in pathconf, -1, not 0, means unsupported.. syncio presumably works, too.

[musl] / src / process / posix_spawnattr_setpgroup.c

#include <spawn.h>

int posix_spawnattr_setpgroup(posix_spawnattr_t *attr, pid_t pgrp)
{
        attr->__pgrp = pgrp;
        return 0;
}