NAME

_Generic - type-generic selection

SYNOPSIS

_Generic(expression, type1: e1, ... /*, default: e */);

DESCRIPTION

_Generic() evaluates the path of code under the type selector that is compatible with the type of the controlling expression, or default: if no type is compatible.

expression is not evaluated.

This is especially useful for writing type-generic macros, that will behave differently depending on the type of the argument.

STANDARDS

C11 and later.

EXAMPLES

The following code demonstrates how to write a macro similar to C++'s 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 bind(2) with sockaddr(3type).

/* This code is in the public domain. */
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/un.h>
#define sockaddr_cast(t, p)                            \

    _Generic(&*(p),                                    \

    struct sockaddr *:                                 \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr_in *:             (t) (p),     \

        struct sockaddr_in6 *:            (t) (p),     \

        struct sockaddr_un *:             (t) (p),     \

        default:                              (p)),    \

    struct sockaddr **:                                \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr_in **:            (t) (p),     \

        struct sockaddr_in6 **:           (t) (p),     \

        struct sockaddr_un **:            (t) (p),     \

        default:                              (p)),    \

    const struct sockaddr *:                           \

        _Generic((t) NULL,                             \

        const struct sockaddr_in *:       (t) (p),     \

        const struct sockaddr_in6 *:      (t) (p),     \

        const struct sockaddr_un *:       (t) (p),     \

        default:                              (p)),    \

                                                       \

    struct sockaddr_in *:                              \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr *:                (t) (p),     \

        default:                              (p)),    \

    struct sockaddr_in **:                             \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr **:               (t) (p),     \

        default:                              (p)),    \

    const struct sockaddr_in *:                        \

        _Generic((t) NULL,                             \

        const struct sockaddr *:          (t) (p),     \

        default:                              (p)),    \

                                                       \

    struct sockaddr_in6 *:                             \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr *:                (t) (p),     \

        default:                              (p)),    \

    struct sockaddr_in6 **:                            \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr **:               (t) (p),     \

        default:                              (p)),    \

    const struct sockaddr_in6 *:                       \

        _Generic((t) NULL,                             \

        const struct sockaddr *:          (t) (p),     \

        default:                              (p)),    \

                                                       \

    struct sockaddr_un *:                              \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr *:                (t) (p),     \

        default:                              (p)),    \

    struct sockaddr_un **:                             \

        _Generic((typeof_unqual(t)) NULL,              \

        struct sockaddr **:               (t) (p),     \

        default:                              (p)),    \

    const struct sockaddr_un *:                        \

        _Generic((t) NULL,                             \

        const struct sockaddr *:          (t) (p),     \

        default:                              (p)),    \

                                                       \

    default:                                           \

        (p)                                            \

    )
socklen_t           slen;
struct sockaddr_un  sun;
slen = sizeof(ss);
getsockname(sfd, sockaddr_cast(struct sockaddr *, &sun), &slen);
    

The following program demonstrates how to write a replacement for the standard imaxabs(3) function, which being a function can't really provide what it promises: seamlessly upgrading to the widest available type.

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define my_imaxabs  _Generic(INTMAX_C(0),  \

    long:           labs,                  \

    long long:      llabs                  \

 /* long long long: lllabs */              \
)
int
main(void)
{

    off_t  a;

    a = -42;

    printf("imaxabs(%jd) == %jd\n", (intmax_t) a, my_imaxabs(a));

    printf("&imaxabs == %p\n", &my_imaxabs);

    printf("&labs    == %p\n", &labs);

    printf("&llabs   == %p\n", &llabs);

    exit(EXIT_SUCCESS);
}