[Adam Shapiro]

The easiest way (in my opinion) is to use the extern keyword. You can do this using two methods:

1. Declare the variable in one file and declare it in each additional file in which it must be used with the extern keyword.
2. Declare the variable in a file and declare it using the extern keyword in a header file to be included in all files requiring the variable.

I always find that it is easiest to use method two as follows:

main.c (or another file)
#include "globals.h"
int iGlobalVar;
iGlobalVar=1;

globals.h
extern int iGlobalVar;

usingglobals.c
#include "globals.h"
if(iGlobalVar==1)printf("Awesome! It worked! ;)");

Hope this helps.