Storage Classes

Storage classes in C are used to determine the visibility, lifetime, memory location and initial value of a variable. Type refers to the data type of a variable. There are four types of storage classes in C.

• Automatic
• External
• Static
• Register

Automatic

• Auto is the default storage class for all the variables and its declared inside a function or a block.
• Automatic variables are initialized to garbage by default.
• Memory assigned to automatic variables get free upon exiting from the block.
• The keyword used for defining automatic variables is auto.
• In C language auto keyword is rarely used in programs.

Let’s see the example

#include <stdio.h>  
int main()  
{  
int a = 10,i;   
printf("%d ",++a);  
{  
int a = 20;   
for (i=0;i<3;i++)  
{  
printf("%d ",a);  
}  
}  
printf("%d ",a);    
}

Output

11 20 20 20 11

External

• External storage class variable is defined elsewhere and not within the same block.
• The default initial value of external integral type is 0 otherwise null.
• External variable can be declared many times but initialized at once.
• External variable are initialize globally and we cannot initialize external variable within any block or method.
• The keyword of external variable is extern.

Let’s see the example of external

#include <stdio.h>  
int a;   
int main()  
{  
extern int a;  
printf("%d",a);  
}

Output

0

Static

• Static variable are visible only to the functions or to the block in which they are defined.
• static variable is default initial value is 0 otherwise null.
• As same static variable can be declared at many times but can be assigned at only once.
• The keyword used to define static variable is static.

Let’s see the example of static

#include<stdio.h>  
void sum()  
{  
static int a = 10;  
static int b = 24;   
printf("%d %d \n",a,b);  
a++;   
b++;  
}  
void main()  
{  
int i;  
for(i = 0; i< 3; i++)  
{  
sum();     
}

Output

10 24 
11 25 
12 26

Register

• The access time of the register variable is faster than automatic variable.
• The initial default value of the register variable is 0.
• We can store pointers into the register and register can store the address of a variable.
• Storage class declares register variables which have the same functionality that of the auto variable.

Let’s see the example of register

#include <stdio.h>  
int main()  
{  
register int a;    
printf("%d",a);  
}

Output

0