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.\" Copyright (C) 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH _Generic 3 (date) "Linux man-pages (unreleased)"
.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 code demonstrates how to write
a macro similar to C++'s
.BR \%static_cast (),
which will allow casting safely between a limited set of types.
It is useful for example when calling
system calls or library functions that use compatible structures,
like for example
.BR bind (2)
with
.BR \%sockaddr (3type).
.IP
.EX
/* This code is in the public domain. */
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/un.h>
#define sockaddr_cast(t, p) \e
_Generic(&*(p), \e
struct sockaddr *: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr_in *: (t) (p), \e
struct sockaddr_in6 *: (t) (p), \e
struct sockaddr_un *: (t) (p), \e
default: (p)), \e
struct sockaddr **: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr_in **: (t) (p), \e
struct sockaddr_in6 **: (t) (p), \e
struct sockaddr_un **: (t) (p), \e
default: (p)), \e
const struct sockaddr *: \e
_Generic((t) NULL, \e
const struct sockaddr_in *: (t) (p), \e
const struct sockaddr_in6 *: (t) (p), \e
const struct sockaddr_un *: (t) (p), \e
default: (p)), \e
\e
struct sockaddr_in *: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr *: (t) (p), \e
default: (p)), \e
struct sockaddr_in **: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr **: (t) (p), \e
default: (p)), \e
const struct sockaddr_in *: \e
_Generic((t) NULL, \e
const struct sockaddr *: (t) (p), \e
default: (p)), \e
\e
struct sockaddr_in6 *: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr *: (t) (p), \e
default: (p)), \e
struct sockaddr_in6 **: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr **: (t) (p), \e
default: (p)), \e
const struct sockaddr_in6 *: \e
_Generic((t) NULL, \e
const struct sockaddr *: (t) (p), \e
default: (p)), \e
\e
struct sockaddr_un *: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr *: (t) (p), \e
default: (p)), \e
struct sockaddr_un **: \e
_Generic((typeof_unqual(t)) NULL, \e
struct sockaddr **: (t) (p), \e
default: (p)), \e
const struct sockaddr_un *: \e
_Generic((t) NULL, \e
const struct sockaddr *: (t) (p), \e
default: (p)), \e
\e
default: \e
(p) \e
)
socklen_t slen;
struct sockaddr_un sun;
slen = sizeof(ss);
getsockname(sfd, sockaddr_cast(struct sockaddr *, &sun), &slen);
.EE
.PP
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.
.IP
.\" SRC BEGIN (_Generic.c)
.EX
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define my_imaxabs _Generic(INTMAX_C(0), \e
long: labs, \e
long long: llabs \e
/* long long long: lllabs */ \e
)
int
main(void)
{
off_t a;
a = \-42;
printf("imaxabs(%jd) == %jd\en", (intmax_t) a, my_imaxabs(a));
printf("&imaxabs == %p\en", &my_imaxabs);
printf("&labs == %p\en", &labs);
printf("&llabs == %p\en", &llabs);
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
|
