c - 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.