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).

Answer 2

In C99, there's nothing called "program scope". In your example variable x has a file scope which terminates at the end of translation unit. Variables y and z which are declared static also have the file scope but with internal linkage.

C99 (6.2.2/3) If the declaration of a file scope identifier for an object or a function contains the storage class specifier static, the identifier has internal linkage

Also, the variable x has an external linkage which means the name x can is accessible to other translation units or throughout the program.

C99 (6.2.2/5) If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.