extern Storage Type

Apart from auto and static types, a variable can also be declared in a manner such that it is available to all functions in a program file, which means it is a global variable. A variable declared outside a function is called a global variable.

An example of this type of data is given below:

//Program 8.4
//global variable usage
#include<iostream.h>
int iVal;
void disp(void)
{
cout<<"Value entered is "<<iVal<<endl;
}
void main(void)
{
cout<<"Enter value: ";
cin>>iVal;
disp();
}

Since the variable iVal has been declared at the begining of the program, it can be accessed by any function. The sample output of Program 8.4 would be:

Enter value: 10
Value entered is 10

The extern type data does not allocate any memory for a variable but merely declares it to have been created elsewhere in the program. This declaration must be used to access a global variable declared in another program file. It must also be used to access a global variable declared later in the same program file. The following program illustrates the usage of global variables and extern type data:

//Pragram 8.5
//Program to illustrate extern type data
#include<iostream.h>
int iVal;
void main(void)
{
cout<<"Enter value: ";
cin>>iVal;
disp();
}

//Program 8.6
//The disp funtion
void disp()
{
extern int iVal; //extern is used to refer to iVal declared in Program 8.5
cout<<"Value entered is "<<iVal;
}

In the above example, Program 8.5 declares a global variable called iVal and also accepts a value from the user. Then, it calls a function disp() which is written in another program file, namely Program 8.6 . Progarm 8.6 refers to the variable iVal using the extern declaration since iVal is declared in Program 8.5 and not in Program 8.6. Program 8.6 should contain a #include statement with the name of the file of Program 8.5. The sample out put after the two programs are compiled together and linked would be:

Enter Value: 10
Value entered is: 10

Summary of the three Storage Types: