EXTERN -- wherever u define variables, it will get access to use them

Extern :

All variables we have seen so far have limited scope (the block in which they are declared) within the program. However, in some applications it may be useful to have data which is accessible from within any other file or which remains in existence for the entire execution of the program.

Suppose if a variable is declared in file1 and we want to access the variable in another file2 and file3, then how to access these variables in other files??? Simple, the answer to this is “Extern”.

Extern keyword is used for the global variables that are defined in one file and can be used those variables in other files by using extern keyword.

extern int a;

Let us see an example:

file1.c                                                                 file2.c

#include<stdio.h>              #include<stdio.h>

int main()                     int a=1;

{                              int main()

    extern int a;              {return 0;}

    printf("%d",a);

    return 0;

}

            In this example, the variable ‘a‘ is created and initialized in file2.c and is used in file1.c as external variable. For information on how to compile a program whose source code is split among multiple files, you can simply type the command gcc -o a file1.c file2.c, and run the program with ./a.

·       When you use extern keyword before the global variable declaration, the compiler understands you want to access a variable being  defined in another program or file, and hence not to allocate any memory for this one. Instead, it simply points to the global variable defined in the other file.

  a)

#include <stdio.h> extern int i;    //extern variable int main() {     printf("%d",i);     return 0; } Output: Compilation error, undefined symbol i.

        

            b)

         #include<stdio.h>

extern int i;

int main()

{

    i = 1;

    return 0;

}

This program throws error in compilation. Because ‘i’ is declared but not defined anywhere. Essentially, the ‘I’ isn’t allocated any memory. In (b), the program is trying to change the value to 1 of a variable that doesn’t exist at all.

·       If the global variables are not initialized then, they are initialized to their default values.

Int – 0 , Float – 0.0, Char - 0 , String – null

#include <stdio.h>

int i; char c; float f; char *str;  

int main()

{

    printf("%d %d %f %s",c,i,f,str);

    return 0;

}

Output: 0 0 0.000000 (null)

·       If we declare a variable as extern, it means that the definition for that variable is defined somewhere else in the program or in other file. It doesn’t allocate the memory for these variables. It’s just the declaration. So we can declare it as many times as we can. 

#include <stdio.h>

extern int i;        //Declaring the variable i.

int i=1;             //Initializing the variable.

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

int main()

{

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

    printf("%d",i);

    return 0;

}

Output: 1

·        A particular extern variable can be declared many times but we can initialize at only one time. (even outside or inside the function of same program)

#include <stdio.h>

extern int i; //Declaring the variable

int i=25;     //Initializing the variable

int main()

{

    printf("%d",i);

    return 0;

}

int i=20; //Initializing the variable

Output: Compilation error: Multiple initialization                 variable i.

·         To define a variable i.e. allocate the memory for extern variables it is necessary to initialize the variables.

#include <stdio.h>

extern int i=1;    //extern variable

int main()

{

    printf("%d",i);

    return 0;

}

Output: 1

·        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.

a)

#include <stdio.h>

int main()

{

    extern int i=1;   // 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=1;         // Try to locally initialization of

                       // extern variable i.

    printf("%d",i);

    return 0;

}

Output: Compilation error: Multiple declaration of variable i.