFL, default handling for that signal will occur. If the value of func is SIG_IGN, the signal will be ignored. Otherwise, func shall point to a function to be called when that signal occurs. An invocation of such a function because of a signal, or (recursively) of any further functions called by that invocation (other than functions in the standard library),251) is called a signal handler.

3 When a signal occurs and func points to a function, it is implementation-defined whether the equivalent of signal(sig, SIG_DFL); is executed or the implementation prevents some implementation-defined set of signals (at least including sig) from occurring until the current signal handling has completed; in the case of SIGILL, the implementation may alternatively define that no action is taken. Then the equivalent of (*func)(sig); is executed. If and when the function returns, if the value of sig is SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a computational exception, the behavior is undefined; otherwise the program will resume execution at the point it was interrupted.

4 If the signal occurs as the result of calling the abort or raise function, the signal handler shall not call the raise function.

5 If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.252)

6 At program startup, the equivalent of

         signal(sig, SIG_IGN);
may be executed for some signals selected in an implementation-defined manner; the equivalent of
         signal(sig, SIG_DFL);
is executed for all other signals defined by the implementation.

7 Use of this function in a multi-threaded program results in undefined behavior. The implementation shall behave as if no library function calls the signal function.

Returns

8 If the request can be honored, the signal function returns the value of func for the most recent successful call to signal for the specified signal sig. Otherwise, a value of SIG_ERR is returned and a positive value is stored in errno.

Forward references: the abort function (7.22.4.1), the exit function (7.22.4.4), the _Exit function (7.22.4.5), the quick_exit function (7.22.4.7).

Footnotes

251) This includes functions called indirectly via standard library functions (e.g., a SIGABRT handler called via the abort function).

252) If any signal is generated by an asynchronous signal handler, the behavior is undefined.

Contents

7.14.2 Send signal

Contents

7.14.2.1 The raise function

Synopsis

1

         #include <signal.h>
         int raise(int sig);

Description

2 The raise function carries out the actions described in 7.14.1.1 for the signal sig. If a signal handler is called, the raise function shall not return until after the signal handler does.

Returns

3 The raise function returns zero if successful, nonzero if unsuccessful.

Contents

7.15 Alignment <stdalign.h>

1 The header <stdalign.h> defines four macros.

2 The macro

        alignas
expands to _Alignas; the macro
        alignof
expands to _Alignof.

3 The remaining macros are suitable for use in #if preprocessing directives. They are

        __alignas_is_defined
and
        __alignof_is_defined
which both expand to the integer constant 1.

Contents

7.16 Variable arguments <stdarg.h>

1 The header <stdarg.h> declares a type and defines four macros, for advancing through a list of arguments whose number and types are not known to the called function when it is translated.

2 A function may be called with a variable number of arguments of varying types. As described in 6.9.1, its parameter list contains one or more parameters. The rightmost parameter plays a special role in the access mechanism, and will be designated parmN in this description.

3 The type declared is

         va_list
which is a complete object type suitable for holding information needed by the macros va_start, va_arg, va_end, and va_copy. If access to the varying arguments is desired, the called function shall declare an object (generally referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap.253)

Footnotes

253) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original function may make further use of the original list after the other function returns.

Contents

7.16.1 Variable argument list access macros

1 The va_start and va_arg macros described in this subclause shall be implemented as macros, not functions. It is unspecified whether va_copy and va_end are macros or identifiers declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the same name, the behavior is undefined. Each invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of the va_end macro in the same function.

Contents

7.16.1.1 The va_arg macro

Synopsis

1

         #include <stdarg.h>
         type va_arg(va_list ap, type);

Description

2 The va_arg macro expands to an expression that has the specified type and the value of the next argument in the call. The parameter ap shall have been initialized by the va_start or va_copy macro (without an intervening invocation of the va_end macro for the same ap). Each invocation of the va_arg macro modifies ap so that the values of successive arguments are returned in turn. The parameter type shall be a type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by postfixing a * to type. If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the behavior is undefined, except for the following cases:

Returns

3 The first invocation of the va_arg macro after that of the va_start macro returns the value of the argument after that specified by parmN . Successive invocations return the values of the remaining arguments in succession.

Contents

7.16.1.2 The va_copy macro

Synopsis

1

        #include <stdarg.h>
        void va_copy(va_list dest, va_list src);

Description

2 The va_copy macro initializes dest as a copy of src, as if the va_start macro had been applied to dest followed by the same sequence of uses of the va_arg macro as had previously been used to reach the present state of src. Neither the va_copy nor va_start macro shall be invoked to reinitialize dest without an intervening invocation of the va_end macro for the same dest.

Returns

3 The va_copy macro returns no value.

Contents

7.16.1.3 The va_end macro

Synopsis

1

        #include <stdarg.h>
        void va_end(va_list ap);

Description

2 The va_end macro facilitates a normal return from the function whose variable argument list was referred to by the expansion of the va_start macro, or the function containing the expansion of the va_copy macro, that initialized the va_list ap. The va_end macro may modify ap so that it is no longer usable (without being reinitialized by the va_start or va_copy macro). If there is no corresponding invocation of the va_start or va_copy macro, or if the va_end macro is not invoked before the return, the behavior is undefined.

Returns

3 The va_end macro returns no value.

Contents

7.16.1.4 The va_start macro

Synopsis

1

         #include <stdarg.h>
         void va_start(va_list ap, parmN);

Description

2 The va_start macro shall be invoked before any access to the unnamed arguments.

3 The va_start macro initializes ap for subsequent use by the va_arg and va_end macros. Neither the va_start nor va_copy macro shall be invoked to reinitialize ap without an intervening invocation of the va_end macro for the same ap.

4 The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ...). If the parameter parmN is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined.

Returns

5 The va_start macro returns no value.

6 EXAMPLE 1 The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS arguments), then passes the array as a single argument to function f2. The number of pointers is specified by the first argument to f1.

         #include <stdarg.h>
         #define MAXARGS   31
         void f1(int n_ptrs, ...)
         {
               va_list ap;
               char *array[MAXARGS];
               int ptr_no = 0;
                   if (n_ptrs > MAXARGS)
                         n_ptrs = MAXARGS;
                   va_start(ap, n_ptrs);
                   while (ptr_no < n_ptrs)
                         array[ptr_no++] = va_arg(ap, char *);
                   va_end(ap);
                   f2(n_ptrs, array);
          }
Each call to f1 is required to have visible the definition of the function or a declaration such as
          void f1(int, ...);

7 EXAMPLE 2 The function f3 is similar, but saves the status of the variable argument list after the indicated number of arguments; after f2 has been called once with the whole list, the trailing part of the list is gathered again and passed to function f4.

          #include <stdarg.h>
          #define MAXARGS 31
          void f3(int n_ptrs, int f4_after, ...)
          {
                va_list ap, ap_save;
                char *array[MAXARGS];
                int ptr_no = 0;
                if (n_ptrs > MAXARGS)
                      n_ptrs = MAXARGS;
                va_start(ap, f4_after);
                while (ptr_no < n_ptrs) {
                      array[ptr_no++] = va_arg(ap, char *);
                      if (ptr_no == f4_after)
                            va_copy(ap_save, ap);
                }
                va_end(ap);
                f2(n_ptrs, array);
                   // Now process the saved copy.
                   n_ptrs -= f4_after;
                   ptr_no = 0;
                   while (ptr_no < n_ptrs)
                         array[ptr_no++] = va_arg(ap_save, char *);
                   va_end(ap_save);
                   f4(n_ptrs, array);
          }

Contents

7.17 Atomics <stdatomic.h>

Contents

7.17.1 Introduction

1 The header <stdatomic.h> defines several macros and declares several types and functions for performing atomic operations on data shared between threads.254)

2 Implementations that define the macro __STDC_NO_ATOMICS__ need not provide this header nor support any of its facilities.

3 The macros defined are the atomic lock-free macros

          ATOMIC_BOOL_LOCK_FREE
          ATOMIC_CHAR_LOCK_FREE
          ATOMIC_CHAR16_T_LOCK_FREE
          ATOMIC_CHAR32_T_LOCK_FREE
          ATOMIC_WCHAR_T_LOCK_FREE
          ATOMIC_SHORT_LOCK_FREE
          ATOMIC_INT_LOCK_FREE
          ATOMIC_LONG_LOCK_FREE
          ATOMIC_LLONG_LOCK_FREE
          ATOMIC_POINTER_LOCK_FREE
which indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and
          ATOMIC_FLAG_INIT
which expands to an initializer for an object of type atomic_flag.

4 The types include

           memory_order
which is an enumerated type whose enumerators identify memory ordering constraints;
           atomic_flag
which is a structure type representing a lock-free, primitive atomic flag; and several atomic analogs of integer types.

5 In the following synopses:

6 NOTE Many operations are volatile-qualified. The ''volatile as device register'' semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects.

Footnotes

254) See ''future library directions'' (7.31.8).

Contents

7.17.2 Initialization

Contents

7.17.2.1 The ATOMIC_VAR_INIT macro

Synopsis

1

         #include <stdatomic.h>
         #define ATOMIC_VAR_INIT(C value)

Description

2 The ATOMIC_VAR_INIT macro expands to a token sequence suitable for initializing an atomic object of a type that is initialization-compatible with value. An atomic object with automatic storage duration that is not explicitly initialized using ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state.

3 Concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race.

4 EXAMPLE

         atomic_int guide = ATOMIC_VAR_INIT(42);

Contents

7.17.2.2 The atomic_init generic function

Synopsis

1

         #include <stdatomic.h>
         void atomic_init(volatile A *obj, C value);

Description

2 The atomic_init generic function initializes the atomic object pointed to by obj to the value value, while also initializing any additional state that the implementation might need to carry for the atomic object.

3 Although this function initializes an atomic object, it does not avoid data races; concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race.

Returns

4 The atomic_init generic function returns no value.

5 EXAMPLE

           atomic_int guide;
           atomic_init(&guide, 42);

Contents

7.17.3 Order and consistency

1 The enumerated type memory_order specifies the detailed regular (non-atomic) memory synchronization operations as defined in 5.1.2.4 and may provide for operation ordering. Its enumeration constants are as follows:255)

          memory_order_relaxed
          memory_order_consume
          memory_order_acquire
          memory_order_release
          memory_order_acq_rel
          memory_order_seq_cst

2 For memory_order_relaxed, no operation orders memory.

3 For memory_order_release, memory_order_acq_rel, and memory_order_seq_cst, a store operation performs a release operation on the affected memory location.

4 For memory_order_acquire, memory_order_acq_rel, and memory_order_seq_cst, a load operation performs an acquire operation on the affected memory location.

5 For memory_order_consume, a load operation performs a consume operation on the affected memory location.

6 There shall be a single total order S on all memory_order_seq_cst operations, consistent with the ''happens before'' order and modification orders for all affected locations, such that each memory_order_seq_cst operation B that loads a value from an atomic object M observes one of the following values:

7 NOTE 1 Although it is not explicitly required that S include lock operations, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the ''happens before'' ordering.

8 NOTE 2 Atomic operations specifying memory_order_relaxed are relaxed only with respect to memory ordering. Implementations must still guarantee that any given atomic access to a particular atomic object be indivisible with respect to all other atomic accesses to that object.

9 For an atomic operation B that reads the value of an atomic object M, if there is a memory_order_seq_cst fence X sequenced before B, then B observes either the last memory_order_seq_cst modification of M preceding X in the total order S or a later modification of M in its modification order.

10 For atomic operations A and B on an atomic object M, where A modifies M and B takes its value, if there is a memory_order_seq_cst fence X such that A is sequenced before X and B follows X in S, then B observes either the effects of A or a later modification of M in its modification order.

11 For atomic operations A and B on an atomic object M, where A modifies M and B takes its value, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B observes either the effects of A or a later modification of M in its modification order.

12 Atomic read-modify-write operations shall always read the last value (in the modification order) stored before the write associated with the read-modify-write operation.

13 An atomic store shall only store a value that has been computed from constants and program input values by a finite sequence of program evaluations, such that each evaluation observes the values of variables as computed by the last prior assignment in the sequence.256) The ordering of evaluations in this sequence shall be such that

14 NOTE 3 The second requirement disallows ''out-of-thin-air'', or ''speculative'' stores of atomics when relaxed atomics are used. Since unordered operations are involved, evaluations may appear in this sequence out of thread order. For example, with x and y initially zero,

           // Thread 1:
           r1 = atomic_load_explicit(&y, memory_order_relaxed);
           atomic_store_explicit(&x, r1, memory_order_relaxed);

           // Thread 2:
           r2 = atomic_load_explicit(&x, memory_order_relaxed);
           atomic_store_explicit(&y, 42, memory_order_relaxed);
is allowed to produce r1 == 42 && r2 == 42. The sequence of evaluations justifying this consists of:
           atomic_store_explicit(&y, 42, memory_order_relaxed);
           r1 = atomic_load_explicit(&y, memory_order_relaxed);
           atomic_store_explicit(&x, r1, memory_order_relaxed);
           r2 = atomic_load_explicit(&x, memory_order_relaxed);
On the other hand,
           // Thread 1:
           r1 = atomic_load_explicit(&y, memory_order_relaxed);
           atomic_store_explicit(&x, r1, memory_order_relaxed);

           // Thread 2:
           r2 = atomic_load_explicit(&x, memory_order_relaxed);
           atomic_store_explicit(&y, r2, memory_order_relaxed);
is not allowed to produce r1 == 42 && r2 = 42, since there is no sequence of evaluations that results in the computation of 42. In the absence of ''relaxed'' operations and read-modify-write operations with weaker than memory_order_acq_rel ordering, the second requirement has no impact.

Recommended practice

15 The requirements do not forbid r1 == 42 && r2 == 42 in the following example, with x and y initially zero:

         // Thread 1:
         r1 = atomic_load_explicit(&x, memory_order_relaxed);
         if (r1 == 42)
              atomic_store_explicit(&y, r1, memory_order_relaxed);

         // Thread 2:
         r2 = atomic_load_explicit(&y, memory_order_relaxed);
         if (r2 == 42)
              atomic_store_explicit(&x, 42, memory_order_relaxed);
However, this is not useful behavior, and implementations should not allow it.

16 Implementations should make atomic stores visible to atomic loads within a reasonable amount of time.

Footnotes

255) See ''future library directions'' (7.31.8).

256) Among other implications, atomic variables shall not decay.

Contents

7.17.3.1 The kill_dependency macro

Synopsis

1

        #include <stdatomic.h>
        type kill_dependency(type y);

Description

2 The kill_dependency macro terminates a dependency chain; the argument does not carry a dependency to the return value.

Returns

3 The kill_dependency macro returns the value of y.

Contents

7.17.4 Fences

1 This subclause introduces synchronization primitives called fences. Fences can have acquire semantics, release semantics, or both. A fence with acquire semantics is called an acquire fence; a fence with release semantics is called a release fence.

2 A release fence A synchronizes with an acquire fence B if there exist atomic operations X and Y , both operating on some atomic object M, such that A is sequenced before X, X modifies M, Y is sequenced before B, and Y reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

3 A release fence A synchronizes with an atomic operation B that performs an acquire operation on an atomic object M if there exists an atomic operation X such that A is sequenced before X, X modifies M, and B reads the value written by X or a value written by any side effect in the hypothetical release sequence X would head if it were a release operation.

4 An atomic operation A that is a release operation on an atomic object M synchronizes with an acquire fence B if there exists some atomic operation X on M such that X is sequenced before B and reads the value written by A or a value written by any side effect in the release sequence headed by A.

Contents

7.17.4.1 The atomic_thread_fence function

Synopsis

1

        #include <stdatomic.h>
        void atomic_thread_fence(memory_order order);

Description

2 Depending on the value of order, this operation:

Returns

3 The atomic_thread_fence function returns no value.

Contents

7.17.4.2 The atomic_signal_fence function

Synopsis

1

         #include <stdatomic.h>
         void atomic_signal_fence(memory_order order);

Description

2 Equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.

3 NOTE 1 The atomic_signal_fence function can be used to specify the order in which actions performed by the thread become visible to the signal handler.

4 NOTE 2 Compiler optimizations and reorderings of loads and stores are inhibited in the same way as with atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not emitted.

Returns

5 The atomic_signal_fence function returns no value.

Contents

7.17.5 Lock-free property

1 The atomic lock-free macros indicate the lock-free property of integer and address atomic types. A value of 0 indicates that the type is never lock-free; a value of 1 indicates that the type is sometimes lock-free; a value of 2 indicates that the type is always lock-free.

2 NOTE Operations that are lock-free should also be address-free. That is, atomic operations on the same memory location via two different addresses will communicate atomically. The implementation should not depend on any per-process state. This restriction enables communication via memory mapped into a process more than once and memory shared between two processes.

Contents

7.17.5.1 The atomic_is_lock_free generic function

Synopsis

1

          #include <stdatomic.h>
          _Bool atomic_is_lock_free(const volatile A *obj);

Description

2 The atomic_is_lock_free generic function indicates whether or not the object pointed to by obj is lock-free.

Returns

3 The atomic_is_lock_free generic function returns nonzero (true) if and only if the object's operations are lock-free. The result of a lock-free query on one object cannot be inferred from the result of a lock-free query on another object.

Contents

7.17.6 Atomic integer types

1 For each line in the following table,257) the atomic type name is declared as a type that has the same representation and alignment requirements as the corresponding direct type.258)

             Atomic type name                      Direct type

           atomic_bool                        _Atomic _Bool
           atomic_char                        _Atomic char
           atomic_schar                       _Atomic signed char
           atomic_uchar                       _Atomic unsigned char
           atomic_short                       _Atomic short
           atomic_ushort                      _Atomic unsigned short
           atomic_int                         _Atomic int
           atomic_uint                        _Atomic unsigned int
           atomic_long                        _Atomic long
           atomic_ulong                       _Atomic unsigned long
           atomic_llong                       _Atomic long long
           atomic_ullong                      _Atomic unsigned long long
           atomic_char16_t                    _Atomic char16_t
           atomic_char32_t                    _Atomic char32_t
           atomic_wchar_t                     _Atomic wchar_t
           atomic_int_least8_t                _Atomic int_least8_t
           atomic_uint_least8_t               _Atomic uint_least8_t
           atomic_int_least16_t               _Atomic int_least16_t
           atomic_uint_least16_t              _Atomic uint_least16_t
           atomic_int_least32_t               _Atomic int_least32_t
           atomic_uint_least32_t              _Atomic uint_least32_t
           atomic_int_least64_t               _Atomic int_least64_t
           atomic_uint_least64_t              _Atomic uint_least64_t
           atomic_int_fast8_t                 _Atomic int_fast8_t
           atomic_uint_fast8_t                _Atomic uint_fast8_t
           atomic_int_fast16_t                _Atomic int_fast16_t
           atomic_uint_fast16_t               _Atomic uint_fast16_t
           atomic_int_fast32_t                _Atomic int_fast32_t
           atomic_uint_fast32_t               _Atomic uint_fast32_t
           atomic_int_fast64_t                _Atomic int_fast64_t
           atomic_uint_fast64_t               _Atomic uint_fast64_t
           atomic_intptr_t                    _Atomic intptr_t
           atomic_uintptr_t                   _Atomic uintptr_t
           atomic_size_t                      _Atomic size_t
           atomic_ptrdiff_t                   _Atomic ptrdiff_t
           atomic_intmax_t                    _Atomic intmax_t
           atomic_uintmax_t                   _Atomic uintmax_t

2 The semantics of the operations on these types are defined in 7.17.7.

3 NOTE The representation of atomic integer types need not have the same size as their corresponding regular types. They should have the same size whenever possible, as it eases effort required to port existing code.

Footnotes

257) See ''future library directions'' (7.31.8).

258) The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

Contents

7.17.7 Operations on atomic types

1 There are only a few kinds of operations on atomic types, though there are many instances of those kinds. This subclause specifies each general kind.

Contents

7.17.7.1 The atomic_store generic functions

Synopsis

1

          #include <stdatomic.h>
          void atomic_store(volatile A *object, C desired);
          void atomic_store_explicit(volatile A *object,
               C desired, memory_order order);

Description

2 The order argument shall not be memory_order_acquire, memory_order_consume, nor memory_order_acq_rel. Atomically replace the value pointed to by object with the value of desired. Memory is affected according to the value of order.

Returns

3 The atomic_store generic functions return no value.

Contents

7.17.7.2 The atomic_load generic functions

Synopsis

1

          #include <stdatomic.h>
          C atomic_load(volatile A *object);
          C atomic_load_explicit(volatile A *object,
               memory_order order);

Description

2 The order argument shall not be memory_order_release nor memory_order_acq_rel. Memory is affected according to the value of order.

Returns Atomically returns the value pointed to by object.

Contents

7.17.7.3 The atomic_exchange generic functions

Synopsis

1

          #include <stdatomic.h>
          C atomic_exchange(volatile A *object, C desired);
          C atomic_exchange_explicit(volatile A *object,
               C desired, memory_order order);

Description

2 Atomically replace the value pointed to by object with desired. Memory is affected according to the value of order. These operations are read-modify-write operations (5.1.2.4).

Returns

3 Atomically returns the value pointed to by object immediately before the effects.

Contents

7.17.7.4 The atomic_compare_exchange generic functions

Synopsis

1

          #include <stdatomic.h>
          _Bool atomic_compare_exchange_strong(volatile A *object,
               C *expected, C desired);
          _Bool atomic_compare_exchange_strong_explicit(
               volatile A *object, C *expected, C desired,
               memory_order success, memory_order failure);
          _Bool atomic_compare_exchange_weak(volatile A *object,
               C *expected, C desired);
          _Bool atomic_compare_exchange_weak_explicit(
               volatile A *object, C *expected, C desired,
               memory_order success, memory_order failure);

Description

2 The failure argument shall not be memory_order_release nor memory_order_acq_rel. The failure argument shall be no stronger than the success argument. Atomically, compares the value pointed to by object for equality with that in expected, and if true, replaces the value pointed to by object with desired, and if false, updates the value in expected with the value pointed to by object. Further, if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure. These operations are atomic read-modify-write operations (5.1.2.4).

3 NOTE 1 For example, the effect of atomic_compare_exchange_strong is

          if (memcmp(object, expected, sizeof (*object)) == 0)
                memcpy(object, &desired, sizeof (*object));
          else
                memcpy(expected, object, sizeof (*object));

4 A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by expected and object are equal, it may return zero and store back to expected the same memory contents that were originally there.

5 NOTE 2 This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g. load-locked store-conditional machines.

6 EXAMPLE A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

          exp = atomic_load(&cur);
          do {
                des = function(exp);
          } while (!atomic_compare_exchange_weak(&cur, &exp, des));
When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

Returns

7 The result of the comparison.

Contents

7.17.7.5 The atomic_fetch and modify generic functions

1 The following operations perform arithmetic and bitwise computations. All of these operations are applicable to an object of any atomic integer type. None of these operations is applicable to atomic_bool. The key, operator, and computation correspondence is:
key op computation
add + addition
sub - subtraction
or | bitwise inclusive or
xor ^ bitwise exclusive or
and & bitwise and

Synopsis

2

          #include <stdatomic.h>
          C atomic_fetch_key(volatile A *object, M operand);
          C atomic_fetch_key_explicit(volatile A *object,
               M operand, memory_order order);

Description

3 Atomically replaces the value pointed to by object with the result of the computation applied to the value pointed to by object and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations (5.1.2.4). For signed integer types, arithmetic is defined to use two's complement representation with silent wrap-around on overflow; there are no undefined results. For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.

Returns

4 Atomically, the value pointed to by object immediately before the effects.

5 NOTE The operation of the atomic_fetch and modify generic functions are nearly equivalent to the operation of the corresponding op= compound assignment operators. The only differences are that the compound assignment operators are not guaranteed to operate atomically, and the value yielded by a compound assignment operator is the updated value of the object, whereas the value returned by the atomic_fetch and modify generic functions is the previous value of the atomic object.

Contents

7.17.8 Atomic flag type and operations

1 The atomic_flag type provides the classic test-and-set functionality. It has two states, set and clear.

2 Operations on an object of type atomic_flag shall be lock free.

3 NOTE Hence the operations should also be address-free. No other type requires lock-free operations, so the atomic_flag type is the minimum hardware-implemented type needed to conform to this International standard. The remaining types can be emulated with atomic_flag, though with less than ideal properties.

4 The macro ATOMIC_FLAG_INIT may be used to initialize an atomic_flag to the clear state. An atomic_flag that is not explicitly initialized with ATOMIC_FLAG_INIT is initially in an indeterminate state.

5 EXAMPLE

         atomic_flag guard = ATOMIC_FLAG_INIT;

Contents

7.17.8.1 The atomic_flag_test_and_set functions

Synopsis

1

         #include <stdatomic.h>
         _Bool atomic_flag_test_and_set(
              volatile atomic_flag *object);
         _Bool atomic_flag_test_and_set_explicit(
              volatile atomic_flag *object, memory_order order);

Description

2 Atomically sets the value pointed to by object to true. Memory is affected according to the value of order. These operations are atomic read-modify-write operations (5.1.2.4).

Returns

3 Atomically, the value of the object immediately before the effects.

Contents

7.17.8.2 The atomic_flag_clear functions

Synopsis

1

        #include <stdatomic.h>
        void atomic_flag_clear(volatile atomic_flag *object);
        void atomic_flag_clear_explicit(
             volatile atomic_flag *object, memory_order order);

Description

2 The order argument shall not be memory_order_acquire nor memory_order_acq_rel. Atomically sets the value pointed to by object to false. Memory is affected according to the value of order.

Returns

3 The atomic_flag_clear functions return no value.

Contents

7.18 Boolean type and values <stdbool.h>

1 The header <stdbool.h> defines four macros.

2 The macro

          bool
expands to _Bool.

3 The remaining three macros are suitable for use in #if preprocessing directives. They are

          true
which expands to the integer constant 1,
          false
which expands to the integer constant 0, and
          __bool_true_false_are_defined
which expands to the integer constant 1.

4 Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the macros bool, true, and false.259)

Footnotes

259) See ''future library directions'' (7.31.9).

Contents

7.19 Common definitions <stddef.h>

1 The header <stddef.h> defines the following macros and declares the following types. Some are also defined in other headers, as noted in their respective subclauses.

2 The types are

        ptrdiff_t
which is the signed integer type of the result of subtracting two pointers;
        size_t
which is the unsigned integer type of the result of the sizeof operator;
        max_align_t
which is an object type whose alignment is as great as is supported by the implementation in all contexts; and
        wchar_t
which is an integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero. Each member of the basic character set shall have a code value equal to its value when used as the lone character in an integer character constant if an implementation does not define __STDC_MB_MIGHT_NEQ_WC__.

3 The macros are

        NULL
which expands to an implementation-defined null pointer constant; and
        offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given
        static type t;
then the expression &(t.member-designator) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.)

Recommended practice

4 The types used for size_t and ptrdiff_t should not have an integer conversion rank greater than that of signed long int unless the implementation supports objects large enough to make this necessary. *

Contents

7.20 Integer types <stdint.h>

1 The header <stdint.h> declares sets of integer types having specified widths, and defines corresponding sets of macros.260) It also defines macros that specify limits of integer types corresponding to types defined in other standard headers.

2 Types are defined in the following categories:

(Some of these types may denote the same type.)

3 Corresponding macros specify limits of the declared types and construct suitable constants.

4 For each type described herein that the implementation provides,261) <stdint.h> shall declare that typedef name and define the associated macros. Conversely, for each type described herein that the implementation does not provide, <stdint.h> shall not declare that typedef name nor shall it define the associated macros. An implementation shall provide those types described as ''required'', but need not provide any of the others (described as ''optional'').

Footnotes

260) See ''future library directions'' (7.31.10).

261) Some of these types may denote implementation-defined extended integer types.

Contents

7.20.1 Integer types

1 When typedef names differing only in the absence or presence of the initial u are defined, they shall denote corresponding signed and unsigned types as described in 6.2.5; an implementation providing one of these corresponding types shall also provide the other.

2 In the following descriptions, the symbol N represents an unsigned decimal integer with no leading zeros (e.g., 8 or 24, but not 04 or 048).

Contents

7.20.1.1 Exact-width integer types

1 The typedef name intN_t designates a signed integer type with width N , no padding bits, and a two's complement representation. Thus, int8_t denotes such a signed integer type with a width of exactly 8 bits.

2 The typedef name uintN_t designates an unsigned integer type with width N and no padding bits. Thus, uint24_t denotes such an unsigned integer type with a width of exactly 24 bits.

3 These types are optional. However, if an implementation provides integer types with widths of 8, 16, 32, or 64 bits, no padding bits, and (for the signed types) that have a two's complement representation, it shall define the corresponding typedef names.

Contents

7.20.1.2 Minimum-width integer types

1 The typedef name int_leastN_t designates a signed integer type with a width of at least N , such that no signed integer type with lesser size has at least the specified width. Thus, int_least32_t denotes a signed integer type with a width of at least 32 bits.

2 The typedef name uint_leastN_t designates an unsigned integer type with a width of at least N , such that no unsigned integer type with lesser size has at least the specified width. Thus, uint_least16_t denotes an unsigned integer type with a width of at least 16 bits.

3 The following types are required:

          int_least8_t                                      uint_least8_t
          int_least16_t                                     uint_least16_t
          int_least32_t                                     uint_least32_t
          int_least64_t                                     uint_least64_t
All other types of this form are optional.

Contents

7.20.1.3 Fastest minimum-width integer types

1 Each of the following types designates an integer type that is usually fastest262) to operate with among all integer types that have at least the specified width.

2 The typedef name int_fastN_t designates the fastest signed integer type with a width of at least N . The typedef name uint_fastN_t designates the fastest unsigned integer type with a width of at least N .

3 The following types are required:

         int_fast8_t                                    uint_fast8_t
         int_fast16_t                                   uint_fast16_t
         int_fast32_t                                   uint_fast32_t
         int_fast64_t                                   uint_fast64_t
All other types of this form are optional.

Footnotes

262) The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements.

Contents

7.20.1.4 Integer types capable of holding object pointers

1 The following type designates a signed integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

         intptr_t
The following type designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:
         uintptr_t
These types are optional.

Contents

7.20.1.5 Greatest-width integer types

1 The following type designates a signed integer type capable of representing any value of any signed integer type:

         intmax_t
The following type designates an unsigned integer type capable of representing any value of any unsigned integer type:
         uintmax_t
These types are required.

Contents

7.20.2 Limits of specified-width integer types

1 The following object-like macros specify the minimum and maximum limits of the types declared in <stdint.h>. Each macro name corresponds to a similar type name in 7.20.1.

2 Each instance of any defined macro shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign, except where stated to be exactly the given value.

Contents

7.20.2.1 Limits of exact-width integer types

1

Contents

7.20.2.2 Limits of minimum-width integer types

1

Contents

7.20.2.3 Limits of fastest minimum-width integer types

1

Contents

7.20.2.4 Limits of integer types capable of holding object pointers

1

Contents

7.20.2.5 Limits of greatest-width integer types

1

Contents

7.20.3 Limits of other integer types

1 The following object-like macros specify the minimum and maximum limits of integer types corresponding to types defined in other standard headers.

2 Each instance of these macros shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign. An implementation shall define only the macros corresponding to those typedef names it actually provides.263)

3 If sig_atomic_t (see 7.14) is defined as a signed integer type, the value of SIG_ATOMIC_MIN shall be no greater than -127 and the value of SIG_ATOMIC_MAX shall be no less than 127; otherwise, sig_atomic_t is defined as an unsigned integer type, and the value of SIG_ATOMIC_MIN shall be 0 and the value of SIG_ATOMIC_MAX shall be no less than 255.

4 If wchar_t (see 7.19) is defined as a signed integer type, the value of WCHAR_MIN shall be no greater than -127 and the value of WCHAR_MAX shall be no less than 127; otherwise, wchar_t is defined as an unsigned integer type, and the value of WCHAR_MIN shall be 0 and the value of WCHAR_MAX shall be no less than 255.264)

5 If wint_t (see 7.29) is defined as a signed integer type, the value of WINT_MIN shall be no greater than -32767 and the value of WINT_MAX shall be no less than 32767; otherwise, wint_t is defined as an unsigned integer type, and the value of WINT_MIN shall be 0 and the value of WINT_MAX shall be no less than 65535.

Footnotes

263) A freestanding implementation need not provide all of these types.

264) The values WCHAR_MIN and WCHAR_MAX do not necessarily correspond to members of the extended character set.

Contents

7.20.4 Macros for integer constants

1 The following function-like macros expand to integer constants suitable for initializing objects that have integer types corresponding to types defined in <stdint.h>. Each macro name corresponds to a similar type name in 7.20.1.2 or 7.20.1.5.

2 The argument in any instance of these macros shall be an unsuffixed integer constant (as defined in 6.4.4.1) with a value that does not exceed the limits for the corresponding type.

3 Each invocation of one of these macros shall expand to an integer constant expression suitable for use in #if preprocessing directives. The type of the expression shall have the same type as would an expression of the corresponding type converted according to the integer promotions. The value of the expression shall be that of the argument.

Contents

7.20.4.1 Macros for minimum-width integer constants

1 The macro INTN_C(value) shall expand to an integer constant expression corresponding to the type int_leastN_t. The macro UINTN_C(value) shall expand to an integer constant expression corresponding to the type uint_leastN_t. For example, if uint_least64_t is a name for the type unsigned long long int, then UINT64_C(0x123) might expand to the integer constant 0x123ULL.

Contents

7.20.4.2 Macros for greatest-width integer constants

1 The following macro expands to an integer constant expression having the value specified by its argument and the type intmax_t:

         INTMAX_C(value)
The following macro expands to an integer constant expression having the value specified by its argument and the type uintmax_t:
         UINTMAX_C(value)

Contents

7.21 Input/output <stdio.h>

Contents

7.21.1 Introduction

1 The header <stdio.h> defines several macros, and declares three types and many functions for performing input and output.

2 The types declared are size_t (described in 7.19);

        FILE
which is an object type capable of recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer (if any), an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached; and
        fpos_t
which is a complete object type other than an array type capable of recording all the information needed to specify uniquely every position within a file.

3 The macros are NULL (described in 7.19);

        _IOFBF
        _IOLBF
        _IONBF
which expand to integer constant expressions with distinct values, suitable for use as the third argument to the setvbuf function;
        BUFSIZ
which expands to an integer constant expression that is the size of the buffer used by the setbuf function;
        EOF
which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;
        FOPEN_MAX
which expands to an integer constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously;
        FILENAME_MAX
which expands to an integer constant expression that is the size needed for an array of char large enough to hold the longest file name string that the implementation guarantees can be opened;265)
         L_tmpnam
which expands to an integer constant expression that is the size needed for an array of char large enough to hold a temporary file name string generated by the tmpnam function;
         SEEK_CUR
         SEEK_END
         SEEK_SET
which expand to integer constant expressions with distinct values, suitable for use as the third argument to the fseek function;
         TMP_MAX
which expands to an integer constant expression that is the minimum number of unique file names that can be generated by the tmpnam function;
         stderr
         stdin
         stdout
which are expressions of type ''pointer to FILE'' that point to the FILE objects associated, respectively, with the standard error, input, and output streams.

4 The header <wchar.h> declares a number of functions useful for wide character input and output. The wide character input/output functions described in that subclause provide operations analogous to most of those described here, except that the fundamental units internal to the program are wide characters. The external representation (in the file) is a sequence of ''generalized'' multibyte characters, as described further in 7.21.3.

5 The input/output functions are given the following collective terms:

Forward references: files (7.21.3), the fseek function (7.21.9.2), streams (7.21.2), the tmpnam function (7.21.4.4), <wchar.h> (7.29).

Footnotes

265) If the implementation imposes no practical limit on the length of file name strings, the value of FILENAME_MAX should instead be the recommended size of an array intended to hold a file name string. Of course, file name string contents are subject to other system-specific constraints; therefore all possible strings of length FILENAME_MAX cannot be expected to be opened successfully.

Contents

7.21.2 Streams

1 Input and output, whether to or from physical devices such as terminals and tape drives, or whether to or from files supported on structured storage devices, are mapped into logical data streams, whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams.266)

2 A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. Characters may have to be added, altered, or deleted on input and output to conform to differing conventions for representing text in the host environment. Thus, there need not be a one- to-one correspondence between the characters in a stream and those in the external representation. Data read in from a text stream will necessarily compare equal to the data that were earlier written out to that stream only if: the data consist only of printing characters and the control characters horizontal tab and new-line; no new-line character is immediately preceded by space characters; and the last character is a new-line character. Whether space characters that are written out immediately before a new-line character appear when read in is implementation-defined.

3 A binary stream is an ordered sequence of characters that can transparently record internal data. Data read in from a binary stream shall compare equal to the data that were earlier written out to that stream, under the same implementation. Such a stream may, however, have an implementation-defined number of null characters appended to the end of the stream.

4 Each stream has an orientation. After a stream is associated with an external file, but before any operations are performed on it, the stream is without orientation. Once a wide character input/output function has been applied to a stream without orientation, the stream becomes a wide-oriented stream. Similarly, once a byte input/output function has been applied to a stream without orientation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide function can otherwise alter the orientation of a stream. (A successful call to freopen removes any orientation.)267)

5 Byte input/output functions shall not be applied to a wide-oriented stream and wide character input/output functions shall not be applied to a byte-oriented stream. The remaining stream operations do not affect, and are not affected by, a stream's orientation, except for the following additional restrictions:

6 Each wide-oriented stream has an associated mbstate_t object that stores the current parse state of the stream. A successful call to fgetpos stores a representation of the value of this mbstate_t object as part of the value of the fpos_t object. A later successful call to fsetpos using the same stored fpos_t value restores the value of the associated mbstate_t object as well as the position within the controlled stream.

7 Each stream has an associated lock that is used to prevent data races when multiple threads of execution access a stream, and to restrict the interleaving of stream operations performed by multiple threads. Only one thread may hold this lock at a time. The lock is reentrant: a single thread may hold the lock multiple times at a given time.

8 All functions that read, write, position, or query the position of a stream lock the stream before accessing it. They release the lock associated with the stream when the access is complete.

Environmental limits

9 An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.

Forward references: the freopen function (7.21.5.4), the fwide function (7.29.3.5), mbstate_t (7.30.1), the fgetpos function (7.21.9.1), the fsetpos function (7.21.9.3).

Footnotes

266) An implementation need not distinguish between text streams and binary streams. In such an implementation, there need be no new-line characters in a text stream nor any limit to the length of a line.

267) The three predefined streams stdin, stdout, and stderr are unoriented at program startup.

Contents

7.21.3 Files

1 A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start (character number zero) of the file, unless the file is opened with append mode in which case it is implementation-defined whether the file position indicator is initially positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file.

2 Binary files are not truncated, except as defined in 7.21.5.3. Whether a write on a text stream causes the associated file to be truncated beyond that point is implementation- defined.

3 When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.

4 A file may be disassociated from a controlling stream by closing the file. Output streams are flushed (any unwritten buffer contents are transmitted to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate after the associated file is closed (including the standard text streams). Whether a file of zero length (on which no characters have been written by an output stream) actually exists is implementation-defined.

5 The file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original caller, or if the exit function is called, all open files are closed (hence all output streams are flushed) before program termination. Other paths to program termination, such as calling the abort function, need not close all files properly.

6 The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original.

7 At program startup, three text streams are predefined and need not be opened explicitly -- standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

8 Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.

9 Although both text and binary wide-oriented streams are conceptually sequences of wide characters, the external file associated with a wide-oriented stream is a sequence of multibyte characters, generalized as follows:

10 Moreover, the encodings used for multibyte characters may differ among files. Both the nature and choice of such encodings are implementation-defined.

11 The wide character input functions read multibyte characters from the stream and convert them to wide characters as if they were read by successive calls to the fgetwc function. Each conversion occurs as if by a call to the mbrtowc function, with the conversion state described by the stream's own mbstate_t object. The byte input functions read characters from the stream as if by successive calls to the fgetc function.

12 The wide character output functions convert wide characters to multibyte characters and write them to the stream as if they were written by successive calls to the fputwc function. Each conversion occurs as if by a call to the wcrtomb function, with the conversion state described by the stream's own mbstate_t object. The byte output functions write characters to the stream as if by successive calls to the fputc function.

13 In some cases, some of the byte input/output functions also perform conversions between multibyte characters and wide characters. These conversions also occur as if by calls to the mbrtowc and wcrtomb functions.

14 An encoding error occurs if the character sequence presented to the underlying mbrtowc function does not form a valid (generalized) multibyte character, or if the code value passed to the underlying wcrtomb does not correspond to a valid (generalized) multibyte character. The wide character input/output functions and the byte input/output functions store the value of the macro EILSEQ in errno if and only if an encoding error occurs.

Environmental limits

15 The value of FOPEN_MAX shall be at least eight, including the three standard text streams.

Forward references: the exit function (7.22.4.4), the fgetc function (7.21.7.1), the fopen function (7.21.5.3), the fputc function (7.21.7.3), the setbuf function (7.21.5.5), the setvbuf function (7.21.5.6), the fgetwc function (7.29.3.1), the fputwc function (7.29.3.3), conversion state (7.29.6), the mbrtowc function (7.29.6.3.2), the wcrtomb function (7.29.6.3.3).

Footnotes

268) Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.

Contents

7.21.4 Operations on files

Contents

7.21.4.1 The remove function

Synopsis

1

        #include <stdio.h>
        int remove(const char *filename);

Description

2 The remove function causes the file whose name is the string pointed to by filename to be no longer accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is created anew. If the file is open, the behavior of the remove function is implementation-defined.

Returns

3 The remove function returns zero if the operation succeeds, nonzero if it fails.

Contents

7.21.4.2 The rename function

Synopsis

1

        #include <stdio.h>
        int rename(const char *old, const char *new);

Description

2 The rename function causes the file whose name is the string pointed to by old to be henceforth known by the name given by the string pointed to by new. The file named old is no longer accessible by that name. If a file named by the string pointed to by new exists prior to the call to the rename function, the behavior is implementation-defined.

Returns

3 The rename function returns zero if the operation succeeds, nonzero if it fails,269) in which case if the file existed previously it is still known by its original name.

Footnotes

269) Among the reasons the implementation may cause the rename function to fail are that the file is open or that it is necessary to copy its contents to effectuate its renaming.

Contents

7.21.4.3 The tmpfile function

Synopsis

1

         #include <stdio.h>
         FILE *tmpfile(void);

Description

2 The tmpfile function creates a temporary binary file that is different from any other existing file and that will automatically be removed when it is closed or at program termination. If the program terminates abnormally, whether an open temporary file is removed is implementation-defined. The file is opened for update with "wb+" mode.

Recommended practice

3 It should be possible to open at least TMP_MAX temporary files during the lifetime of the program (this limit may be shared with tmpnam) and there should be no limit on the number simultaneously open other than this limit and any limit on the number of open files (FOPEN_MAX).

Returns

4 The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be created, the tmpfile function returns a null pointer.

Forward references: the fopen function (7.21.5.3).

Contents

7.21.4.4 The tmpnam function

Synopsis

1

         #include <stdio.h>
         char *tmpnam(char *s);

Description

2 The tmpnam function generates a string that is a valid file name and that is not the same as the name of an existing file.270) The function is potentially capable of generating at least TMP_MAX different strings, but any or all of them may already be in use by existing files and thus not be suitable return values.

3 The tmpnam function generates a different string each time it is called.

4 Calls to the tmpnam function with a null pointer argument may introduce data races with each other. The implementation shall behave as if no library function calls the tmpnam function.

Returns

5 If no suitable string can be generated, the tmpnam function returns a null pointer. Otherwise, if the argument is a null pointer, the tmpnam function leaves its result in an internal static object and returns a pointer to that object (subsequent calls to the tmpnam function may modify the same object). If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam chars; the tmpnam function writes its result in that array and returns the argument as its value.

Environmental limits

6 The value of the macro TMP_MAX shall be at least 25.

Footnotes

270) Files created using strings generated by the tmpnam function are temporary only in the sense that their names should not collide with those generated by conventional naming rules for the implementation. It is still necessary to use the remove function to remove such files when their use is ended, and before program termination.

Contents

7.21.5 File access functions

Contents

7.21.5.1 The fclose function

Synopsis

1

        #include <stdio.h>
        int fclose(FILE *stream);

Description

2 A successful call to the fclose function causes the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded. Whether or not the call succeeds, the stream is disassociated from the file and any buffer set by the setbuf or setvbuf function is disassociated from the stream (and deallocated if it was automatically allocated).

Returns

3 The fclose function returns zero if the stream was successfully closed, or EOF if any errors were detected.

Contents

7.21.5.2 The fflush function

Synopsis

1

         #include <stdio.h>
         int fflush(FILE *stream);

Description

2 If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

3 If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above.

Returns

4 The fflush function sets the error indicator for the stream and returns EOF if a write error occurs, otherwise it returns zero.

Forward references: the fopen function (7.21.5.3).

Contents

7.21.5.3 The fopen function

Synopsis

1

         #include <stdio.h>
         FILE *fopen(const char * restrict filename,
              const char * restrict mode);

Description

2 The fopen function opens the file whose name is the string pointed to by filename, and associates a stream with it.

3 The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behavior is undefined.271)

r
open text file for reading
w
truncate to zero length or create text file for writing
wx
create text file for writing
a
append; open or create text file for writing at end-of-file
rb
open binary file for reading
wb
truncate to zero length or create binary file for writing
wbx
create binary file for writing
ab
append; open or create binary file for writing at end-of-file
r+
open text file for update (reading and writing)
w+
truncate to zero length or create text file for update
w+x
create text file for update
a+
append; open or create text file for update, writing at end-of-file
r+b or rb+
open binary file for update (reading and writing)
w+b or wb+
truncate to zero length or create binary file for update
w+bx or wb+x
create binary file for update
a+b or ab+
append; open or create binary file for update, writing at end-of-file

4 Opening a file with read mode ('r' as the first character in the mode argument) fails if the file does not exist or cannot be read.

5 Opening a file with exclusive mode ('x' as the last character in the mode argument) fails if the file already exists or cannot be created. Otherwise, the file is created with exclusive (also known as non-shared) access to the extent that the underlying system supports exclusive access.

6 Opening a file with append mode ('a' as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to the fseek function. In some implementations, opening a binary file with append mode ('b' as the second or third character in the above list of mode argument values) may initially position the file position indicator for the stream beyond the last data written, because of null character padding.

7 When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

8 When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared.

Returns

9 The fopen function returns a pointer to the object controlling the stream. If the open operation fails, fopen returns a null pointer.

Forward references: file positioning functions (7.21.9).

Footnotes

271) If the string begins with one of the above sequences, the implementation might choose to ignore the remaining characters, or it might use them to select different kinds of a file (some of which might not conform to the properties in 7.21.2).

Contents

7.21.5.4 The freopen function

Synopsis

1

         #include <stdio.h>
         FILE *freopen(const char * restrict filename,
              const char * restrict mode,
              FILE * restrict stream);

Description

2 The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function.272)

3 If filename is a null pointer, the freopen function attempts to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

4 The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file is ignored. The error and end-of-file indicators for the stream are cleared.

Returns

5 The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns the value of stream.

Footnotes

272) The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin, or stdout), as those identifiers need not be modifiable lvalues to which the value returned by the fopen function may be assigned.

Contents

7.21.5.5 The setbuf function

Synopsis

1

         #include <stdio.h>
         void setbuf(FILE * restrict stream,
              char * restrict buf);

Description

2 Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked with the values _IOFBF for mode and BUFSIZ for size, or (if buf is a null pointer), with the value _IONBF for mode.

Returns

3 The setbuf function returns no value.

Forward references: the setvbuf function (7.21.5.6).

Contents

7.21.5.6 The setvbuf function

Synopsis

1

         #include <stdio.h>
         int setvbuf(FILE * restrict stream,
              char * restrict buf,
              int mode, size_t size);

Description

2 The setvbuf function may be used only after the stream pointed to by stream has been associated with an open file and before any other operation (other than an unsuccessful call to setvbuf) is performed on the stream. The argument mode determines how stream will be buffered, as follows: _IOFBF causes input/output to be fully buffered; _IOLBF causes input/output to be line buffered; _IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the setvbuf function273) and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf function. The contents of the array at any time are indeterminate.

Returns

3 The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if the request cannot be honored.

Footnotes

273) The buffer has to have a lifetime at least as great as the open stream, so the stream should be closed before a buffer that has automatic storage duration is deallocated upon block exit.

Contents

7.21.6 Formatted input/output functions

1 The formatted input/output functions shall behave as if there is a sequence point after the actions associated with each specifier.274)

Footnotes

274) The fprintf functions perform writes to memory for the %n specifier.

Contents

7.21.6.1 The fprintf function

Synopsis

1

          #include <stdio.h>
          int fprintf(FILE * restrict stream,
               const char * restrict format, ...);

Description

2 The fprintf function writes output to the stream pointed to by stream, under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered.

3 The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: ordinary multibyte characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments, converting them, if applicable, according to the corresponding conversion specifier, and then writing the result to the output stream.

4 Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

5 As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. The arguments specifying field width, or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A negative precision argument is taken as if the precision were omitted.

6 The flag characters and their meanings are:

-
The result of the conversion is left-justified within the field. (It is right-justified if this flag is not specified.)
+
The result of a signed conversion always begins with a plus or minus sign. (It begins with a sign only when a negative value is converted if this flag is not specified.)276)
space
If the first character of a signed conversion is not a sign, or if a signed conversion results in no characters, a space is prefixed to the result. If the space and + flags both appear, the space flag is ignored.
#
The result is converted to an ''alternative form''. For o conversion, it increases the precision, if and only if necessary, to force the first digit of the result to be a zero (if the value and precision are both 0, a single 0 is printed). For x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G conversions, the result of converting a floating-point number always contains a decimal-point character, even if no digits follow it. (Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.) For g and G conversions, trailing zeros are not removed from the result. For other conversions, the behavior is undefined.
0
For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the 0 flag is ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag is ignored. For other conversions, the behavior is undefined.

7 The length modifiers and their meanings are:

hh
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to signed char or unsigned char before printing); or that a following n conversion specifier applies to a pointer to a signed char argument.
h
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short int or unsigned short int argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short int or unsigned short int before printing); or that a following n conversion specifier applies to a pointer to a short int argument.
l (ell)
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer to a long int argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.
ll (ell-ell)
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long long int or unsigned long long int argument; or that a following n conversion specifier applies to a pointer to a long long int argument.
j
Specifies that a following d, i, o, u, x, or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument.
z
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument.
t
Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned integer type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.
L
Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.
If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

8 The conversion specifiers and their meanings are:

d,i
The int argument is converted to signed decimal in the style [-]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.
o,u,x,X
The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; the letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.
f,F
A double argument representing a floating-point number is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. If a decimal-point character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits. A double argument representing an infinity is converted in one of the styles [-]inf or [-]infinity -- which style is implementation-defined. A double argument representing a NaN is converted in one of the styles [-]nan or [-]nan(n-char-sequence) -- which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.277)
e,E
A double argument representing a floating-point number is converted in the style [-]d.ddd e(+-)dd, where there is one digit (which is nonzero if the argument is nonzero) before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. The value is rounded to the appropriate number of digits. The E conversion specifier produces a number with E instead of e introducing the exponent. The exponent always contains at least two digits, and only as many more digits as necessary to represent the exponent. If the value is zero, the exponent is zero. A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.
g,G
A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let P equal the precision if nonzero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X: Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion of the result and the decimal-point character is removed if there is no fractional portion remaining. A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.
a,A
A double argument representing a floating-point number is converted in the style [-]0xh.hhhh p(+-)d, where there is one hexadecimal digit (which is nonzero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character278) and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision is sufficient to distinguish279) values of type double, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal- point character appears. The letters abcdef are used for a conversion and the letters ABCDEF for A conversion. The A conversion specifier produces a number with X and P instead of x and p. The exponent always contains at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent is zero. A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.
c
If no l length modifier is present, the int argument is converted to an unsigned char, and the resulting character is written. If an l length modifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to the initial element of a two-element array of wchar_t, the first element containing the wint_t argument to the lc conversion specification and the second a null wide character.
s
If no l length modifier is present, the argument shall be a pointer to the initial element of an array of character type.280) Characters from the array are written up to (but not including) the terminating null character. If the precision is specified, no more than that many bytes are written. If the precision is not specified or is greater than the size of the array, the array shall contain a null character. If an l length modifier is present, the argument shall be a pointer to the initial element of an array of wchar_t type. Wide characters from the array are converted to multibyte characters (each as if by a call to the wcrtomb function, with the conversion state described by an mbstate_t object initialized to zero before the first wide character is converted) up to and including a terminating null wide character. The resulting multibyte characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array shall contain a null wide character. If a precision is specified, no more than that many bytes are written (including shift sequences, if any), and the array shall contain a null wide character if, to equal the multibyte character sequence length given by the precision, the function would need to access a wide character one past the end of the array. In no case is a partial multibyte character written.281)
p
The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.
n
The argument shall be a pointer to signed integer into which is written the number of characters written to the output stream so far by this call to fprintf. No argument is converted, but one is consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined.
%
A % character is written. No argument is converted. The complete conversion specification shall be %%.

9 If a conversion specification is invalid, the behavior is undefined.282) If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

10 In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

11 For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal floating number with the given precision.

Recommended practice

12 For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

13 For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded.283) If the number of significant decimal digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the result should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the current rounding direction.

Returns

14 The fprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

Environmental limits

15 The number of characters that can be produced by any single conversion shall be at least 4095.

16 EXAMPLE 1 To print a date and time in the form ''Sunday, July 3, 10:02'' followed by pi to five decimal places:

          #include <math.h>
          #include <stdio.h>
          /* ... */
          char *weekday, *month;      // pointers to strings
          int day, hour, min;
          fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
                  weekday, month, day, hour, min);
          fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

17 EXAMPLE 2 In this example, multibyte characters do not have a state-dependent encoding, and the members of the extended character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a # and the second by an uppercase letter.

18 Given the following wide string with length seven,

          static wchar_t wstr[] = L"#X#Yabc#Z#W";
the seven calls
          fprintf(stdout, "|1234567890123|\n");
          fprintf(stdout, "|%13ls|\n", wstr);
          fprintf(stdout, "|%-13.9ls|\n", wstr);
          fprintf(stdout, "|%13.10ls|\n", wstr);
          fprintf(stdout, "|%13.11ls|\n", wstr);
          fprintf(stdout, "|%13.15ls|\n", &wstr[2]);
          fprintf(stdout, "|%13lc|\n", (wint_t) wstr[5]);
will print the following seven lines:
          |1234567890123|
          |  #X#Yabc#Z#W|
          |#X#Yabc#Z    |
          |    #X#Yabc#Z|
          |  #X#Yabc#Z#W|
          |      abc#Z#W|
          |           #Z|

Forward references: conversion state (7.29.6), the wcrtomb function (7.29.6.3.3).

Footnotes

275) Note that 0 is taken as a flag, not as the beginning of a field width.

276) The results of all floating conversions of a negative zero, and of negative values that round to zero, include a minus sign.

277) When applied to infinite and NaN values, the -, +, and space flag characters have their usual meaning; the # and 0 flag characters have no effect.

278) Binary implementations can choose the hexadecimal digit to the left of the decimal-point character so that subsequent digits align to nibble (4-bit) boundaries.

279) The precision p is sufficient to distinguish values of the source type if 16 p-1 > b n where b is FLT_RADIX and n is the number of base-b digits in the significand of the source type. A smaller p might suffice depending on the implementation's scheme for determining the digit to the left of the decimal-point character.

280) No special provisions are made for multibyte characters.

281) Redundant shift sequences may result if multibyte characters have a state-dependent encoding.

282) See ''future library directions'' (7.31.11).

283) For binary-to-decimal conversion, the result format's values are the numbers representable with the given format specifier. The number of significant digits is determined by the format specifier, and in the case of fixed-point conversion by the source value as well.

Contents

7.21.6.2 The fscanf function

Synopsis

1

         #include <stdio.h>
         int fscanf(FILE * restrict stream,
              const char * restrict format, ...);

Description

2 The fscanf function reads input from the stream pointed to by stream, under control of the string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.

3 The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: one or more white-space characters, an ordinary multibyte character (neither % nor a white-space character), or a conversion specification. Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

4 The fscanf function executes each directive of the format in turn. When all directives have been executed, or if a directive fails (as detailed below), the function returns. Failures are described as input failures (due to the occurrence of an encoding error or the unavailability of input characters), or matching failures (due to inappropriate input).

5 A directive composed of white-space character(s) is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read. The directive never fails.

6 A directive that is an ordinary multibyte character is executed by reading the next characters of the stream. If any of those characters differ from the ones composing the directive, the directive fails and the differing and subsequent characters remain unread. Similarly, if end-of-file, an encoding error, or a read error prevents a character from being read, the directive fails.

7 A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

8 Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c, or n specifier.284)

9 An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence.285) The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

10 Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input characters) is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

11 The length modifiers and their meanings are:

hh
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to signed char or unsigned char.
h
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to short int or unsigned short int.
l (ell)
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to double; or that a following c, s, or [ conversion specifier applies to an argument with type pointer to wchar_t.
ll (ell-ell)
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long long int or unsigned long long int.
j
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.
z
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to size_t or the corresponding signed integer type.
t
Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned integer type.
L
Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to long double.
If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

12 The conversion specifiers and their meanings are:

d
Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol function with the value 10 for the base argument. The corresponding argument shall be a pointer to signed integer.
i
Matches an optionally signed integer, whose format is the same as expected for the subject sequence of the strtol function with the value 0 for the base argument. The corresponding argument shall be a pointer to signed integer.
o
Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 8 for the base argument. The corresponding argument shall be a pointer to unsigned integer.
u
Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 10 for the base argument. The corresponding argument shall be a pointer to unsigned integer.
x
Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 16 for the base argument. The corresponding argument shall be a pointer to unsigned integer.
a,e,f,g
Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the strtod function. The corresponding argument shall be a pointer to floating.
c
Matches a sequence of characters of exactly the number specified by the field width (1 if no field width is present in the directive).286) If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence. No null character is added. If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character in the sequence is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the resulting sequence of wide characters. No null wide character is added.
s
Matches a sequence of non-white-space characters.286) If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically. If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.
[
Matches a nonempty sequence of characters from a set of expected characters (the scanset).286) If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically. If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically. The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (]). The characters between the brackets (the scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the right bracket character is in the scanlist and the next following right bracket character is the matching right bracket that ends the specification; otherwise the first following right bracket character is the one that ends the specification. If a - character is in the scanlist and is not the first, nor the second where the first character is a ^, nor the last character, the behavior is implementation-defined.
p
Matches an implementation-defined set of sequences, which should be the same as the set of sequences that may be produced by the %p conversion of the fprintf function. The corresponding argument shall be a pointer to a pointer to void. The input item is converted to a pointer value in an implementation-defined manner. If the input item is a value converted earlier during the same program execution, the pointer that results shall compare equal to that value; otherwise the behavior of the %p conversion is undefined.
n
No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of characters read from the input stream so far by this call to the fscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fscanf function. No argument is converted, but one is consumed. If the conversion specification includes an assignment- suppressing character or a field width, the behavior is undefined.
%
Matches a single % character; no conversion or assignment occurs. The complete conversion specification shall be %%.

13 If a conversion specification is invalid, the behavior is undefined.287)

14 The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f, g, and x.

15 Trailing white space (including new-line characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive.

Returns

16 The fscanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

17 EXAMPLE 1 The call:

          #include <stdio.h>
          /* ... */
          int n, i; float x; char name[50];
          n = fscanf(stdin, "%d%f%s", &i, &x, name);
with the input line:
          25 54.32E-1 thompson
will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.

18 EXAMPLE 2 The call:

          #include <stdio.h>
          /* ... */
          int i; float x; char name[50];
          fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);
with input:
          56789 0123 56a72
will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the sequence 56\0. The next character read from the input stream will be a.

19 EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:

          #include <stdio.h>
          /* ... */
          int count; float quant; char units[21], item[21];
          do {
                  count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
                  fscanf(stdin,"%*[^\n]");
          } while (!feof(stdin) && !ferror(stdin));

20 If the stdin stream contains the following lines:

          2 quarts of oil
          -12.8degrees Celsius
          lots of luck
          10.0LBS     of
          dirt
          100ergs of energy
the execution of the above example will be analogous to the following assignments:
           quant     =   2; strcpy(units, "quarts"); strcpy(item, "oil");
           count     =   3;
           quant     =   -12.8; strcpy(units, "degrees");
           count     =   2; // "C" fails to match "o"
           count     =   0; // "l" fails to match "%f"
           quant     =   10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
           count     =   3;
           count     =   0; // "100e" fails to match "%f"
           count     =   EOF;

21 EXAMPLE 4 In:

           #include <stdio.h>
           /* ... */
           int d1, d2, n1, n2, i;
           i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);
the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure, the value of 3 is also assigned to n2. The value of d2 is not affected. The value 1 is assigned to i.

22 EXAMPLE 5 The call:

           #include <stdio.h>
           /* ... */
           int n, i;
           n = sscanf("foo  %  bar  42", "foo%%bar%d", &i);
will assign to n the value 1 and to i the value 42 because input white-space characters are skipped for both the % and d conversion specifiers.

23 EXAMPLE 6 In these examples, multibyte characters do have a state-dependent encoding, and the members of the extended character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a # and the second by an uppercase letter, but are only recognized as such when in the alternate shift state. The shift sequences are denoted by ^ and $, in which the first causes entry into the alternate shift state.

24 After the call:

           #include <stdio.h>
           /* ... */
           char str[50];
           fscanf(stdin, "a%s", str);
with the input line:
           a^#X#Y$ bc
str will contain ^#X#Y$\0 assuming that none of the bytes of the shift sequences (or of the multibyte characters, in the more general case) appears to be a single-byte white-space character.

25 In contrast, after the call:

         #include <stdio.h>
         #include <stddef.h>
         /* ... */
         wchar_t wstr[50];
         fscanf(stdin, "a%ls", wstr);
with the same input line, wstr will contain the two wide characters that correspond to #X and #Y and a terminating null wide character.

26 However, the call:

         #include <stdio.h>
         #include <stddef.h>
         /* ... */
         wchar_t wstr[50];
         fscanf(stdin, "a^#X$%ls", wstr);
with the same input line will return zero due to a matching failure against the $ sequence in the format string.

27 Assuming that the first byte of the multibyte character #X is the same as the first byte of the multibyte character #Y, after the call:

         #include <stdio.h>
         #include <stddef.h>
         /* ... */
         wchar_t wstr[50];
         fscanf(stdin, "a^#Y$%ls", wstr);
with the same input line, zero will again be returned, but stdin will be left with a partially consumed multibyte character.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3), the strtol, strtoll, strtoul, and strtoull functions (7.22.1.4), conversion state (7.29.6), the wcrtomb function (7.29.6.3.3).

Footnotes

284) These white-space characters are not counted against a specified field width.

285) fscanf pushes back at most one input character onto the input stream. Therefore, some sequences that are acceptable to strtod, strtol, etc., are unacceptable to fscanf.

286) No special provisions are made for multibyte characters in the matching rules used by the c, s, and [ conversion specifiers -- the extent of the input field is determined on a byte-by-byte basis. The resulting field is nevertheless a sequence of multibyte characters that begins in the initial shift state.

287) See ''future library directions'' (7.31.11).

Contents

7.21.6.3 The printf function

Synopsis

1

         #include <stdio.h>
         int printf(const char * restrict format, ...);

Description

2 The printf function is equivalent to fprintf with the argument stdout interposed before the arguments to printf.

Returns

3 The printf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

Contents

7.21.6.4 The scanf function

Synopsis

1

         #include <stdio.h>
         int scanf(const char * restrict format, ...);

Description

2 The scanf function is equivalent to fscanf with the argument stdin interposed before the arguments to scanf.

Returns

3 The scanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the scanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Contents

7.21.6.5 The snprintf function

Synopsis

1

         #include <stdio.h>
         int snprintf(char * restrict s, size_t n,
              const char * restrict format, ...);

Description

2 The snprintf function is equivalent to fprintf, except that the output is written into an array (specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written to the array, and a null character is written at the end of the characters actually written into the array. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The snprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than n.

Contents

7.21.6.6 The sprintf function

Synopsis

1

         #include <stdio.h>
         int sprintf(char * restrict s,
              const char * restrict format, ...);

Description

2 The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The sprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

Contents

7.21.6.7 The sscanf function

Synopsis

1

        #include <stdio.h>
        int sscanf(const char * restrict s,
             const char * restrict format, ...);

Description

2 The sscanf function is equivalent to fscanf, except that input is obtained from a string (specified by the argument s) rather than from a stream. Reaching the end of the string is equivalent to encountering end-of-file for the fscanf function. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The sscanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the sscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Contents

7.21.6.8 The vfprintf function

Synopsis

1

        #include <stdarg.h>
        #include <stdio.h>
        int vfprintf(FILE * restrict stream,
             const char * restrict format,
             va_list arg);

Description

2 The vfprintf function is equivalent to fprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfprintf function does not invoke the va_end macro.288)

Returns

3 The vfprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

4 EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.

         #include <stdarg.h>
         #include <stdio.h>
         void error(char *function_name, char *format, ...)
         {
               va_list args;
                 va_start(args, format);
                 // print out name of function causing error
                 fprintf(stderr, "ERROR in %s: ", function_name);
                 // print out remainder of message
                 vfprintf(stderr, format, args);
                 va_end(args);
         }

Footnotes

288) As the functions vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf, and vsscanf invoke the va_arg macro, the value of arg after the return is indeterminate.

Contents

7.21.6.9 The vfscanf function

Synopsis

1

         #include <stdarg.h>
         #include <stdio.h>
         int vfscanf(FILE * restrict stream,
              const char * restrict format,
              va_list arg);

Description

2 The vfscanf function is equivalent to fscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfscanf function does not invoke the va_end macro.288)

Returns

3 The vfscanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the vfscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Contents

7.21.6.10 The vprintf function

Synopsis

1

        #include <stdarg.h>
        #include <stdio.h>
        int vprintf(const char * restrict format,
             va_list arg);

Description

2 The vprintf function is equivalent to printf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vprintf function does not invoke the va_end macro.288)

Returns

3 The vprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

Contents

7.21.6.11 The vscanf function

Synopsis

1

        #include <stdarg.h>
        #include <stdio.h>
        int vscanf(const char * restrict format,
             va_list arg);

Description

2 The vscanf function is equivalent to scanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vscanf function does not invoke the va_end macro.288)

Returns

3 The vscanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the vscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Contents

7.21.6.12 The vsnprintf function

Synopsis

1

         #include <stdarg.h>
         #include <stdio.h>
         int vsnprintf(char * restrict s, size_t n,
              const char * restrict format,
              va_list arg);

Description

2 The vsnprintf function is equivalent to snprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsnprintf function does not invoke the va_end macro.288) If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The vsnprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than n.

Contents

7.21.6.13 The vsprintf function

Synopsis

1

         #include <stdarg.h>
         #include <stdio.h>
         int vsprintf(char * restrict s,
              const char * restrict format,
              va_list arg);

Description

2 The vsprintf function is equivalent to sprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsprintf function does not invoke the va_end macro.288) If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The vsprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

Contents

7.21.6.14 The vsscanf function

Synopsis

1

         #include <stdarg.h>
         #include <stdio.h>
         int vsscanf(const char * restrict s,
              const char * restrict format,
              va_list arg);

Description

2 The vsscanf function is equivalent to sscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsscanf function does not invoke the va_end macro.288)

Returns

3 The vsscanf function returns the value of the macro EOF if an input failure occurs before the first conversion (if any) has completed. Otherwise, the vsscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Contents

7.21.7 Character input/output functions

Contents

7.21.7.1 The fgetc function

Synopsis

1

         #include <stdio.h>
         int fgetc(FILE *stream);

Description

2 If the end-of-file indicator for the input stream pointed to by stream is not set and a next character is present, the fgetc function obtains that character as an unsigned char converted to an int and advances the associated file position indicator for the stream (if defined).

Returns

3 If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end- of-file indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function returns the next character from the input stream pointed to by stream. If a read error occurs, the error indicator for the stream is set and the fgetc function returns EOF.289)

Footnotes

289) An end-of-file and a read error can be distinguished by use of the feof and ferror functions.

Contents

7.21.7.2 The fgets function

Synopsis

1

         #include <stdio.h>
         char *fgets(char * restrict s, int n,
              FILE * restrict stream);

Description

2 The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

Returns

3 The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

Contents

7.21.7.3 The fputc function

Synopsis

1

         #include <stdio.h>
         int fputc(int c, FILE *stream);

Description

2 The fputc function writes the character specified by c (converted to an unsigned char) to the output stream pointed to by stream, at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream.

Returns

3 The fputc function returns the character written. If a write error occurs, the error indicator for the stream is set and fputc returns EOF.

Contents

7.21.7.4 The fputs function

Synopsis

1

         #include <stdio.h>
         int fputs(const char * restrict s,
              FILE * restrict stream);

Description

2 The fputs function writes the string pointed to by s to the stream pointed to by stream. The terminating null character is not written.

Returns

3 The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

Contents

7.21.7.5 The getc function

Synopsis

1

        #include <stdio.h>
        int getc(FILE *stream);

Description

2 The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects.

Returns

3 The getc function returns the next character from the input stream pointed to by stream. If the stream is at end-of-file, the end-of-file indicator for the stream is set and getc returns EOF. If a read error occurs, the error indicator for the stream is set and getc returns EOF.

Contents

7.21.7.6 The getchar function

Synopsis

1

        #include <stdio.h>
        int getchar(void);

Description

2 The getchar function is equivalent to getc with the argument stdin.

Returns

3 The getchar function returns the next character from the input stream pointed to by stdin. If the stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF. If a read error occurs, the error indicator for the stream is set and getchar returns EOF.

Contents

7.21.7.7 The putc function

Synopsis

1

         #include <stdio.h>
         int putc(int c, FILE *stream);

Description

2 The putc function is equivalent to fputc, except that if it is implemented as a macro, it may evaluate stream more than once, so that argument should never be an expression with side effects.

Returns

3 The putc function returns the character written. If a write error occurs, the error indicator for the stream is set and putc returns EOF.

Contents

7.21.7.8 The putchar function

Synopsis

1

         #include <stdio.h>
         int putchar(int c);

Description

2 The putchar function is equivalent to putc with the second argument stdout.

Returns

3 The putchar function returns the character written. If a write error occurs, the error indicator for the stream is set and putchar returns EOF.

Contents

7.21.7.9 The puts function

Synopsis

1

         #include <stdio.h>
         int puts(const char *s);

Description

2 The puts function writes the string pointed to by s to the stream pointed to by stdout, and appends a new-line character to the output. The terminating null character is not written.

Returns

3 The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

Contents

7.21.7.10 The ungetc function

Synopsis

1

          #include <stdio.h>
          int ungetc(int c, FILE *stream);

Description

2 The ungetc function pushes the character specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged.

3 One character of pushback is guaranteed. If the ungetc function is called too many times on the same stream without an intervening read or file positioning operation on that stream, the operation may fail.

4 If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.

5 A successful call to the ungetc function clears the end-of-file indicator for the stream. The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back. For a text stream, the value of its file position indicator after a successful call to the ungetc function is unspecified until all pushed-back characters are read or discarded. For a binary stream, its file position indicator is decremented by each successful call to the ungetc function; if its value was zero before a call, it is indeterminate after the call.290)

Returns

6 The ungetc function returns the character pushed back after conversion, or EOF if the operation fails.

Forward references: file positioning functions (7.21.9).

Footnotes

290) See ''future library directions'' (7.31.11).

Contents

7.21.8 Direct input/output functions

Contents

7.21.8.1 The fread function

Synopsis

1

         #include <stdio.h>
         size_t fread(void * restrict ptr,
              size_t size, size_t nmemb,
              FILE * restrict stream);

Description

2 The fread function reads, into the array pointed to by ptr, up to nmemb elements whose size is specified by size, from the stream pointed to by stream. For each object, size calls are made to the fgetc function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) is advanced by the number of characters successfully read. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.

Returns

3 The fread function returns the number of elements successfully read, which may be less than nmemb if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the contents of the array and the state of the stream remain unchanged.

Contents

7.21.8.2 The fwrite function

Synopsis

1

         #include <stdio.h>
         size_t fwrite(const void * restrict ptr,
              size_t size, size_t nmemb,
              FILE * restrict stream);

Description

2 The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is specified by size, to the stream pointed to by stream. For each object, size calls are made to the fputc function, taking the values (in order) from an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) is advanced by the number of characters successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.

Returns

3 The fwrite function returns the number of elements successfully written, which will be less than nmemb only if a write error is encountered. If size or nmemb is zero, fwrite returns zero and the state of the stream remains unchanged.

Contents

7.21.9 File positioning functions

Contents

7.21.9.1 The fgetpos function

Synopsis

1

        #include <stdio.h>
        int fgetpos(FILE * restrict stream,
             fpos_t * restrict pos);

Description

2 The fgetpos function stores the current values of the parse state (if any) and file position indicator for the stream pointed to by stream in the object pointed to by pos. The values stored contain unspecified information usable by the fsetpos function for repositioning the stream to its position at the time of the call to the fgetpos function.

Returns

3 If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero and stores an implementation-defined positive value in errno.

Forward references: the fsetpos function (7.21.9.3).

Contents

7.21.9.2 The fseek function

Synopsis

1

        #include <stdio.h>
        int fseek(FILE *stream, long int offset, int whence);

Description

2 The fseek function sets the file position indicator for the stream pointed to by stream. If a read or write error occurs, the error indicator for the stream is set and fseek fails.

3 For a binary stream, the new position, measured in characters from the beginning of the file, is obtained by adding offset to the position specified by whence. The specified position is the beginning of the file if whence is SEEK_SET, the current value of the file position indicator if SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

4 For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier successful call to the ftell function on a stream associated with the same file and whence shall be SEEK_SET.

5 After determining the new position, a successful call to the fseek function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new position. After a successful fseek call, the next operation on an update stream may be either input or output.

Returns

6 The fseek function returns nonzero only for a request that cannot be satisfied.

Forward references: the ftell function (7.21.9.4).

Contents

7.21.9.3 The fsetpos function

Synopsis

1

         #include <stdio.h>
         int fsetpos(FILE *stream, const fpos_t *pos);

Description

2 The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier successful call to the fgetpos function on a stream associated with the same file. If a read or write error occurs, the error indicator for the stream is set and fsetpos fails.

3 A successful call to the fsetpos function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new parse state and position. After a successful fsetpos call, the next operation on an update stream may be either input or output.

Returns

4 If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero and stores an implementation-defined positive value in errno.

Contents

7.21.9.4 The ftell function

Synopsis

1

         #include <stdio.h>
         long int ftell(FILE *stream);

Description

2 The ftell function obtains the current value of the file position indicator for the stream pointed to by stream. For a binary stream, the value is the number of characters from the beginning of the file. For a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call; the difference between two such return values is not necessarily a meaningful measure of the number of characters written or read.

Returns

3 If successful, the ftell function returns the current value of the file position indicator for the stream. On failure, the ftell function returns -1L and stores an implementation-defined positive value in errno.

Contents

7.21.9.5 The rewind function

Synopsis

1

        #include <stdio.h>
        void rewind(FILE *stream);

Description

2 The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to

        (void)fseek(stream, 0L, SEEK_SET)
except that the error indicator for the stream is also cleared.

Returns

3 The rewind function returns no value.

Contents

7.21.10 Error-handling functions

Contents

7.21.10.1 The clearerr function

Synopsis

1

        #include <stdio.h>
        void clearerr(FILE *stream);

Description

2 The clearerr function clears the end-of-file and error indicators for the stream pointed to by stream.

Returns

3 The clearerr function returns no value.

Contents

7.21.10.2 The feof function

Synopsis

1

         #include <stdio.h>
         int feof(FILE *stream);

Description

2 The feof function tests the end-of-file indicator for the stream pointed to by stream.

Returns

3 The feof function returns nonzero if and only if the end-of-file indicator is set for stream.

Contents

7.21.10.3 The ferror function

Synopsis

1

         #include <stdio.h>
         int ferror(FILE *stream);

Description

2 The ferror function tests the error indicator for the stream pointed to by stream.

Returns

3 The ferror function returns nonzero if and only if the error indicator is set for stream.

Contents

7.21.10.4 The perror function

Synopsis

1

         #include <stdio.h>
         void perror(const char *s);

Description

2 The perror function maps the error number in the integer expression errno to an error message. It writes a sequence of characters to the standard error stream thus: first (if s is not a null pointer and the character pointed to by s is not the null character), the string pointed to by s followed by a colon (:) and a space; then an appropriate error message string followed by a new-line character. The contents of the error message strings are the same as those returned by the strerror function with argument errno.

Returns

3 The perror function returns no value.

Forward references: the strerror function (7.24.6.2).

Contents

7.22 General utilities <stdlib.h>

1 The header <stdlib.h> declares five types and several functions of general utility, and defines several macros.291)

2 The types declared are size_t and wchar_t (both described in 7.19),

          div_t
which is a structure type that is the type of the value returned by the div function,
          ldiv_t
which is a structure type that is the type of the value returned by the ldiv function, and
          lldiv_t
which is a structure type that is the type of the value returned by the lldiv function.

3 The macros defined are NULL (described in 7.19);

          EXIT_FAILURE
and
          EXIT_SUCCESS
which expand to integer constant expressions that can be used as the argument to the exit function to return unsuccessful or successful termination status, respectively, to the host environment;
          RAND_MAX
which expands to an integer constant expression that is the maximum value returned by the rand function; and
          MB_CUR_MAX
which expands to a positive integer expression with type size_t that is the maximum number of bytes in a multibyte character for the extended character set specified by the current locale (category LC_CTYPE), which is never greater than MB_LEN_MAX.

Footnotes

291) See ''future library directions'' (7.31.12).

Contents

7.22.1 Numeric conversion functions

1 The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno on an error. If the value of the result cannot be represented, the behavior is undefined.

Contents

7.22.1.1 The atof function

Synopsis

1

         #include <stdlib.h>
         double atof(const char *nptr);

Description

2 The atof function converts the initial portion of the string pointed to by nptr to double representation. Except for the behavior on error, it is equivalent to

         strtod(nptr, (char **)NULL)

Returns

3 The atof function returns the converted value.

Forward references: the strtod, strtof, and strtold functions (7.22.1.3).

Contents

7.22.1.2 The atoi, atol, and atoll functions

Synopsis

1

         #include <stdlib.h>
         int atoi(const char *nptr);
         long int atol(const char *nptr);
         long long int atoll(const char *nptr);

Description

2 The atoi, atol, and atoll functions convert the initial portion of the string pointed to by nptr to int, long int, and long long int representation, respectively. Except for the behavior on error, they are equivalent to

         atoi: (int)strtol(nptr, (char **)NULL, 10)
         atol: strtol(nptr, (char **)NULL, 10)
         atoll: strtoll(nptr, (char **)NULL, 10)

Returns

3 The atoi, atol, and atoll functions return the converted value.

Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.22.1.4).

Contents

7.22.1.3 The strtod, strtof, and strtold functions

Synopsis

1

        #include <stdlib.h>
        double strtod(const char * restrict nptr,
             char ** restrict endptr);
        float strtof(const char * restrict nptr,
             char ** restrict endptr);
        long double strtold(const char * restrict nptr,
             char ** restrict endptr);

Description

2 The strtod, strtof, and strtold functions convert the initial portion of the string pointed to by nptr to double, float, and long double representation, respectively. First, they decompose the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by the isspace function), a subject sequence resembling a floating-point constant or representing an infinity or NaN; and a final string of one or more unrecognized characters, including the terminating null character of the input string. Then, they attempt to convert the subject sequence to a floating-point number, and return the result.

3 The expected form of the subject sequence is an optional plus or minus sign, then one of the following:

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is not of the expected form.

4 If the subject sequence has the expected form for a floating-point number, the sequence of characters starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a floating constant according to the rules of 6.4.4.2, except that the decimal-point character is used in place of a period, and that if neither an exponent part nor a decimal-point character appears in a decimal floating point number, or if a binary exponent part does not appear in a hexadecimal floating point number, an exponent part of the appropriate type with value zero is assumed to follow the last digit in the string. If the subject sequence begins with a minus sign, the sequence is interpreted as negated.292) A character sequence INF or INFINITY is interpreted as an infinity, if representable in the return type, else like a floating constant that is too large for the range of the return type. A character sequence NAN or NAN(n-char-sequenceopt) is interpreted as a quiet NaN, if supported in the return type, else like a subject sequence part that does not have the expected form; the meaning of the n-char sequence is implementation-defined.293) A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

5 If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting from the conversion is correctly rounded.

6 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.

7 If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Recommended practice

8 If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2, and the result is not exactly representable, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.

9 If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in <float.h>) significant digits, the result should be correctly rounded. If the subject sequence D has the decimal form and more than DECIMAL_DIG significant digits, consider the two bounding, adjacent decimal strings L and U, both having DECIMAL_DIG significant digits, such that the values of L, D, and U satisfy L <= D <= U. The result should be one of the (equal or adjacent) values that would be obtained by correctly rounding L and U according to the current rounding direction, with the extra stipulation that the error with respect to D should have a correct sign for the current rounding direction.294)

Returns

10 The functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value overflows and default rounding is in effect (7.12.1), plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value), and the value of the macro ERANGE is stored in errno. If the result underflows (7.12.1), the functions return a value whose magnitude is no greater than the smallest normalized positive number in the return type; whether errno acquires the value ERANGE is implementation-defined.

Footnotes

292) It is unspecified whether a minus-signed sequence is converted to a negative number directly or by negating the value resulting from converting the corresponding unsigned sequence (see F.5); the two methods may yield different results if rounding is toward positive or negative infinity. In either case, the functions honor the sign of zero if floating-point arithmetic supports signed zeros.

293) An implementation may use the n-char sequence to determine extra information to be represented in the NaN's significand.

294) DECIMAL_DIG, defined in <float.h>, should be sufficiently large that L and U will usually round to the same internal floating value, but if not will round to adjacent values.

Contents

7.22.1.4 The strtol, strtoll, strtoul, and strtoull functions

Synopsis

1

         #include <stdlib.h>
         long int strtol(
              const char * restrict nptr,
              char ** restrict endptr,
              int base);
         long long int strtoll(
              const char * restrict nptr,
              char ** restrict endptr,
              int base);
         unsigned long int strtoul(
              const char * restrict nptr,
              char ** restrict endptr,
              int base);
         unsigned long long int strtoull(
              const char * restrict nptr,
              char ** restrict endptr,
              int base);

Description

2 The strtol, strtoll, strtoul, and strtoull functions convert the initial portion of the string pointed to by nptr to long int, long long int, unsigned long int, and unsigned long long int representation, respectively. First, they decompose the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by the isspace function), a subject sequence resembling an integer represented in some radix determined by the value of base, and a final string of one or more unrecognized characters, including the terminating null character of the input string. Then, they attempt to convert the subject sequence to an integer, and return the result.

3 If the value of base is zero, the expected form of the subject sequence is that of an integer constant as described in 6.4.4.1, optionally preceded by a plus or minus sign, but not including an integer suffix. If the value of base is between 2 and 36 (inclusive), the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a plus or minus sign, but not including an integer suffix. The letters from a (or A) through z (or Z) are ascribed the values 10 through 35; only letters and digits whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

4 The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign or a permissible letter or digit.

5 If the subject sequence has the expected form and the value of base is zero, the sequence of characters starting with the first digit is interpreted as an integer constant according to the rules of 6.4.4.1. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated (in the return type). A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

6 In other than the "C" locale, additional locale-specific subject sequence forms may be accepted.

7 If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Returns

8 The strtol, strtoll, strtoul, and strtoull functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, LONG_MIN, LONG_MAX, LLONG_MIN, LLONG_MAX, ULONG_MAX, or ULLONG_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

Contents

7.22.2 Pseudo-random sequence generation functions

Contents

7.22.2.1 The rand function

Synopsis

1

         #include <stdlib.h>
         int rand(void);

Description

2 The rand function computes a sequence of pseudo-random integers in the range 0 to RAND_MAX.295)

3 The rand function is not required to avoid data races with other calls to pseudo-random sequence generation functions. The implementation shall behave as if no library function calls the rand function.

Returns

4 The rand function returns a pseudo-random integer.

Environmental limits

5 The value of the RAND_MAX macro shall be at least 32767.

Footnotes

295) There are no guarantees as to the quality of the random sequence produced and some implementations are known to produce sequences with distressingly non-random low-order bits. Applications with particular requirements should use a generator that is known to be sufficient for their needs.

Contents

7.22.2.2 The srand function

Synopsis

1

         #include <stdlib.h>
         void srand(unsigned int seed);

Description

2 The srand function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand. If srand is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. If rand is called before any calls to srand have been made, the same sequence shall be generated as when srand is first called with a seed value of 1.

3 The srand function is not required to avoid data races with other calls to pseudo- random sequence generation functions. The implementation shall behave as if no library function calls the srand function.

Returns

4 The srand function returns no value.

5 EXAMPLE The following functions define a portable implementation of rand and srand.

         static unsigned long int next = 1;
         int rand(void)   // RAND_MAX assumed to be 32767
         {
               next = next * 1103515245 + 12345;
               return (unsigned int)(next/65536) % 32768;
         }
         void srand(unsigned int seed)
         {
               next = seed;
         }

Contents

7.22.3 Memory management functions

1 The order and contiguity of storage allocated by successive calls to the aligned_alloc, calloc, malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated). The lifetime of an allocated object extends from the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

2 For purposes of determining the existence of a data race, memory allocation functions behave as though they accessed only memory locations accessible through their arguments and not other static duration storage. These functions may, however, visibly modify the storage that they allocate or deallocate. A call to free or realloc that deallocates a region p of memory synchronizes with any allocation call that allocates all or part of the region p. This synchronization occurs after any access of p by the deallocating function, and before any such access by the allocating function.

Contents

7.22.3.1 The aligned_alloc function

Synopsis

1

         #include <stdlib.h>
         void *aligned_alloc(size_t alignment, size_t size);

Description

2 The aligned_alloc function allocates space for an object whose alignment is specified by alignment, whose size is specified by size, and whose value is indeterminate. The value of alignment shall be a valid alignment supported by the implementation and the value of size shall be an integral multiple of alignment.

Returns

3 The aligned_alloc function returns either a null pointer or a pointer to the allocated space.

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7.22.3.2 The calloc function

Synopsis

1

         #include <stdlib.h>
         void *calloc(size_t nmemb, size_t size);

Description

2 The calloc function allocates space for an array of nmemb objects, each of whose size is size. The space is initialized to all bits zero.296)

Returns

3 The calloc function returns either a null pointer or a pointer to the allocated space.

Footnotes

296) Note that this need not be the same as the representation of floating-point zero or a null pointer constant.

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7.22.3.3 The free function

Synopsis

1

         #include <stdlib.h>
         void free(void *ptr);

Description

2 The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

Returns

3 The free function returns no value.

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7.22.3.4 The malloc function

Synopsis

1

         #include <stdlib.h>
         void *malloc(size_t size);

Description

2 The malloc function allocates space for an object whose size is specified by size and whose value is indeterminate.

Returns

3 The malloc function returns either a null pointer or a pointer to the allocated space.

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7.22.3.5 The realloc function

Synopsis

1

         #include <stdlib.h>
         void *realloc(void *ptr, size_t size);

Description

2 The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size. The contents of the new object shall be the same as that of the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new object beyond the size of the old object have indeterminate values.

3 If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size. Otherwise, if ptr does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to the free or realloc function, the behavior is undefined. If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

Returns

4 The realloc function returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object could not be allocated.

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7.22.4 Communication with the environment

Contents

7.22.4.1 The abort function

Synopsis

1

        #include <stdlib.h>
        _Noreturn void abort(void);

Description

2 The abort function causes abnormal program termination to occur, unless the signal SIGABRT is being caught and the signal handler does not return. Whether open streams with unwritten buffered data are flushed, open streams are closed, or temporary files are removed is implementation-defined. An implementation-defined form of the status unsuccessful termination is returned to the host environment by means of the function call raise(SIGABRT).

Returns

3 The abort function does not return to its caller.

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7.22.4.2 The atexit function

Synopsis

1

        #include <stdlib.h>
        int atexit(void (*func)(void));

Description

2 The atexit function registers the function pointed to by func, to be called without arguments at normal program termination.297) It is unspecified whether a call to the atexit function that does not happen before the exit function is called will succeed.

Environmental limits

3 The implementation shall support the registration of at least 32 functions.

Returns

4 The atexit function returns zero if the registration succeeds, nonzero if it fails.

Forward references: the at_quick_exit function (7.22.4.3), the exit function (7.22.4.4).

Footnotes

297) The atexit function registrations are distinct from the at_quick_exit registrations, so applications may need to call both registration functions with the same argument.

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7.22.4.3 The at_quick_exit function

Synopsis

1

         #include <stdlib.h>
         int at_quick_exit(void (*func)(void));

Description

2 The at_quick_exit function registers the function pointed to by func, to be called without arguments should quick_exit be called.298) It is unspecified whether a call to the at_quick_exit function that does not happen before the quick_exit function is called will succeed.

Environmental limits

3 The implementation shall support the registration of at least 32 functions.

Returns

4 The at_quick_exit function returns zero if the registration succeeds, nonzero if it fails.

Forward references: the quick_exit function (7.22.4.7).

Footnotes

298) The at_quick_exit function registrations are distinct from the atexit registrations, so applications may need to call both registration functions with the same argument.

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7.22.4.4 The exit function

Synopsis

1

         #include <stdlib.h>
         _Noreturn void exit(int status);

Description

2 The exit function causes normal program termination to occur. No functions registered by the at_quick_exit function are called. If a program calls the exit function more than once, or calls the quick_exit function in addition to the exit function, the behavior is undefined.

3 First, all functions registered by the atexit function are called, in the reverse order of their registration,299) except that a function is called after any previously registered functions that had already been called at the time it was registered. If, during the call to any such function, a call to the longjmp function is made that would terminate the call to the registered function, the behavior is undefined.

4 Next, all open streams with unwritten buffered data are flushed, all open streams are closed, and all files created by the tmpfile function are removed.

5 Finally, control is returned to the host environment. If the value of status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If the value of status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.

Returns

6 The exit function cannot return to its caller.

Footnotes

299) Each function is called as many times as it was registered, and in the correct order with respect to other registered functions.

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7.22.4.5 The _Exit function

Synopsis

1

         #include <stdlib.h>
         _Noreturn void _Exit(int status);

Description

2 The _Exit function causes normal program termination to occur and control to be returned to the host environment. No functions registered by the atexit function, the at_quick_exit function, or signal handlers registered by the signal function are called. The status returned to the host environment is determined in the same way as for the exit function (7.22.4.4). Whether open streams with unwritten buffered data are flushed, open streams are closed, or temporary files are removed is implementation- defined.

Returns

3 The _Exit function cannot return to its caller.

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7.22.4.6 The getenv function

Synopsis

1

         #include <stdlib.h>
         char *getenv(const char *name);

Description

2 The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation-defined. The getenv function need not avoid data races with other threads of execution that modify the environment list.300)

3 The implementation shall behave as if no library function calls the getenv function.

Returns

4 The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned.

Footnotes

300) Many implementations provide non-standard functions that modify the environment list.

Contents

7.22.4.7 The quick_exit function

Synopsis

1

         #include <stdlib.h>
         _Noreturn void quick_exit(int status);

Description

2 The quick_exit function causes normal program termination to occur. No functions registered by the atexit function or signal handlers registered by the signal function are called. If a program calls the quick_exit function more than once, or calls the exit function in addition to the quick_exit function, the behavior is undefined. If a signal is raised while the quick_exit function is executing, the behavior is undefined.

3 The quick_exit function first calls all functions registered by the at_quick_exit function, in the reverse order of their registration,301) except that a function is called after any previously registered functions that had already been called at the time it was registered. If, during the call to any such function, a call to the longjmp function is made that would terminate the call to the registered function, the behavior is undefined.

4 Then control is returned to the host environment by means of the function call _Exit(status).

Returns

5 The quick_exit function cannot return to its caller.

Footnotes

301) Each function is called as many times as it was registered, and in the correct order with respect to other registered functions.

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7.22.4.8 The system function

Synopsis

1

         #include <stdlib.h>
         int system(const char *string);

Description

2 If string is a null pointer, the system function determines whether the host environment has a command processor. If string is not a null pointer, the system function passes the string pointed to by string to that command processor to be executed in a manner which the implementation shall document; this might then cause the program calling system to behave in a non-conforming manner or to terminate.

Returns

3 If the argument is a null pointer, the system function returns nonzero only if a command processor is available. If the argument is not a null pointer, and the system function does return, it returns an implementation-defined value.

Contents

7.22.5 Searching and sorting utilities

1 These utilities make use of a comparison function to search or sort arrays of unspecified type. Where an argument declared as size_t nmemb specifies the length of the array for a function, nmemb can have the value zero on a call to that function; the comparison function is not called, a search finds no matching element, and sorting performs no rearrangement. Pointer arguments on such a call shall still have valid values, as described in 7.1.4.

2 The implementation shall ensure that the second argument of the comparison function (when called from bsearch), or both arguments (when called from qsort), are pointers to elements of the array.302) The first argument when called from bsearch shall equal key.

3 The comparison function shall not alter the contents of the array. The implementation may reorder elements of the array between calls to the comparison function, but shall not alter the contents of any individual element.

4 When the same objects (consisting of size bytes, irrespective of their current positions in the array) are passed more than once to the comparison function, the results shall be consistent with one another. That is, for qsort they shall define a total ordering on the array, and for bsearch the same object shall always compare the same way with the key.

5 A sequence point occurs immediately before and immediately after each call to the comparison function, and also between any call to the comparison function and any movement of the objects passed as arguments to that call.

Footnotes

302) That is, if the value passed is p, then the following expressions are always nonzero:

          ((char *)p - (char *)base) % size == 0
          (char *)p >= (char *)base
          (char *)p < (char *)base + nmemb * size

Contents

7.22.5.1 The bsearch function

Synopsis

1

          #include <stdlib.h>
          void *bsearch(const void *key, const void *base,
               size_t nmemb, size_t size,
               int (*compar)(const void *, const void *));

Description

2 The bsearch function searches an array of nmemb objects, the initial element of which is pointed to by base, for an element that matches the object pointed to by key. The size of each element of the array is specified by size.

3 The comparison function pointed to by compar is called with two arguments that point to the key object and to an array element, in that order. The function shall return an integer less than, equal to, or greater than zero if the key object is considered, respectively, to be less than, to match, or to be greater than the array element. The array shall consist of: all the elements that compare less than, all the elements that compare equal to, and all the elements that compare greater than the key object, in that order.303)

Returns

4 The bsearch function returns a pointer to a matching element of the array, or a null pointer if no match is found. If two elements compare as equal, which element is matched is unspecified.

Footnotes

303) In practice, the entire array is sorted according to the comparison function.

Contents

7.22.5.2 The qsort function

Synopsis

1

          #include <stdlib.h>
          void qsort(void *base, size_t nmemb, size_t size,
               int (*compar)(const void *, const void *));

Description

2 The qsort function sorts an array of nmemb objects, the initial element of which is pointed to by base. The size of each object is specified by size.

3 The contents of the array are sorted into ascending order according to a comparison function pointed to by compar, which is called with two arguments that point to the objects being compared. The function shall return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

4 If two elements compare as equal, their order in the resulting sorted array is unspecified.

Returns

5 The qsort function returns no value.

Contents

7.22.6 Integer arithmetic functions

Contents

7.22.6.1 The abs, labs and llabs functions

Synopsis

1

         #include <stdlib.h>
         int abs(int j);
         long int labs(long int j);
         long long int llabs(long long int j);

Description

2 The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot be represented, the behavior is undefined.304)

Returns

3 The abs, labs, and llabs, functions return the absolute value.

Footnotes

304) The absolute value of the most negative number cannot be represented in two's complement.

Contents

7.22.6.2 The div, ldiv, and lldiv functions

Synopsis

1

         #include <stdlib.h>
         div_t div(int numer, int denom);
         ldiv_t ldiv(long int numer, long int denom);
         lldiv_t lldiv(long long int numer, long long int denom);

Description

2 The div, ldiv, and lldiv, functions compute numer / denom and numer % denom in a single operation.

Returns

3 The div, ldiv, and lldiv functions return a structure of type div_t, ldiv_t, and lldiv_t, respectively, comprising both the quotient and the remainder. The structures shall contain (in either order) the members quot (the quotient) and rem (the remainder), each of which has the same type as the arguments numer and denom. If either part of the result cannot be represented, the behavior is undefined.

Contents

7.22.7 Multibyte/wide character conversion functions

1 The behavior of the multibyte character functions is affected by the LC_CTYPE category of the current locale. For a state-dependent encoding, each function is placed into its initial conversion state at program startup and can be returned to that state by a call for which its character pointer argument, s, is a null pointer. Subsequent calls with s as other than a null pointer cause the internal conversion state of the function to be altered as necessary. A call with s as a null pointer causes these functions to return a nonzero value if encodings have state dependency, and zero otherwise.305) Changing the LC_CTYPE category causes the conversion state of these functions to be indeterminate.

Footnotes

305) If the locale employs special bytes to change the shift state, these bytes do not produce separate wide character codes, but are grouped with an adjacent multibyte character.

Contents

7.22.7.1 The mblen function

Synopsis

1

         #include <stdlib.h>
         int mblen(const char *s, size_t n);

Description

2 If s is not a null pointer, the mblen function determines the number of bytes contained in the multibyte character pointed to by s. Except that the conversion state of the mbtowc function is not affected, it is equivalent to

         mbtowc((wchar_t *)0, (const char *)0, 0);
         mbtowc((wchar_t *)0, s, n);

3 The implementation shall behave as if no library function calls the mblen function.

Returns

4 If s is a null pointer, the mblen function returns a nonzero or zero value, if multibyte character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the mblen function either returns 0 (if s points to the null character), or returns the number of bytes that are contained in the multibyte character (if the next n or fewer bytes form a valid multibyte character), or returns -1 (if they do not form a valid multibyte character).

Forward references: the mbtowc function (7.22.7.2).

Contents

7.22.7.2 The mbtowc function

Synopsis

1

        #include <stdlib.h>
        int mbtowc(wchar_t * restrict pwc,
             const char * restrict s,
             size_t n);

Description

2 If s is not a null pointer, the mbtowc function inspects at most n bytes beginning with the byte pointed to by s to determine the number of bytes needed to complete the next multibyte character (including any shift sequences). If the function determines that the next multibyte character is complete and valid, it determines the value of the corresponding wide character and then, if pwc is not a null pointer, stores that value in the object pointed to by pwc. If the corresponding wide character is the null wide character, the function is left in the initial conversion state.

3 The implementation shall behave as if no library function calls the mbtowc function.

Returns

4 If s is a null pointer, the mbtowc function returns a nonzero or zero value, if multibyte character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the mbtowc function either returns 0 (if s points to the null character), or returns the number of bytes that are contained in the converted multibyte character (if the next n or fewer bytes form a valid multibyte character), or returns -1 (if they do not form a valid multibyte character).

5 In no case will the value returned be greater than n or the value of the MB_CUR_MAX macro.

Contents

7.22.7.3 The wctomb function

Synopsis

1

        #include <stdlib.h>
        int wctomb(char *s, wchar_t wc);

Description

2 The wctomb function determines the number of bytes needed to represent the multibyte character corresponding to the wide character given by wc (including any shift sequences), and stores the multibyte character representation in the array whose first element is pointed to by s (if s is not a null pointer). At most MB_CUR_MAX characters are stored. If wc is a null wide character, a null byte is stored, preceded by any shift sequence needed to restore the initial shift state, and the function is left in the initial conversion state.

3 The implementation shall behave as if no library function calls the wctomb function.

Returns

4 If s is a null pointer, the wctomb function returns a nonzero or zero value, if multibyte character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the wctomb function returns -1 if the value of wc does not correspond to a valid multibyte character, or returns the number of bytes that are contained in the multibyte character corresponding to the value of wc.

5 In no case will the value returned be greater than the value of the MB_CUR_MAX macro.

Contents

7.22.8 Multibyte/wide string conversion functions

1 The behavior of the multibyte string functions is affected by the LC_CTYPE category of the current locale.

Contents

7.22.8.1 The mbstowcs function

Synopsis

1

          #include <stdlib.h>
          size_t mbstowcs(wchar_t * restrict pwcs,
               const char * restrict s,
               size_t n);

Description

2 The mbstowcs function converts a sequence of multibyte characters that begins in the initial shift state from the array pointed to by s into a sequence of corresponding wide characters and stores not more than n wide characters into the array pointed to by pwcs. No multibyte characters that follow a null character (which is converted into a null wide character) will be examined or converted. Each multibyte character is converted as if by a call to the mbtowc function, except that the conversion state of the mbtowc function is not affected.

3 No more than n elements will be modified in the array pointed to by pwcs. If copying takes place between objects that overlap, the behavior is undefined.

Returns

4 If an invalid multibyte character is encountered, the mbstowcs function returns (size_t)(-1). Otherwise, the mbstowcs function returns the number of array elements modified, not including a terminating null wide character, if any.306)

Footnotes

306) The array will not be null-terminated if the value returned is n.

Contents

7.22.8.2 The wcstombs function

Synopsis

1

        #include <stdlib.h>
        size_t wcstombs(char * restrict s,
             const wchar_t * restrict pwcs,
             size_t n);

Description

2 The wcstombs function converts a sequence of wide characters from the array pointed to by pwcs into a sequence of corresponding multibyte characters that begins in the initial shift state, and stores these multibyte characters into the array pointed to by s, stopping if a multibyte character would exceed the limit of n total bytes or if a null character is stored. Each wide character is converted as if by a call to the wctomb function, except that the conversion state of the wctomb function is not affected.

3 No more than n bytes will be modified in the array pointed to by s. If copying takes place between objects that overlap, the behavior is undefined.

Returns

4 If a wide character is encountered that does not correspond to a valid multibyte character, the wcstombs function returns (size_t)(-1). Otherwise, the wcstombs function returns the number of bytes modified, not including a terminating null character, if any.306)

Contents

7.23 _Noreturn <stdnoreturn.h>

1 The header <stdnoreturn.h> defines the macro

         noreturn
which expands to _Noreturn.

Contents

7.24 String handling <string.h>

Contents

7.24.1 String function conventions

1 The header <string.h> declares one type and several functions, and defines one macro useful for manipulating arrays of character type and other objects treated as arrays of character type.307) The type is size_t and the macro is NULL (both described in 7.19). Various methods are used for determining the lengths of the arrays, but in all cases a char * or void * argument points to the initial (lowest addressed) character of the array. If an array is accessed beyond the end of an object, the behavior is undefined.

2 Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.

3 For all functions in this subclause, each character shall be interpreted as if it had the type unsigned char (and therefore every possible object representation is valid and has a different value).

Footnotes

307) See ''future library directions'' (7.31.13).

Contents

7.24.2 Copying functions

Contents

7.24.2.1 The memcpy function

Synopsis

1

          #include <string.h>
          void *memcpy(void * restrict s1,
               const void * restrict s2,
               size_t n);

Description

2 The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The memcpy function returns the value of s1.

Contents

7.24.2.2 The memmove function

Synopsis

1

         #include <string.h>
         void *memmove(void *s1, const void *s2, size_t n);

Description

2 The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1.

Returns

3 The memmove function returns the value of s1.

Contents

7.24.2.3 The strcpy function

Synopsis

1

         #include <string.h>
         char *strcpy(char * restrict s1,
              const char * restrict s2);

Description

2 The strcpy function copies the string pointed to by s2 (including the terminating null character) into the array pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The strcpy function returns the value of s1.

Contents

7.24.2.4 The strncpy function

Synopsis

1

         #include <string.h>
         char *strncpy(char * restrict s1,
              const char * restrict s2,
              size_t n);

Description

2 The strncpy function copies not more than n characters (characters that follow a null character are not copied) from the array pointed to by s2 to the array pointed to by s1.308) If copying takes place between objects that overlap, the behavior is undefined.

3 If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended to the copy in the array pointed to by s1, until n characters in all have been written.

Returns

4 The strncpy function returns the value of s1.

Footnotes

308) Thus, if there is no null character in the first n characters of the array pointed to by s2, the result will not be null-terminated.

Contents

7.24.3 Concatenation functions

Contents

7.24.3.1 The strcat function

Synopsis

1

          #include <string.h>
          char *strcat(char * restrict s1,
               const char * restrict s2);

Description

2 The strcat function appends a copy of the string pointed to by s2 (including the terminating null character) to the end of the string pointed to by s1. The initial character of s2 overwrites the null character at the end of s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The strcat function returns the value of s1.

Contents

7.24.3.2 The strncat function

Synopsis

1

          #include <string.h>
          char *strncat(char * restrict s1,
               const char * restrict s2,
               size_t n);

Description

2 The strncat function appends not more than n characters (a null character and characters that follow it are not appended) from the array pointed to by s2 to the end of the string pointed to by s1. The initial character of s2 overwrites the null character at the end of s1. A terminating null character is always appended to the result.309) If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The strncat function returns the value of s1.

Forward references: the strlen function (7.24.6.3).

Footnotes

309) Thus, the maximum number of characters that can end up in the array pointed to by s1 is strlen(s1)+n+1.

Contents

7.24.4 Comparison functions

1 The sign of a nonzero value returned by the comparison functions memcmp, strcmp, and strncmp is determined by the sign of the difference between the values of the first pair of characters (both interpreted as unsigned char) that differ in the objects being compared.

Contents

7.24.4.1 The memcmp function

Synopsis

1

         #include <string.h>
         int memcmp(const void *s1, const void *s2, size_t n);

Description

2 The memcmp function compares the first n characters of the object pointed to by s1 to the first n characters of the object pointed to by s2.310)

Returns

3 The memcmp function returns an integer greater than, equal to, or less than zero, accordingly as the object pointed to by s1 is greater than, equal to, or less than the object pointed to by s2.

Footnotes

310) The contents of ''holes'' used as padding for purposes of alignment within structure objects are indeterminate. Strings shorter than their allocated space and unions may also cause problems in comparison.

Contents

7.24.4.2 The strcmp function

Synopsis

1

         #include <string.h>
         int strcmp(const char *s1, const char *s2);

Description

2 The strcmp function compares the string pointed to by s1 to the string pointed to by s2.

Returns

3 The strcmp function returns an integer greater than, equal to, or less than zero, accordingly as the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2.

Contents

7.24.4.3 The strcoll function

Synopsis

1

        #include <string.h>
        int strcoll(const char *s1, const char *s2);

Description

2 The strcoll function compares the string pointed to by s1 to the string pointed to by s2, both interpreted as appropriate to the LC_COLLATE category of the current locale.

Returns

3 The strcoll function returns an integer greater than, equal to, or less than zero, accordingly as the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2 when both are interpreted as appropriate to the current locale.

Contents

7.24.4.4 The strncmp function

Synopsis

1

        #include <string.h>
        int strncmp(const char *s1, const char *s2, size_t n);

Description

2 The strncmp function compares not more than n characters (characters that follow a null character are not compared) from the array pointed to by s1 to the array pointed to by s2.

Returns

3 The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly null-terminated array pointed to by s2.

Contents

7.24.4.5 The strxfrm function

Synopsis

1

        #include <string.h>
        size_t strxfrm(char * restrict s1,
             const char * restrict s2,
             size_t n);

Description

2 The strxfrm function transforms the string pointed to by s2 and places the resulting string into the array pointed to by s1. The transformation is such that if the strcmp function is applied to two transformed strings, it returns a value greater than, equal to, or less than zero, corresponding to the result of the strcoll function applied to the same two original strings. No more than n characters are placed into the resulting array pointed to by s1, including the terminating null character. If n is zero, s1 is permitted to be a null pointer. If copying takes place between objects that overlap, the behavior is undefined.

Returns

3 The strxfrm function returns the length of the transformed string (not including the terminating null character). If the value returned is n or more, the contents of the array pointed to by s1 are indeterminate.

4 EXAMPLE The value of the following expression is the size of the array needed to hold the transformation of the string pointed to by s.

         1 + strxfrm(NULL, s, 0)

Contents

7.24.5 Search functions

Contents

7.24.5.1 The memchr function

Synopsis

1

         #include <string.h>
         void *memchr(const void *s, int c, size_t n);

Description

2 The memchr function locates the first occurrence of c (converted to an unsigned char) in the initial n characters (each interpreted as unsigned char) of the object pointed to by s. The implementation shall behave as if it reads the characters sequentially and stops as soon as a matching character is found.

Returns

3 The memchr function returns a pointer to the located character, or a null pointer if the character does not occur in the object.

Contents

7.24.5.2 The strchr function

Synopsis

1

         #include <string.h>
         char *strchr(const char *s, int c);

Description

2 The strchr function locates the first occurrence of c (converted to a char) in the string pointed to by s. The terminating null character is considered to be part of the string.

Returns

3 The strchr function returns a pointer to the located character, or a null pointer if the character does not occur in the string.

Contents

7.24.5.3 The strcspn function

Synopsis

1

        #include <string.h>
        size_t strcspn(const char *s1, const char *s2);

Description

2 The strcspn function computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters not from the string pointed to by s2.

Returns

3 The strcspn function returns the length of the segment.

Contents

7.24.5.4 The strpbrk function

Synopsis

1

        #include <string.h>
        char *strpbrk(const char *s1, const char *s2);

Description

2 The strpbrk function locates the first occurrence in the string pointed to by s1 of any character from the string pointed to by s2.

Returns

3 The strpbrk function returns a pointer to the character, or a null pointer if no character from s2 occurs in s1.

Contents

7.24.5.5 The strrchr function

Synopsis

1

        #include <string.h>
        char *strrchr(const char *s, int c);

Description

2 The strrchr function locates the last occurrence of c (converted to a char) in the string pointed to by s. The terminating null character is considered to be part of the string.

Returns

3 The strrchr function returns a pointer to the character, or a null pointer if c does not occur in the string.

Contents

7.24.5.6 The strspn function

Synopsis

1

         #include <string.h>
         size_t strspn(const char *s1, const char *s2);

Description

2 The strspn function computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters from the string pointed to by s2.

Returns

3 The strspn function returns the length of the segment.

Contents

7.24.5.7 The strstr function

Synopsis

1

         #include <string.h>
         char *strstr(const char *s1, const char *s2);

Description

2 The strstr function locates the first occurrence in the string pointed to by s1 of the sequence of characters (excluding the terminating null character) in the string pointed to by s2.

Returns

3 The strstr function returns a pointer to the located string, or a null pointer if the string is not found. If s2 points to a string with zero length, the function returns s1.

Contents

7.24.5.8 The strtok function

Synopsis

1

         #include <string.h>
         char *strtok(char * restrict s1,
              const char * restrict s2);

Description

2 A sequence of calls to the strtok function breaks the string pointed to by s1 into a sequence of tokens, each of which is delimited by a character from the string pointed to by s2. The first call in the sequence has a non-null first argument; subsequent calls in the sequence have a null first argument. The separator string pointed to by s2 may be different from call to call.

3 The first call in the sequence searches the string pointed to by s1 for the first character that is not contained in the current separator string pointed to by s2. If no such character is found, then there are no tokens in the string pointed to by s1 and the strtok function returns a null pointer. If such a character is found, it is the start of the first token.

4 The strtok function then searches from there for a character that is contained in the current separator string. If no such character is found, the current token extends to the end of the string pointed to by s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is overwritten by a null character, which terminates the current token. The strtok function saves a pointer to the following character, from which the next search for a token will start.

5 Each subsequent call, with a null pointer as the value of the first argument, starts searching from the saved pointer and behaves as described above.

6 The strtok function is not required to avoid data races with other calls to the strtok function.311) The implementation shall behave as if no library function calls the strtok function.

Returns

7 The strtok function returns a pointer to the first character of a token, or a null pointer if there is no token.

8 EXAMPLE

         #include <string.h>
         static char str[] = "?a???b,,,#c";
         char *t;
         t   =   strtok(str, "?");         //   t   points to the token "a"
         t   =   strtok(NULL, ",");        //   t   points to the token "??b"
         t   =   strtok(NULL, "#,");       //   t   points to the token "c"
         t   =   strtok(NULL, "?");        //   t   is a null pointer

Forward references: The strtok_s function (K.3.7.3.1).

Footnotes

311) The strtok_s function can be used instead to avoid data races.

Contents

7.24.6 Miscellaneous functions

Contents

7.24.6.1 The memset function

Synopsis

1

         #include <string.h>
         void *memset(void *s, int c, size_t n);

Description

2 The memset function copies the value of c (converted to an unsigned char) into each of the first n characters of the object pointed to by s.

Returns

3 The memset function returns the value of s.

Contents

7.24.6.2 The strerror function

Synopsis

1

         #include <string.h>
         char *strerror(int errnum);

Description

2 The strerror function maps the number in errnum to a message string. Typically, the values for errnum come from errno, but strerror shall map any value of type int to a message.

3 The strerror function is not required to avoid data races with other calls to the strerror function.312) The implementation shall behave as if no library function calls the strerror function.

Returns

4 The strerror function returns a pointer to the string, the contents of which are locale- specific. The array pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the strerror function.

Forward references: The strerror_s function (K.3.7.4.2).

Footnotes

312) The strerror_s function can be used instead to avoid data races.

Contents

7.24.6.3 The strlen function

Synopsis

1

        #include <string.h>
        size_t strlen(const char *s);

Description

2 The strlen function computes the length of the string pointed to by s.

Returns

3 The strlen function returns the number of characters that precede the terminating null character.

Contents

7.25 Type-generic math <tgmath.h>

1 The header <tgmath.h> includes the headers <math.h> and <complex.h> and defines several type-generic macros.

2 Of the <math.h> and <complex.h> functions without an f (float) or l (long double) suffix, several have one or more parameters whose corresponding real type is double. For each such function, except modf, there is a corresponding type-generic macro.313) The parameters whose corresponding real type is double in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters.314)

3 Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:

4 For each unsuffixed function in <math.h> for which there is a function in <complex.h> with the same name except for a c prefix, the corresponding type- generic macro (for both functions) has the same name as the function in <math.h>. The corresponding type-generic macro for fabs and cabs is fabs.

         <math.h>          <complex.h>              type-generic
          function            function                 macro

          acos               cacos                    acos
          asin               casin                    asin
          atan               catan                    atan
          acosh              cacosh                   acosh
          asinh              casinh                   asinh
          atanh              catanh                   atanh
          cos                ccos                     cos
          sin                csin                     sin
          tan                ctan                     tan
          cosh               ccosh                    cosh
          sinh               csinh                    sinh
          tanh               ctanh                    tanh
          exp                cexp                     exp
          log                clog                     log
          pow                cpow                     pow
          sqrt               csqrt                    sqrt
          fabs               cabs                     fabs
If at least one argument for a generic parameter is complex, then use of the macro invokes a complex function; otherwise, use of the macro invokes a real function.

5 For each unsuffixed function in <math.h> without a c-prefixed counterpart in <complex.h> (except modf), the corresponding type-generic macro has the same name as the function. These type-generic macros are:

       atan2                fma                  llround              remainder
       cbrt                 fmax                 log10                remquo
       ceil                 fmin                 log1p                rint
       copysign             fmod                 log2                 round
       erf                  frexp                logb                 scalbn
       erfc                 hypot                lrint                scalbln
       exp2                 ilogb                lround               tgamma
       expm1                ldexp                nearbyint            trunc
       fdim                 lgamma               nextafter
       floor                llrint               nexttoward
If all arguments for generic parameters are real, then use of the macro invokes a real function; otherwise, use of the macro results in undefined behavior.

6 For each unsuffixed function in <complex.h> that is not a c-prefixed counterpart to a function in <math.h>, the corresponding type-generic macro has the same name as the function. These type-generic macros are:

         carg                    conj                     creal
         cimag                   cproj
Use of the macro with any real or complex argument invokes a complex function.

7 EXAMPLE With the declarations

         #include <tgmath.h>
         int n;
         float f;
         double d;
         long double ld;
         float complex fc;
         double complex dc;
         long double complex ldc;
functions invoked by use of type-generic macros are shown in the following table:
                  macro use                                  invokes

             exp(n)                              exp(n), the function
             acosh(f)                            acoshf(f)
             sin(d)                              sin(d), the function
             atan(ld)                            atanl(ld)
             log(fc)                             clogf(fc)
             sqrt(dc)                            csqrt(dc)
             pow(ldc, f)                         cpowl(ldc, f)
             remainder(n, n)                     remainder(n, n), the function
             nextafter(d, f)                     nextafter(d, f), the function
             nexttoward(f, ld)                   nexttowardf(f, ld)
             copysign(n, ld)                     copysignl(n, ld)
             ceil(fc)                            undefined behavior
             rint(dc)                            undefined behavior
             fmax(ldc, ld)                       undefined behavior
             carg(n)                             carg(n), the function
             cproj(f)                            cprojf(f)
             creal(d)                            creal(d), the function
             cimag(ld)                           cimagl(ld)
             fabs(fc)                            cabsf(fc)
             carg(dc)                            carg(dc), the function
             cproj(ldc)                          cprojl(ldc)

Footnotes

313) Like other function-like macros in Standard libraries, each type-generic macro can be suppressed to make available the corresponding ordinary function.

314) If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is undefined.

Contents

7.26 Threads <threads.h>

Contents

7.26.1 Introduction

1 The header <threads.h> includes the header <time.h>, defines macros, and declares types, enumeration constants, and functions that support multiple threads of execution.315)

2 Implementations that define the macro __STDC_NO_THREADS__ need not provide this header nor support any of its facilities.

3 The macros are

          thread_local
which expands to _Thread_local;
          ONCE_FLAG_INIT
which expands to a value that can be used to initialize an object of type once_flag; and
          TSS_DTOR_ITERATIONS
which expands to an integer constant expression representing the maximum number of times that destructors will be called when a thread terminates.

4 The types are

          cnd_t
which is a complete object type that holds an identifier for a condition variable;
          thrd_t
which is a complete object type that holds an identifier for a thread;
          tss_t
which is a complete object type that holds an identifier for a thread-specific storage pointer;
          mtx_t
which is a complete object type that holds an identifier for a mutex;
          tss_dtor_t
which is the function pointer type void (*)(void*), used for a destructor for a thread-specific storage pointer;
         thrd_start_t
which is the function pointer type int (*)(void*) that is passed to thrd_create to create a new thread; and
           once_flag
which is a complete object type that holds a flag for use by call_once.

5 The enumeration constants are

         mtx_plain
which is passed to mtx_init to create a mutex object that supports neither timeout nor test and return;
           mtx_recursive
which is passed to mtx_init to create a mutex object that supports recursive locking;
           mtx_timed
which is passed to mtx_init to create a mutex object that supports timeout;
         thrd_timedout
which is returned by a timed wait function to indicate that the time specified in the call was reached without acquiring the requested resource;
         thrd_success
which is returned by a function to indicate that the requested operation succeeded;
         thrd_busy
which is returned by a function to indicate that the requested operation failed because a resource requested by a test and return function is already in use;
         thrd_error
which is returned by a function to indicate that the requested operation failed; and
         thrd_nomem
which is returned by a function to indicate that the requested operation failed because it was unable to allocate memory.

Forward references: date and time (7.27).

Footnotes

315) See ''future library directions'' (7.31.15).

Contents

7.26.2 Initialization functions

Contents

7.26.2.1 The call_once function

Synopsis

1

        #include <threads.h>
        void call_once(once_flag *flag, void (*func)(void));

Description

2 The call_once function uses the once_flag pointed to by flag to ensure that func is called exactly once, the first time the call_once function is called with that value of flag. Completion of an effective call to the call_once function synchronizes with all subsequent calls to the call_once function with the same value of flag.

Returns

3 The call_once function returns no value.

Contents

7.26.3 Condition variable functions

Contents

7.26.3.1 The cnd_broadcast function

Synopsis

1

        #include <threads.h>
        int cnd_broadcast(cnd_t *cond);

Description

2 The cnd_broadcast function unblocks all of the threads that are blocked on the condition variable pointed to by cond at the time of the call. If no threads are blocked on the condition variable pointed to by cond at the time of the call, the function does nothing.

Returns

3 The cnd_broadcast function returns thrd_success on success, or thrd_error if the request could not be honored.

Contents

7.26.3.2 The cnd_destroy function

Synopsis

1

        #include <threads.h>
        void cnd_destroy(cnd_t *cond);

Description

2 The cnd_destroy function releases all resources used by the condition variable pointed to by cond. The cnd_destroy function requires that no threads be blocked waiting for the condition variable pointed to by cond.

Returns

3 The cnd_destroy function returns no value.

Contents

7.26.3.3 The cnd_init function

Synopsis

1

         #include <threads.h>
         int cnd_init(cnd_t *cond);

Description

2 The cnd_init function creates a condition variable. If it succeeds it sets the variable pointed to by cond to a value that uniquely identifies the newly created condition variable. A thread that calls cnd_wait on a newly created condition variable will block.

Returns

3 The cnd_init function returns thrd_success on success, or thrd_nomem if no memory could be allocated for the newly created condition, or thrd_error if the request could not be honored.

Contents

7.26.3.4 The cnd_signal function

Synopsis

1

         #include <threads.h>
         int cnd_signal(cnd_t *cond);

Description

2 The cnd_signal function unblocks one of the threads that are blocked on the condition variable pointed to by cond at the time of the call. If no threads are blocked on the condition variable at the time of the call, the function does nothing and return success.

Returns

3 The cnd_signal function returns thrd_success on success or thrd_error if the request could not be honored.

Contents

7.26.3.5 The cnd_timedwait function

Synopsis

1

         #include <threads.h>
         int cnd_timedwait(cnd_t *restrict cond,
              mtx_t *restrict mtx,
              const struct timespec *restrict ts);

Description

2 The cnd_timedwait function atomically unlocks the mutex pointed to by mtx and endeavors to block until the condition variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast, or until after the TIME_UTC-based calendar time pointed to by ts. When the calling thread becomes unblocked it locks the variable pointed to by mtx before it returns. The cnd_timedwait function requires that the mutex pointed to by mtx be locked by the calling thread.

Returns

3 The cnd_timedwait function returns thrd_success upon success, or thrd_timedout if the time specified in the call was reached without acquiring the requested resource, or thrd_error if the request could not be honored.

Contents

7.26.3.6 The cnd_wait function

Synopsis

1

        #include <threads.h>
        int cnd_wait(cnd_t *cond, mtx_t *mtx);

Description

2 The cnd_wait function atomically unlocks the mutex pointed to by mtx and endeavors to block until the condition variable pointed to by cond is signaled by a call to cnd_signal or to cnd_broadcast. When the calling thread becomes unblocked it locks the mutex pointed to by mtx before it returns. The cnd_wait function requires that the mutex pointed to by mtx be locked by the calling thread.

Returns

3 The cnd_wait function returns thrd_success on success or thrd_error if the request could not be honored.

Contents

7.26.4 Mutex functions

Contents

7.26.4.1 The mtx_destroy function

Synopsis

1

        #include <threads.h>
        void mtx_destroy(mtx_t *mtx);

Description

2 The mtx_destroy function releases any resources used by the mutex pointed to by mtx. No threads can be blocked waiting for the mutex pointed to by mtx.

Returns

3 The mtx_destroy function returns no value.

Contents

7.26.4.2 The mtx_init function

Synopsis

1

         #include <threads.h>
         int mtx_init(mtx_t *mtx, int type);

Description

2 The mtx_init function creates a mutex object with properties indicated by type, which must have one of the six values:

mtx_plain
for a simple non-recursive mutex,
mtx_timed
for a non-recursive mutex that supports timeout,
mtx_plain | mtx_recursive
for a simple recursive mutex, or
mtx_timed | mtx_recursive
for a recursive mutex that supports timeout.

3 If the mtx_init function succeeds, it sets the mutex pointed to by mtx to a value that uniquely identifies the newly created mutex.

Returns

4 The mtx_init function returns thrd_success on success, or thrd_error if the request could not be honored.

Contents

7.26.4.3 The mtx_lock function

Synopsis

1

         #include <threads.h>
         int mtx_lock(mtx_t *mtx);

Description

2 The mtx_lock function blocks until it locks the mutex pointed to by mtx. If the mutex is non-recursive, it shall not be locked by the calling thread. Prior calls to mtx_unlock on the same mutex shall synchronize with this operation.

Returns

3 The mtx_lock function returns thrd_success on success, or thrd_error if the request could not be honored.

Contents

7.26.4.4 The mtx_timedlock function

Synopsis

1

         #include <threads.h>
         int mtx_timedlock(mtx_t *restrict mtx,
              const struct timespec *restrict ts);

Description

2 The mtx_timedlock function endeavors to block until it locks the mutex pointed to by mtx or until after the TIME_UTC-based calendar time pointed to by ts. The specified mutex shall support timeout. If the operation succeeds, prior calls to mtx_unlock on the same mutex shall synchronize with this operation.

Returns

3 The mtx_timedlock function returns thrd_success on success, or thrd_timedout if the time specified was reached without acquiring the requested resource, or thrd_error if the request could not be honored.

Contents

7.26.4.5 The mtx_trylock function

Synopsis

1

        #include <threads.h>
        int mtx_trylock(mtx_t *mtx);

Description

2 The mtx_trylock function endeavors to lock the mutex pointed to by mtx. If the mutex is already locked, the function returns without blocking. If the operation succeeds, prior calls to mtx_unlock on the same mutex shall synchronize with this operation.

Returns

3 The mtx_trylock function returns thrd_success on success, or thrd_busy if the resource requested is already in use, or thrd_error if the request could not be honored.

Contents

7.26.4.6 The mtx_unlock function

Synopsis

1

        #include <threads.h>
        int mtx_unlock(mtx_t *mtx);

Description

2 The mtx_unlock function unlocks the mutex pointed to by mtx. The mutex pointed to by mtx shall be locked by the calling thread.

Returns

3 The mtx_unlock function returns thrd_success on success or thrd_error if the request could not be honored.

Contents

7.26.5 Thread functions

Contents

7.26.5.1 The thrd_create function

Synopsis

1

         #include <threads.h>
         int thrd_create(thrd_t *thr, thrd_start_t func,
              void *arg);

Description

2 The thrd_create function creates a new thread executing func(arg). If the thrd_create function succeeds, it sets the object pointed to by thr to the identifier of the newly created thread. (A thread's identifier may be reused for a different thread once the original thread has exited and either been detached or joined to another thread.) The completion of the thrd_create function synchronizes with the beginning of the execution of the new thread.

Returns

3 The thrd_create function returns thrd_success on success, or thrd_nomem if no memory could be allocated for the thread requested, or thrd_error if the request could not be honored.

Contents

7.26.5.2 The thrd_current function

Synopsis

1

         #include <threads.h>
         thrd_t thrd_current(void);

Description

2 The thrd_current function identifies the thread that called it.

Returns

3 The thrd_current function returns the identifier of the thread that called it.

Contents

7.26.5.3 The thrd_detach function

Synopsis

1

         #include <threads.h>
         int thrd_detach(thrd_t thr);

Description

2 The thrd_detach function tells the operating system to dispose of any resources allocated to the thread identified by thr when that thread terminates. The thread identified by thr shall not have been previously detached or joined with another thread.

Returns

3 The thrd_detach function returns thrd_success on success or thrd_error if the request could not be honored.

Contents

7.26.5.4 The thrd_equal function

Synopsis

1

        #include <threads.h>
        int thrd_equal(thrd_t thr0, thrd_t thr1);

Description

2 The thrd_equal function will determine whether the thread identified by thr0 refers to the thread identified by thr1.

Returns

3 The thrd_equal function returns zero if the thread thr0 and the thread thr1 refer to different threads. Otherwise the thrd_equal function returns a nonzero value.

Contents

7.26.5.5 The thrd_exit function

Synopsis

1

        #include <threads.h>
        _Noreturn void thrd_exit(int res);

Description

2 The thrd_exit function terminates execution of the calling thread and sets its result code to res.

3 The program shall terminate normally after the last thread has been terminated. The behavior shall be as if the program called the exit function with the status EXIT_SUCCESS at thread termination time.

Returns

4 The thrd_exit function returns no value.

Contents

7.26.5.6 The thrd_join function

Synopsis

1

        #include <threads.h>
        int thrd_join(thrd_t thr, int *res);

Description

2 The thrd_join function joins the thread identified by thr with the current thread by blocking until the other thread has terminated. If the parameter res is not a null pointer, it stores the thread's result code in the integer pointed to by res. The termination of the other thread synchronizes with the completion of the thrd_join function. The thread identified by thr shall not have been previously detached or joined with another thread.

Returns

3 The thrd_join function returns thrd_success on success or thrd_error if the request could not be honored.

Contents

7.26.5.7 The thrd_sleep function

Synopsis

1

         #include <threads.h>
         int thrd_sleep(const struct timespec *duration,
              struct timespec *remaining);

Description

2 The thrd_sleep function suspends execution of the calling thread until either the interval spec