Extern Keyword in C/C++ Programming Languages - Geeks4Coding

Extern keyword in C / C++

Keyword extern is used for declaring extern variables in c. This modifier is used with all data types like int, float, double, array, pointer, structure, function etc.

Facts about extern keyword:

1. It is default storage class of all global variables as well all functions.

For example, Analyze following second.c code and its output:

(a)

#include <stdio.h>

int a; //By default it is extern variable

int main(){

printf("%d",a);

return 0;

}

Output: 0

(b)

#include <stdio.h>

extern int a;//extern variable

int main(){

printf("%d",a);

return 0;

}

Output: Compilation error, undefined symbol i.

In Both program variable a is extern variable. Read second and third points for explaination.

(c)
#include <stdio.h>

void add(int,int) //extern by default.

int main(){

int x=15,y=10;

add(x,y);

return 0;

}

void sum(int x,int y){

printf("%d”",x+y);

}

Output: 25

1. When we use extern with any variables it is only declaration that is memory is not allocated for that variable. Hence in second case compiler is showing error unknown symbol a. To define a variable i.e. allocate the memory for extern variables it is necessary to initialize the variables. For example:

#include <stdio.h>

extern int a=1000; //extern variable

int main(){

printf("%d",a);

return 0;

}

Output: 1000

3. If you will not use extern keyword with global variables then compiler will automatically initialize with default value to extern variable.

4. Default initial value of extern integral type variable is zero otherwise null. For example:

#include <stdio.h>

char c;

int i;

float f;

char *str;

int main(){

printf("%d %d %f %s",c,i,f,str);

return 0;

}

Output: 0 0 0.000000 (null)

5. We cannot initialize extern variable locally i.e. within any block either at the time of declaration or separately. We can only initialize extern variable globally. For example:

(a)

#include <stdio.h>

int main(){

extern int i=10; //Try to initialize extern variable

//locally.

printf("%d",i);

return 0;

}

Output: Compilation error: Cannot initialize extern variable.

(b)

#include <stdio.h>

int main(){

extern int i; //Declaration of extern variable i.

int i=10; //Try to locally initialization of

//extern variable i.

printf("%d",i);

return 0;

}

Output: Compilation error: Multiple declaration of variable i.

6. If we declare any variable as extern variable then it searches that variable either it has been initialized or not. If it has been initialized which may be either extern or static* then it is ok otherwise compiler will show an error. For example:

(a)

#include <stdio.h>

int main(){

extern int i; //It will search the initialization of

//variable i.

printf("%d",i);

return 0;

}

int i=20; //Initialization of variable i.
Output: 20

(b)

#include <stdio.h>

int main(){

extern int i; //It will search the any initialized

//variable i which may be static or

//extern.

printf("%d",i);

return 0;

}

extern int i=20; //Initialization of extern variable i.

Output: 20

(c)

#include <stdio.h>

int main(){

extern int i; //It will search the any initialized

//variable i which may be static or

//extern.

printf("%d",i);

return 0;

}

static int i=20; //Initialization of static variable i.

Output: 20

(d)

#include <stdio.h>

int main(){

extern int i; //variable i has declared but not

//initialized

printf("%d",i);

return 0;

}

Output: Compilation error: Unknown symbol i.


7. A particular extern variable can be declared many times but we can initialize at only one time. For example:

(a)

extern int i; //Declaring the variable i.

int i=25; //Initializing the variable.

extern int i; //Again declaring the variable i.

#include

int main(){

extern int i; //Again declaring the variable i.

printf("%d",i);

return 0;

}

Output: 25

(b)

extern int i; //Declaring the variable

int i=25; //Initializing the variable

#include

int main(){

printf("%d",i);

return 0;

}

int i=20; //Initializing the variable

Output: Compilation error: Multiple initialization variable i.

8. We cannot write any assignment statement globally. For example:

#include <stdio.h>

extern int i;

int i=10; //Initialization statement

i=25; //Assignment statement

int main(){

printf("%d",i);

return 0;

}

Output: Compilation error

Note: Assigning any value to the variable at the time of declaration is known as initialization while assigning any value to variable not at the time of declaration is known assignment.

(b)

#include <stdio.h>

extern int i;

int main(){

i=25; //Assignment statement

printf("%d",i);

return 0;

}

int i=10; //Initialization statement

Output: 25

9. If declared an extern variables or function globally then its visibility will whole the program which may contain one file or many files. For example consider a c program which has written in two files named as first.c and second.c:

(a)
//first.c
#include

int i=25; //By default extern variable

int j=5; //By default extern variable

/**

Above two line is initialization of variable i and j.

*/

void main(){

clrscr();

sum();

getch();

}



//second.c

#include

extern int i; //Declaration of variable i.

extern int j; //Declaration of variable j.

void sum(){

int s;

s=i+j;

printf("%d",s);

}

Compile and execute above two file first.c and second.c at the same time: