NULL(3const) — Linux manual page

NULL(3const)                                                NULL(3const)

NAME

       NULL - null pointer constant

LIBRARY

       Standard C library (libc)

SYNOPSIS

       #include <stddef.h>

       #define NULL  ((void *) 0)

DESCRIPTION

       NULL represents a null pointer constant, that is, a pointer that
       does not point to anything.

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       C89, POSIX.1-2001.

NOTES

       The following headers also provide NULL: <locale.h>, <stdio.h>,
       <stdlib.h>, <string.h>, <time.h>, <unistd.h>, and <wchar.h>.

CAVEATS

       It is undefined behavior to dereference a null pointer, and that
       usually causes a segmentation fault in practice.

       It is also undefined behavior to perform pointer arithmetic on
       it.

       NULL - NULL is undefined behavior, according to ISO C, but is
       defined to be 0 in C++.

       To avoid confusing human readers of the code, do not compare
       pointer variables to 0, and do not assign 0 to them.  Instead,
       always use NULL.

       NULL shouldn't be confused with NUL, which is an ascii(7)
       character, represented in C as '\0'.

BUGS

       When it is necessary to set a pointer variable to a null pointer,
       it is not enough to use memset(3) to zero the pointer (this is
       usually done when zeroing a struct that contains pointers), since
       ISO C and POSIX don't guarantee that a bit pattern of all 0s
       represent a null pointer.  See the EXAMPLES section in
       getaddrinfo(3) for an example program that does this correctly.

SEE ALSO

       void(3type)

COLOPHON

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

Linux man-pages 6.9.1          2024-06-15                   NULL(3const)