NULL

Section: Misc. Reference Manual Pages (3const)
Updated: 2023-02-05
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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.

CONFORMING TO

C99 and later; POSIX.1-2001 and later.

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.

This document was created by man2html, using the manual pages.
Time: 18:09:15 GMT, May 18, 2024