C – Storage Class specifiers Extern

main()
{
    extern int x;  //Tells compiler that it is defined somewhere else
    printf("%d",x);   
}
int x=10;    //Global variable x

Extern Variable Initializer

Extern as global variable initializer

• The extern keyword variable can be initialize when its global

Example : External variable always initialize (Zero) ‘0’

#include "stdio.h"
extern int a;
main(){
    printf("a=%d",a);
    return 0;
}
int a;

/* Output : a=0 */

Example : variable initialize in  global variable

#include "stdio.h"
extern int a;
main(){
    printf("a=%d",a);
    return 0;
}
int a =5;

/* Output : a=5 */

Example :

#include <stdio.h> 
extern int a;
int main()
{
    void fun();
    printf("\n a=%d",a);
    fun();
    return 0;
}
int a=7;
    
void fun()
{
    printf("\n in fun a=%d",a);
}

/* Output : 
 a=7
 in fun a=7
*/

Example : variable initialize in  global variable

#include <stdio.h> 
extern int a=5;
int main()
{   
    printf("%d",a);
    return 0;
}

/* Output : 5*/
/* Explanation pritf function print a value as 5. */

Example : variable initialize in  global variable

#include <stdio.h> 
extern int a=5;
int main()
{
    void fun();
    printf("\n a=%d",a);
    fun();
    return 0;
}
int a;
    
void fun()
{
    printf("\n in fun a=%d",a);
}

/* Output : 
 a=5
 in fun a=5
*/

Example : Error while running Linker

#include <stdio.h> 
extern int a;
int main()
{
    printf("\na=%d",a);
    return 0;
}

/* Output : // Error */

/*
  Error[e46]: Undefined external "a" referred in main 
  Error while running Linker 
*/

Extern as local variable initializer

#include <stdio.h> 
int main()
{
  extern int a=5;
  printf("%d",a);
  return 0;
}

/* Output */
// Error[Pe2442]: an initializer is not allowed on a local declaration of an extern variable main.c