The extern Storage Class

 The extern Storage Class

The extern storage class is used to give a reference of a global variable that is visible to ALL the program files

When you use 'extern', the variable cannot be initialized as all it does is point the variable name at a storage location that has been previously defined. 

When you have multiple files and you define a global variable or function, which will be used in other files also, then extern will be used in another file to give reference of defined variable or function.

 Just for understanding, extern is used to declare a global variable or function in another file. 

The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

First File: main.c

#include<stdio.h>

  int count ;

 extern void write_extern(); 

main() 

{

    write_extern();

 } 

Second File: write.c

#include<stdio.h>

  extern int count; 

 void write_extern(void)

 { 

    count = 5;

    printf("count is %d\n", count);

 } 

Here, extern keyword is being used to declare count in the second file where as it has its definition in the first file, main.c. Now, compile these two files as follows:

$gcc main.c write.c 

This will produce a.out executable program, when this program is executed, it produces the following result: 

5