Storage Class

 

STORAGE CLASSES

● Storage classes are used to define the scope (visibility) and life-time of

variables and/or functions

● Every variable and function in C has two attributes: type and storage class.

● There are four storage classes:

• auto

• static

• extern or global

• register

THE AUTO STORAGE CLASS :

The auto keyword is applied to all local variables automatically. It is the default storage

class that is why it is known as automatic variable.

EXAMPLE PROBLEM:

#include<stdio.h>

int main()

{

int a=10;

auto int b=10;//same like above

printf("%d %d",a,b);

return 0;

}

Output:

10 10

The register storage class :

● Is used to define local variables that should be stored in a register instead of RAM

● Variable has a maximum size equal to the register size (usually one word)

● Can't have the unary '&' operator applied to it (as it does not have a memory

location).

{

register int miles;

}

● The register should only be used for variables that require quick access such as

counters.

● Defining 'register' does not mean that the variable will be stored in a register

● Means that it might be stored in a register depending on hardware and

implementation restrictions.

The Static Storage Class :

● Instructs the compiler to keep a local variable in existence during the life-time of the

program instead of creating and destroying it each time it comes into and goes out of

scope

● Making local variables static allows them to maintain their values between function

calls

● The static modifier may also be applied to global variables and it causes that

variable's scope to be restricted to the file in which it is declared

● When static is used on a global variable, it causes only one copy of that member to be

shared by all the objects of its class

The Static Storage Class (contd.,):

The static variable has the default value 0 which is provided by compiler.

#include<stdio.h>

int func()

{

static int i=0;//static variable

int j=0;//local variable

i++;

j++;

printf("i= %d and j= %d\n", i, j);

}

int main()

{

func();

func();

func();

return 0;

}

Output:

i= 1 and j= 1

i= 2 and j= 1

i= 3 and j= 1

EXTERN STORAGE CLASS:

● is used to give a reference of a global variable that is visible to ALL the program

files

● When Extern is used the variable cannot be initialized however, it points the

variable name at a storage location that has been previously defined

When you have multiple files and you define a global variable or function, which

will also be used in other files, then extern will be used in another file to provide

the reference of defined variable or function. Just for understanding, extern is

used to declare a global variable or function in another file.

The extern modifier is most commonly used when there are two or more files sharing the

same global variables or functions as explained below.

First File: main.c

#include <stdio.h>

int count ;

extern void write_extern();

main()

{

count = 5;

write_extern();

}

Second File: support.c

#include <stdio.h>

extern int count;

void write_extern(void) {

printf("count is %d\n", count);

}