Reported-by: "G. Branden Robinson" <g.branden.robinson@xxxxxxxxx> Cc: Ralph Corderoy <ralph@xxxxxxxxxxxxxxx> Cc: Ingo Schwarze <schwarze@xxxxxxx> Cc: Jakub Wilk <jwilk@xxxxxxxxx> Signed-off-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> --- v2: - Move to man3const [Ralph, Branden] - Added LIBRARY section - Added #include [Ralph] - Note that it can also be used as a function pointer [Ralph] - Document that 0 is another null pointer constant [Ralph] But note that it's to be avoided by most coding standards [alx] - Note that NULL is not NUL - Improve wording about zeroing a pointer [Ralph] And refer to getaddrinfo(3) for an example. This probably can be further improved; I'm not convinced. - Trim SEE ALSO to just void(3type) - Other minor fixes v3: - Don't boldface 0s, since it doesn't refer to the literal constant 0, but to the bit pattern of 0s. - Add list of headers that also define NULL (per POSIX.1-2008). v4: - Remove details about POSIX defining NULL as (void*)0. [Ingo] All Unix systems already define it that way, so it's irrelevant for us that ISO C or old versions of POSIX didn't define it that way. - Reword the remaining DESCRIPTION [Ingo] - Move a big part of NOTES into a new CAVEATS section [Ingo] NOTES is a generic "doesn't fit elsewhere" section. Those things fitted very well a CAVEATS section. - Simplify mention of 0 as a null pointer constant, and change it to make clear that it's a thing to avoid. [Ingo] - Keep extra headers in NOTES, as it's a thing that few readers will be interested in. - Reworded a few things. [Ingo] man3const/NULL.3const | 76 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 76 insertions(+) create mode 100644 man3const/NULL.3const diff --git a/man3const/NULL.3const b/man3const/NULL.3const new file mode 100644 index 000000000..28a6f67aa --- /dev/null +++ b/man3const/NULL.3const @@ -0,0 +1,76 @@ +.\" Copyright (c) 2022 by Alejandro Colomar <colomar.6.4.3@xxxxxxxxx> +.\" +.\" 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 arithmetics 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 +would represent a null pointer. +Instead, pointer variables need to be explicitly set to a null pointer +when they need to hold a null pointer. +See the EXAMPLES section in +.BR getaddrinfo (3) +for an example program that does this. +.SH SEE ALSO +.BR void (3type) -- 2.36.1
