can I make functions with unresolved dependency in C?

Question

what is the SHORTEST or Easiest way to solve the following dependency problem. Given, I have to keep xx in a separate file.

file1.h

    static inline void xx(){
      yy();//yy is not defined in this file but defined in file2.c;
    }

file2.c

  #include "file1.h"
  void yy(){
     printf("Hello");
   }

    void main(){
      xx();
    }

Compiler error in file1 yy not defined.

Answer 1

A declaration is required but not a definition:

// file1.h

static inline void xx() {
    void yy(); // Just a declaration

    yy(); 
}

// file2.c

#include "file1.h"
void yy() {
    printf("Hello");
}

int main() { // void main is not legal C
    xx(); // Works fine.
}