C Foundation

What is C? C Compiler Installation C Extensions C Compiler C Interpreter C Program Structure

C Basics

C Keywords C Data Types C Identifiers C Variables C Constant C Escape Sequences C Constant and Volatile C Typecast


What is Operator C Comma Operator C Arithmetic Operators C Relational Operators C Logical Operators C Bitwise Operators C Conditional Operators C : : Operator C Operator Priority

Basic IO's

Basic IO's C Formatted Functions C Unformatted Functions C Common Functions

Control Statements

What is Control Statement C if Statement C if else Statement C Nested if Statement C Else if Statement C Break Statement C Continue Statement C Switch Statement C Goto Statement


What is Control Loop C for Loop C Nested for Loop C while Loop C Nested while Loop C do while Loop C Nested do while loop


What is Function C User Defined Functions C Recursion C Passing Parameters


Scope C Local Scope C Global Scope

Storage Classes

What is Storage Class C Auto C Extern C Static C Register


What is Array C One Dimensional Array C Two Dimensional Array C Multi Dimensional Array C Arrays Of Strings


What is String C String Functions


What is Pointer C Pointers Arithmetic C Pointer to Pointer C Pointers and Arrays C Pointers and Strings C Pointer to Functions Void Pointers Null Pointers C Null and Void Pointer


What is Structure C Struct within Struct C Array within Structure C Pointer to Structure C Structure and Function C Enum C Bitfield Structure C Type def


What is Union


What is File C read a file C write a file C File Handling C Error Handling C Low Level Disk I/O C Other file functions

Memory Allocation

What is Memory Allocation C Malloc() C Calloc() C Free() C Realloc() C Coreleft()

C Reference

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The ones who are crazy enough to think they can change the world are the ones who do.
- Steve Jobs

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
C Extern Storage Class

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;
  • 9 + 1 = 10


The program illustrates that an external variables are declared in seperate file with an extension .h which is then including in demanded file.

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;
  • 3 + 4 = 7


extern variables num1 and num2 are redeclared inside main function.


Keyword extern
Storage Type Memory
Scope Block Scope
Life Time No validity
Visibility Global
Default Value nothing (ends in error if value is not initialized)
Default Storage Class No

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