NULL(3const) - Linux manual page (original) (raw)

NULL(3const) NULL(3const)

NAME top

   NULL - null pointer constant

LIBRARY top

   Standard C library (_libc_)

SYNOPSIS top

   **#include <stddef.h>**

   **#define NULL  ((void *) 0)**

DESCRIPTION top

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

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   C89, POSIX.1-2001.

NOTES top

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

CAVEATS top

   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)](../man7/ascii.7.html)
   character, represented in C as **'\0'**.

BUGS top

   When it is necessary to set a pointer variable to a null pointer,
   it is not enough to use [memset(3)](../man3/memset.3.html) 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)](../man3/getaddrinfo.3.html) for an example program that does this correctly.

SEE ALSO top

   [void(3type)](../man3/void.3type.html)

COLOPHON top

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Linux man-pages 6.10 2024-06-15 NULL(3const)