c example to override functions using `#define` macros

define-override-function

#include <stdio.h>

int fun0(void) { return 5; }
int fun1(int x) { return x * 2; }

#define OVERRIDE  1

#if OVERRIDE == 1
    #define fun0() (9)
    #define fun1(x) (99)
    
    // To override functions, these `#define funXX` macros
    // must be placed *after* the function definitions overridden
    // regardless of their prototype declarations or not.
#endif

int main(void)
{
    printf("OVERRIDE = %d: fun0 => %d, fun1 => %d\n",
           OVERRIDE, fun0(), fun1(1));
    return 0;
}

/*
OVERRIDE = 0: fun0 => 5, fun1 => 2
OVERRIDE = 1: fun0 => 9, fun1 => 99
 */