Reported-by: "G. Branden Robinson" <g.branden.robinson@xxxxxxxxx> Cc: Ralph Corderoy <ralph@xxxxxxxxxxxxxxx> Cc: Ingo Schwarze <schwarze@xxxxxxx> 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 man3const/NULL.3const | 70 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 70 insertions(+) create mode 100644 man3const/NULL.3const diff --git a/man3const/NULL.3const b/man3const/NULL.3const new file mode 100644 index 000000000..1b0cb7948 --- /dev/null +++ b/man3const/NULL.3const @@ -0,0 +1,70 @@ +.\" 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. +.PP +According to POSIX, +it shall expand to an integer constant expression with the value +.B 0 +cast to type +.IR "void *" . +.PP +A null pointer is one that doesn't point to a valid object or function. +.SH CONFORMING TO +C99 and later; +POSIX.1-2001 and later. +.SH NOTES +It is undefined behavior to dereference a null pointer +or to perform pointer arithmetics on it. +.I NULL \- NULL +is, surprisingly, undefined behavior, according to ISO C. +.PP +.B 0 +also represents a null pointer constant +when used in a context where a pointer is expected. +This is considered bad practise by most coding guidelines, +since it can be very confusing, +and so +.B NULL +is preferred. +.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 +.BR 0 s +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
