wcrtomb should be declared in <wchar.h> not in <stdio.h>
see
http://pubs.opengroup.org/onlinepubs/9699919799/functions/wcrtomb.html
vs
http://repo.or.cz/w/c-standard.git/blob_plain/HEAD:/n1256.html#7.24.6.3.3

pthread_kill(invalid_value, sig); required to return ESRCH which is not possible
http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_kill.html

restrict in wcpncpy prototype is at the wrong place
wchar_t *wcpncpy(wchar_t restrict *ws1, const wchar_t *restrict ws2, size_t n);

http://pubs.opengroup.org/onlinepubs/9699919799/functions/dirname.html
dirname of "usr/" is not specified clearly (it could be "" or "." based on the specs)

INET_ADDRSTRLEN etc is a symbolic constant not a macro
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/arpa_inet.h.html

in pthread the usual text is used:
"The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided."
pthread_cleanup_[push|pop] should not need a prototype
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html

PTHREAD_CANCELED symbolic const should be marked as void* (otherwise it's assumed to be int)
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html

"..to access an actual object, or a progrm defines an identifier with the name.."
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html

cx option in unistd.h
[CX][Option Start]
char        *ctermid(char *);
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html

gets "get a string from a .I stdin stream"
http://pubs.opengroup.org/onlinepubs/9699919799/functions/gets.html

quo is not specified when the return value is nan only if y is zero
http://pubs.opengroup.org/onlinepubs/9699919799/functions/remquo.html

"If x is 0.0, -HUGE_VAL shall be returned" is specified for y0,y1,yn
but yn(1,x) == -yn(-1,x) so this is incorrect mathematically
http://pubs.opengroup.org/onlinepubs/9699919799/functions/yn.html

what if syslog fails (bad printf args..)
http://pubs.opengroup.org/onlinepubs/9699919799/functions/syslog.html

duplocale example typo: '}'

fputc('x',stdin) should be explicitly ub

unspecified: owner thread of locked recursive/errorchecking mutex exits,
then another thread with same thread id locks it again

scandir: cmp type (qsort cannot be used), non-total order?

pow: iso c cases:
 pow((+-)0, y) returns (+-)(inf) and raises the ''divide-by-zero'' floating-point exception for y an odd integer < 0.
 pow((+-)0, y) returns +(inf) and raises the ''divide-by-zero'' floating-point exception for y < 0 and not an odd integer.
"For y < 0, if x is zero, .. +-HUGE_VAL .." should be "For y < 0, if x is +-0,"