C Extern Storage Class

What Is Extern Storage Class

• extern variable is a programmer's shorthand to represent external variable.
• extern variables are also known as global variables because extern variables are declared above the main function. So, the variables can be accessed by any function.
• We can also access extern variables of one file to another file. But make sure both files are in same folder.
• The keyword for a variable to declared under extern storage class is extern

Syntax - Extern Storage Class

extern int m;

C program - Extern Storage Class

Let's workout a program to demonstrate extern storage class in C. First create a file named variable.h where you put all your variables with extern keyword which can be used by any program by simply including the file name in it.

extern int num1 = 9;
extern int num2 = 1;

#include <stdio.h>
#include "variable.h"
int main()
{
int add = num1 + num2;
printf("%d + %d = %d ", num1, num2, add);
return 0;
}

Did you know?

Though extern varibale is declared above the main function. We can redeclare the extern variable in the main function which will overwrite the global extern variable only within the function.

#include <stdio.h>
extern int num1 = 1;
extern int num2 = 2;
int main()
{
int num1 = 3;
int num2 = 4;
int add = num1 + num2;
printf("%d + %d = %d ", num1, num2, add);
return 0;
}

Reference

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