What is the difference between "File scope" and "program scope"

Question

A variable declared globally is said to having program scope
A variable declared globally with static keyword is said to have file scope.

For example:

int x = 0;             // **program scope**   
static int y = 0;      // **file scope**  
static float z = 0.0;  // **file scope** 

int main()  
{  
   int i;   /* block scope */  
   /* .
      .
      .
   */ 
   return 0;  
}  

What is the difference between these two?

Answer 1

Variables declared as static cannot be directly accessed from other files. On the contrary, non-static ones can be accessed from other files if declared as extern in those other files.

Example:

foo.c

int foodata;
static int foodata_private;

void foo()
{
    foodata = 1;
    foodata_private = 2;
}

foo.h

void foo();

main.c

#include "foo.h"
#include <stdio.h>

int main()
{
    extern int foodata; /* OK */
    extern int foodata_private; /* error, won't compile */

    foo();

    printf("%d\n", foodata); /* OK */

    return 0;
}

Generally, one should avoid global variables. However, in real-world applications those are often useful. It is common to move the extern int foo; declarations to a shared header file (foo.h in the example).