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| author | Alejandro Colomar <alx.manpages@gmail.com> | 2022-07-22 17:29:48 +0200 |
|---|---|---|
| committer | Alejandro Colomar <alx.manpages@gmail.com> | 2022-09-05 00:47:18 +0200 |
| commit | bea5dac263377600c0a5b5fbf89f2b84a4186acb (patch) | |
| tree | f5258636645ce57bb7ef084acbc2c5ca0bab6f6b | |
| parent | 23461f5347cb564812b2a1eedac8a16a87257fe2 (diff) | |
NULL.3const: Add documentation for NULL
Reported-by: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Cc: Ralph Corderoy <ralph@inputplus.co.uk>
Cc: Ingo Schwarze <schwarze@usta.de>
Cc: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
| -rw-r--r-- | man3const/NULL.3const | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/man3const/NULL.3const b/man3const/NULL.3const new file mode 100644 index 000000000..44dda1d64 --- /dev/null +++ b/man3const/NULL.3const @@ -0,0 +1,74 @@ +.\" Copyright (c) 2022 by Alejandro Colomar <colomar.6.4.3@gmail.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" +.TH NULL 3const 2022-07-22 Linux "Linux Programmer's Manual" +.SH NAME +NULL \- null pointer constant +.SH LIBRARY +Standard C library +.RI ( libc ) +.SH SYNOPSIS +.nf +.B #include <stddef.h> +.PP +.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 CONFORMING TO +C99 and later; +POSIX.1-2001 and later. +.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. +.PP +It is also undefined behavior to perform pointer arithmetic on it. +.PP +.I NULL \- NULL +is undefined behavior, according to ISO C, but is defined to be 0 in C++. +.PP +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 . +.PP +.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) |