External Storage Class

 External Storage Class:

·       Values stores in memory only.

·       Default value of external storage value is zero.

·       Variable scope is global.

·       Life of variable is as long as the program’s execution doesn’t come to end

·       External variables declared outside function, So that function may use it. These variables are available to all functions.

#include<stdio.h> void useFile(); /* this variable is external variable, its default value will be zero */ char* fileName="test.txt";     main( ) {        printf("File main function  - %s\n",fileName);              useFile(); } void useFile() {        printf("File useFile function  - %s\n\n",fileName); }

In this program, we define an external variable (fileName) which is a char pointer. This variable is available in all function. We use filename in both function and we can see output in attached image.

Example 2:

#include<stdio.h> int x = 1 ; main( ) {    extern int y ;    // declaration    printf ( "\n%d %d", x, y ) ; } int y = 2 ;

Here x and y both are external variables as both are defined outside of functions.

-        In main function we defined extern int y. this is declaration of y variable and int y=2 is definition. As we declare a variable it will not reserve memory.

-        In this function we required declaration of y variable because its definition is after the main function and we are using this variable in main function. If we did not declare it before we use , it will show compile error.

-        To declare a variable we use extern keyword.

-        Local variable have more preference then global variable.