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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | STANDARDS | HISTORY | NOTES | CAVEATS | BUGS | SEE ALSO | COLOPHON |
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NULL(3const) NULL(3const)
NULL - null pointer constant
Standard C library (libc)
#include <stddef.h>
#define NULL ((void *) 0)
NULL represents a null pointer constant, that is, a pointer that
does not point to anything.
C11, POSIX.1-2008.
C89, POSIX.1-2001.
The following headers also provide NULL: <locale.h>, <stdio.h>,
<stdlib.h>, <string.h>, <time.h>, <unistd.h>, and <wchar.h>.
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'.
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.
void(3type)
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
[email protected]
Linux man-pages 6.9.1 2024-06-15 NULL(3const)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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