Storage classes in C Language | auto , register, extern, static in C Language

Storage class specifies the scope of a variable in a program. It specifies the whether the variable will be stored in RAM or registers. It specifies which parts of a program can access the variable and how long the variable stays in the memory. Storage class in C Language is of four types.

• Storage class auto 
• Storage class register 
• Storage Class static 
• Storage Class extern 

1.  Storage Class auto in C Language

 Keyword auto is used to specify automatic variables in C Language. Variables declared without any storage class are also automatic variables. They are declared inside a function in which they should be used.  They are local to the function in which they are declared.

Example:

int var;

or

auto int var;

#include<stdio.h>
int main()
{
auto int var=20;  /*or int var;*/
printf("\nvar=%d",var);
return(0);
}

i=10

2. Storage Class register in C Language

 Keyword register is used to specify register variables in C Language. They are stored in registers inside CPU in spite of RAM where other variables are declared. Their processing is fast as compared to other variables. 

Only those variables should be of register type which are used  again and again in program just like loop control variables.

Example

register int var;

In the above declaration, a is an int type variable which has been defined in the computer register.

#include<stdio.h>
int main()
{
register int var=100;
printf("\ni=%d",var);
return(0);
}

i=10

3. Storage Class static in C Language

Keyword static can be used to declare static variables in C Language. they are special variables which keep their values alive throughout the program. They get initialized only once.

#include<stdio.h>
void showstatic()
{
static int var=10;
var++;
printf("\ni=%d",var);
}
int main()
{
showstatic();
showstatic();
showstatic();
return(0);
}

var=10
var=11
var=12

 4.  Storage Class extern in C Language

Keyword extern can be used to declare an external variable. They are special types of variables which keeps their values alive throughout the program. They are always declared outside a function.

We can make any variable global by using the keyword extern anywhere in a program. Eternal variables can be accessed across multiple files. 

#include<stdio.h>
void show();
void  demo();
int main()
{
extern int var;
printf("\nvar=%d",var);
show();
demo();
return(0);
}
void show()
{
extern int var;
printf("\nvar=%d",var);
}
int var=10;
void demo()
{
var++;
printf("\nvar=%d",var);
}

var=10
var=10
var=11