The following functions are compatible with POSIX.1:
fork, execl, execle, execlp, execv, execve, execvp, wait, waitpid, _exit, kill, sigemptyset, sigfillset, sigaddset, sigdelset, sigismember, sigaction, pthread_sigmask, sigprocmask, sigpending, sigsuspend, alarm, pause, sleep, pthread_kill, pthread_sigmask
getpid, getppid, getuid, geteuid, getgid, getegid, setuid, setgid, getgroups, getlogin, getpgrp, setsid, setpgid, uname, time, times, getenv, ctermid, ttyname, isatty, sysconf
opendir, readdir, rewinddir, closedir, chdir, getcwd, open, creat, umask, link, mkdir, unlink, rmdir, rename, stat, fstat, access, chmod, fchmod, chown, utime, ftruncate, pathconf, fpathconf
pipe, dup, dup2, close, read, write, fcntl, lseek, fsync
cfgetispeed, cfgetospeed, cfsetispeed, cfsetospeed, tcdrain, tcflow, tcflush, tcgetattr, tcgetpgrp, tcsendbreak, tcsetattr, tcsetpgrp
abort, exit, fclose, fdopen, fflush, fgetc, fgets, fileno, fopen, fprintf, fputc, fputs, fread, freopen, fscanf, fseek, ftell, fwrite, getc, getchar, gets, perror, printf, putc, putchar, puts, remove, rewind, scanf, setlocale, siglongjmp, sigsetjmp, tmpfile, tmpnam, tzset
getgrgid, getgrnam, getpwnam, getpwuid
sem_init, sem_destroy, sem_wait, sem_trywait, sem_post, pthread_mutex_init, pthread_mutex_destroy, pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock
mmap, mprotect, msync, munmap
pthread_attr_init, pthread_attr_destroy, pthread_attr_setstacksize, pthread_attr_getstacksize, pthread_create, pthread_exit, pthread_self, pthread_equal
pthread_key_create, pthread_setspecific, pthread_getspecific, pthread_key_delete
setuid and setgid always return ENOSYS.
link will copy the file if it can't implement a true symbolic link. Currently, symbolic links work, if at all, only under Windows NT.
chown always returns zero.
fcntl doesn't support F_GETLK - it returns -1 and sets errno to ENOSYS.
lseek only works properly on binary files.