Can I re-define a function or check if it exists?

Question

I have a question about (re-)defining functions. My goal is to have a script where I can choose to define a function or not. Like this:

void func(){}

int main(){
   if (func)func();
}

AND without the function, just:

int main(){
   if (func)func();
}

Anybody an idea?

Answer 1

You can do this in GCC using its weak function attribute extension:

void func() __attribute__((weak)); // weak declaration must always be present

int main() {
  if (func) func();
  // ...
}

// optional definition:
void func() { ... }

This works even if func() is defined in another .c file or a library.

Answer 2

Something like this, I think. Haven't used function pointers much, so I may have gotten the syntax slightly wrong.

  void func()
  {
#define FUNC_PRESENT
      // code
  }

  void (*funcptr)();

#ifdef FUNC_PRESENT
      funcptr = func;
#else
      funcptr = NULL;
#endif

  int main()
  {
      if (funcptr)
          funcptr();
  }