QUESTION 25 · Storage Classes

What does extern int total; declare?

2 marks

Answer

It declares an int named total, normally referring to a definition supplied elsewhere.

It does not by itself supply a new definition when written without an initialiser in this form.
A definition must exist when the program requires that object.

Explain extern declarations with an example.

5 marks

Answer in brief

extern is commonly used to declare an object or function whose definition is supplied elsewhere, possibly in another source file.

Declaration and definition

extern int total; /* declaration */
int total = 10;   /* definition at file scope */

Working

The declaration tells the compiler the name and type. The definition supplies the object. When compatible declarations have external linkage, they can refer to the same object across translation units. A header can contain an extern declaration while one source file contains the definition.

Example explanation

In the complete single-file example, total is declared before main and defined after main. The earlier declaration makes the name available when main is compiled. The definition gives total the initial value 10, which main prints. The order is chosen to show the difference clearly.

Important rules

Keep the declared type compatible with the definition.
Provide one required external definition rather than putting an ordinary initialised definition in every included header.
Do not assume extern makes a previously hidden name visible automatically: a declaration must appear in an appropriate scope.
An extern declaration with an initialiser at file scope is a definition, so the initialiser changes the situation.

Linkage and storage duration

An object declared with external linkage has static storage duration. However, extern is not simply another word for global: a block-scope extern declaration can refer to an object defined outside that block. Linkage also depends on prior visible declarations, so avoid the oversimplification that extern always overrides an earlier static declaration.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
extern int total;
int main(void)
{
    printf("Total = %d\n", total);
    return 0;
}
int total = 10;

Sample input and output

This example uses values already written in the program.

Output

Total = 10