c example to override functions using `#define` macros for inline functions

define-override-function-inline.c

#include <stdio.h>

int fun0(void) { return 5; }
int fun1(int x) { return x * 10; }

// inline int fun1_inline(int x) { return x * 100; } // => undefined reference
extern inline int fun1_inline_extern(int x) { return x * 100; }
static inline int fun1_inline_static(int x) { return x * 1000; }

#define OVERRIDE 1

#if OVERRIDE == 1
    #define fun0() (9)
    #define fun1(x) (99)
    #define fun1_inline_extern(x) (999)
    #define fun1_inline_static(x) (9999)
    // To override functions, these `#define funXX` macros
    // must be placed *after* the function definitions overridden
    // regardless of thier prototype declarations or not.
#endif

int main(void)
{
    printf("OVERRIDE = %d: fun0 => %d, fun1 => %d\n",
           OVERRIDE, fun0(), fun1(2));
    printf("OVERRIDE = %d: fun1_inline_extern => %d, fun1_inline_static => %d\n",
           OVERRIDE, fun1_inline_extern(2), fun1_inline_static(2));
    return 0;
}

/*
OVERRIDE = 0: fun0 => 5, fun1 => 20
OVERRIDE = 0: fun1_inline_extern => 200, fun1_inline_static => 2000

OVERRIDE = 1: fun0 => 9, fun1 => 99
OVERRIDE = 1: fun1_inline_extern => 999, fun1_inline_static => 9999
 */