::::::::::::::
f.h
::::::::::::::
#ifndef F_H
#define F_H
extern int a; 
void foo(); 
#endif
::::::::::::::
f.c
::::::::::::::
#include "f.h"
int a=7;

void foo() {
  a++;
}
::::::::::::::
main.c
::::::::::::::
#include "f.h"
#include <stdio.h>

int main() {
  printf("%d\n",a);
  foo();
  printf("%d\n",a);
}