_Generic.3

_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.

       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);
              }

Linux man-pages 6.9.1                              2024-06-15                                        _Generic(3)

       C11.

       _Generic - type-generic selection

       C11.

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