NULL(3const) — Linux manual page

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)
       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) 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         top

       void(3type)

COLOPHON         top

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Linux man-pages 6.15            2025-05-17                   NULL(3const)

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