_Generic - Man Page

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

_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.

C11.

C11.

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);
}