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.