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| author | Alejandro Colomar <alx.manpages@gmail.com> | 2022-08-20 18:57:51 +0200 |
|---|---|---|
| committer | Alejandro Colomar <alx.manpages@gmail.com> | 2022-08-21 14:29:13 +0200 |
| commit | 112770f9ecd7d68762a5a81fd16a7d3f9fb10374 (patch) | |
| tree | 0e8ac2b982c9a7b78ec7b8bda5a61bf88bcf3fac | |
| parent | 9859afefcd41901a41a1ef520b6b5c95077333bb (diff) | |
_Generic.3: New page documenting _Generic()
Also add a hint of how intmax(3) and other functions using
[u]intmax_t types could be better defined by ISO C, by requiring
that they're implemented as type-generic macros, to avoid having
problems with the ABI.
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Cc: JeanHeyd Meneide <wg14@soasis.org>
Cc: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Cc: Ingo Schwarze <schwarze@openbsd.de>
| -rw-r--r-- | man3/_Generic.3 | 60 |
1 files changed, 60 insertions, 0 deletions
diff --git a/man3/_Generic.3 b/man3/_Generic.3 new file mode 100644 index 000000000..3e1c159ab --- /dev/null +++ b/man3/_Generic.3 @@ -0,0 +1,60 @@ +.\" Copyright (C) 2022 Alejandro Colomar <alx.manpages@gmail.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH _Generic 3 2022-08-20 "Linux man-pages (unreleased)" "Linux Programmer's Manual" +.SH NAME +_Generic \- type-generic selection +.SH SYNOPSIS +.nf +.BR _Generic( \fIexpression\fP ", type1: " e1 ", " "... /*" \ +", default: " "e */" ); +.fi +.SH DESCRIPTION +.BR _Generic () +evaluates the path of code under the type selector +that is compatible with the type of the controlling +.IR expression , +or +.B default: +if no type is compatible. +.PP +.I expression +is not evaluated. +.PP +This is especially useful for writing type-generic macros, +that will behave differently depending on the type of the argument. +.SH STANDARDS +C11 and later. +.SH EXAMPLES +The following program demonstrates how to write +a replacement for the standard +.BR imaxabs (3) +function, which being a function can't really provide what it promises: +seamlessly upgrading to the widest available type. +.PP +.\" SRC BEGIN (_Generic.c) +.EX +#include <stdint.h> +#include <stdio.h> +#include <stdlib.h> + +#define my_imaxabs(j) _Generic((intmax_t) 0, \e + int: abs(j), \e + long: labs(j), \e + long long: llabs(j) \e + /* long long long: lllabs(j) */ \e +) + +int +main(void) +{ + off_t a; + + a = \-42; + printf("imaxabs(%jd) == %jd\en", (intmax_t) a, my_imaxabs(a)); + + exit(EXIT_SUCCESS); +} +.EE +.\" SRC END |