How can I choose function behavior at compile time? - c

Question

I have 4 files in the following example ; a header, a default implementation file, and 2 platform-specific implementation files.I define two functions; get_value_1 and get_value_2. The 'default' behavior is to return -1, but some of these functions have special implementations. I would like each function to return -1 only if another file didn't implement it.
/* interface.h *
***************/
int get_value_1();
int get_value_2();
/* default.c *
*************/
#include "interface.h"
#ifndef GET_VALUE_1
int get_value_1() { return -1; }
#endif
#ifndef GET_VALUE_2
int get_value_2() { return -1; }
#endif
/* platform1.c *
***************/
#include "interface.h"
#ifndef GET_VALUE_1
#define GET_VALUE_1
int get_value_1() { return 1; }
#endif
/* platform2.c *
***************/
#include "interface.h"
#ifndef GET_VALUE_2
#define GET_VALUE_2
int get_value_2() { return 2; }
#endif
But when I run the command gcc default.c platform1.c -shared -fpic -o platform1.so, it tells me that I've multiply defined the get_value_1 function, and that it was originally defined in platform1.c.
So how can I have a set of functions where a subset of those functions can have their behavior chosen at compile time?

Answer 1

make them weak in the default.c and "normal" in your platform files. So if the platform.c file implements this function as not weak, the weak one dfrom default.c will be replaced link time.
It is not the part of the standard but most compilers support it (it can be pragma, attribute or something else - you need to check in the compiler documentation)
gcc version:
#define __weak __attribute__((weak))
/* interface.h *
***************/
int get_value_1();
int get_value_2();
/* default.c *
*************/
#include "interface.h"
#ifndef GET_VALUE_1
__weak int get_value_1() { return -1; }
#endif
#ifndef GET_VALUE_2
__weak int get_value_2() { return -1; }
#endif
/* platform1.c *
***************/
#include "interface.h"
#ifndef GET_VALUE_1
#define GET_VALUE_1
int get_value_1() { return 1; }
#endif
/* platform2.c *
***************/
#include "interface.h"
#ifndef GET_VALUE_2
#define GET_VALUE_2
int get_value_2() { return 2; }
#endif