C Storage Classes

In C programming language, storage classes are used to define things like storage location (whether RAM or REGISTER), scope, life time and default value of a variable.

In C programming language, memory of variables is allocated either in computer memory (RAM) or CPU Registers. The allocation of memory always depends on the storage classes.

In C programmig language, there are FOUR(4) storage classes.

 These are :

1. auto storage class
2. extern storage class
3. static storage class
4. register storage class
5. auto storage class

The default storage class of all local varibles (variables declared inside block or function) is auto storage class. Variable of auto storage class has the following properties…

Property	Description
Keyword	auto
Storage	Computer Memory (RAM)
Default Value	Garbage Value
Scope	Local to the block in which the variable is defined
Life time	Till the control remains within the block in which variable is                  defined.

Example Program 1

#include<stdio.h>
#include<conio.h>
int main(){
int i;
auto char c;
float f;
printf(“i = %d\tc = %c\tf = %f”,i,c,f);
return 0;
}

Example Program 2

#include<stdio.h>
#include<conio.h>
int main(){
int a=10;
{
int a=20;
printf(“%d”,a);
}
printf(” %d”,a);
return 0;
}

Example Program 3

#include<stdio.h>
#include<conio.h>
int main(){
{
int a=20;
printf(“%d”,a);
}
printf(” %d”,a); 
return 0;
}

External storage class

The default storage class of all global varibles (variables declared outside function) is external storage class. Variable of external storage class has the following properties…

Property	Description
Keyword	extern
Storage	Computer Memory (RAM)
Default Value	Zero
Scope	Global to the program (i.e., Throughout the program)
Life time	As long as the program’s execution does not comes to end

Example Program 1

#include<stdio.h>

#include<conio.h>

int i;    //By default it is extern variable

int main(){

    printf(“%d”,i);

    return 0;

}

Example Program 2

#include<stdio.h>

#include<conio.h>

extern int i;   

int main(){

    printf(“%d”,i);

    return 0;

}

Static storage class

The static storage class is used to create variables that holds value beyond its scope till the end of the program. Static variable allows to initialize only once and can be modified any number of times. Variable of static storage class has the following properties…

Property	Description
Keyword	static
Storage	Computer Memory (RAM)
Default Value	Zero
Scope	Local to the block in which the variable is defined
Life time	The value of the persists between different function calls (i.e., Initialization is done only once)

Example Program 1

#include<stdio.h>

#include<conio.h>

static int a;

int main(){

    printf(“%d”,a);

    return 0;

}

Example Program 2

#include<stdio.h>

#include<conio.h>

static int i=10;

int main(){

    i=25;       //Assignment statement

    printf(“%d”,i);

    return 0;

}

Register storage class

The register storage class is used to specify the memory of variable has to be allocated in CPU Registers. The memory of register variable is allocated in CPU Register but not in RAM. The register variables enables the faster accessability compared to other storage class variables. As the number of registers inside CPU are very less we can use very less number of register variables. Variable of rregister storage class has the following properties…

Property	Description
Keyword	register
Storage	CPU Register
Default Value	Garbage Value
Scope	Local to the block in which the variable is defined
Life time	Till the control remains within the block in which variable is defined

Example Program 1

#include<stdio.h>

#include<conio.h>

int main(){

    register int a,b;

    scanf(“%d%d”,&a,&b);

    printf(“%d  %d”,a,b);

}

The following table provide detailed properties of all storage classes…

Storage Class	Keyword	Memory Location	Default Value	Scope	Life Time
Automatic	auto	Computer Memory (RAM)	Garbage Value	Local to the block in which the variable is defined	Till the control remains within the block in which variable is defined
External	extern	Computer Memory (RAM)	Zero	Global to the program (i.e., Throughout the program)	As long as the program’s execution does not comes to end
Static	static	Computer Memory (RAM)	Zero	Local to the block in which the variables are defined	The value of the persists between different function calls (i.e., Initialization is done only once)
Register	register	CPU Register	Garbage Value	Local to the block in which the variable is defined	Till the control remains within the block in which variable is defined