.\" Copyright (c) 2022 by Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\"
.TH NULL 3const (date) "Linux man-pages (unreleased)"
.SH NAME
NULL \- null pointer constant
.SH LIBRARY
Standard C library
.RI ( libc )
.SH SYNOPSIS
.nf
.B #include <stddef.h>
.P
.B "#define NULL ((void *) 0)"
.fi
.SH DESCRIPTION
.B NULL
represents a null pointer constant,
that is, a pointer that does not point to anything.
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C89, POSIX.1-2001.
.SH NOTES
The following headers also provide
.BR NULL :
.IR <locale.h> ,
.IR <stdio.h> ,
.IR <stdlib.h> ,
.IR <string.h> ,
.IR <time.h> ,
.IR <unistd.h> ,
and
.IR <wchar.h> .
.SH CAVEATS
It is undefined behavior to dereference a null pointer,
and that usually causes a segmentation fault in practice.
.P
It is also undefined behavior to perform pointer arithmetic on it.
.P
.I NULL \- NULL
is undefined behavior, according to ISO C, but is defined to be 0 in C++.
.P
To avoid confusing human readers of the code,
do not compare pointer variables to
.BR 0 ,
and do not assign
.B 0
to them.
Instead, always use
.BR NULL .
.P
.B NULL
shouldn't be confused with
.BR NUL ,
which is an
.BR ascii (7)
character,
represented in C as
.BR \[aq]\e0\[aq] .
.SH BUGS
When it is necessary to set a pointer variable to a null pointer,
it is not enough to use
.BR 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
.BR getaddrinfo (3)
for an example program that does this correctly.
.SH SEE ALSO
.BR void (3type)
