C: Passing variables between files - c

Question

I'm trying to pass a local variable (in func1) to a function (func2) in another file, but func2 requires that as a global variable. To explain things better, here are the two files:
file1.c:
#include <something.h>
extern void func2();
void func1(){
int a=0;
func2();
}
file2.c:
#include <something.h>
extern int a; //this will fail
void func2(){
printf("%d\n",a);
}
The variable int a can't be declared as global in file1, as func1 is called recursively. Is there a better way to do this?

Answer 1

In file1.c:
#include <something.h>
#include "file1.h"
int a;
void func1(){
a = 0;
}
In file1.h
extern int a;
In file2.c:
#include <something.h>
#include "file1.h"
void func2(){
printf("%d\n",a);
}
So:
The variable is in file1.c
file1.h allows others to know that it exists and its type is int.
file2.c includes file1.h so that the compiler knows about var a existence before file2.c tries to use it.

Answer 2

Sorry I can’t comment on Ciro Pedrini’s answer, so I would say: do as he says, but also:
Make the argument list of func1 explicitly (void) (and set your compiler to complain about empty argument lists), because () just means you are not specifying what the arguments are, at least in C, which your question is tagged as.
Declare void func1(void); as well as a in file1.h.
#include file1.h in file1.c, so the compiler checks that the definitions (in file1.c) of a and func1 are consistent with their declarations (in file1.h).
But, although you may have to do this as an exercise, try to avoid passing information in global variables: arguments are usually more reliable, as it is easier to ensure that no other part of the programme alters them. And you talk about passing information to a recursive function through a global variable: in that case you must be especially careful to pick up and save the value in func1 before a recursive call alters the value - passing an argument is so much easier and more reliable!

Answer 3

Ciro's solution would have worked if I am only using func1 once; however func1 is called recursively so variable a cannot be global in file1 (i.e. the variable has to live on the stack and not on the heap). Here's the solution I ended up using:
File1.c:
#include <something.h>
extern void func2_new(int b);
void func1(){
int a=0;
func2_new(a);
}
File2.c:
#include <something.h>
int a;
void func2(){
printf("%d\n",a);
}
void func2_new(int b){
a=b;
func2();
}
As many have pointed out, the only answer would be to refactor func2 completely. I welcome more input to this solution.