Manual browser: unistd(3)

UNISTD(3) Library Functions Manual UNISTD(3)


unistdstandard symbolic constants and types


#include <unistd.h>


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 NULL pointer 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_VERSION that 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_IPV6 for inet6(4) functionality and _POSIX_ASYNCHRONOUS_IO for aio(3).


The <unistd.h> header conforms to IEEE Std 1003.1-2001 (“POSIX.1”).
August 9, 2011 NetBSD 7.0