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
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
mmap, mprotect, msync, munmap
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.