1. extern

The extern keyword tells the compiler that a variable or function is defined elsewhere, possibly in another file. As long as the variable or fucntion is not declared as static. 

// file1.c
int counter = 0;

// file2.c
extern int counter;  // refers to the same counter defined in file1.c
extern void print_result();  // function defined in another file

// It is still global (lives the whole program), but only visible within that file.
static int count = 0;