External Storage Class : (Extern) in C Programming

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1. Variables of this storage class are “Global variables”
2. Global Variables are declared outside the function and are accessible to all functions in the program
3. Generally , External variables are declared again in the function using keyword extern
4. In order to Explicit declaration of variable use ‘extern’ keyword

extern int num1 ;   // Explicit Declaration  

Storage	Memory
Scope	Global / File Scope
Life time	Exists as long as variable is running Retains value within the function
Default initial Value	Zero

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int num =  75 ;  

void display();

void main()
{
 extern int mum ;
             printf("nNum : %d",num);
 display();
}

void display()
{
 extern int mum ;
             printf("nNum : %d",num);
}

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Num : 75
Num : 75

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Note :

1. Declaration within the function indicates that the function uses external variable
2. Functions belonging to same source code , does not require declaration (no need to write extern)
   
3. If variable is defined outside the source code , then declaration using extern keyword is required
   

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