unistd —
standard symbolic constants and types
The <unistd.h> header forms the
  basis of the Portable Operating System Interface for Unix (POSIX) API. It
  includes definitions for numerous functions, symbolic constants, and types.
  Among these are:
  - The implementation-defined NULLpointer
    constant.
- Types such as size_t, ssize_t,
      pid_t, and intptr_t.
- Various limits and other symbolic constants described in
      pathconf(2) and
      sysconf(3).
- Prototypes for several important system calls and library routines such as
      chown(2),
      dup(2),
      execl(3),
      fork(2),
      pipe(2),
      rmdir(2),
      setuid(2), and
      write(2).
- The symbolic constant _POSIX_VERSIONthat defines
      the version of the POSIX standard to which the implementation
    conforms.
- Various other _POSIX-prefixed symbolic constants
      that indicate whether the system implements some optional part of the
      standard. Examples include such constants as_POSIX_IPV6for
      inet6(4) functionality and_POSIX_ASYNCHRONOUS_IOfor
      aio(3).
The<unistd.h> header conforms
  to IEEE Std 1003.1-2001 (“POSIX.1”).