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Storage class specifiers in C language tells the compiler where to store a variable, how to store the variable, what is the initial value of the variable and life time of the variable.

Syntax:

storage_specifier data_type variable _name;

Types of Storage Class Specifiers in C:

There are 4 storage class specifiers available in C language. They are,

auto
extern
static
register

Storage Specifier	Description
auto	Storage place: CPU Memory Initial/default value: Garbage value Scope: local Life: Within the function only.
extern	Storage place: CPU memory Initial/default value: Zero Scope: Global Life: Till the end of the main program. Variable definition might be anywhere in the C program.
static	Storage place: CPU memory Initial/default value: Zero Scope: local Life: Retains the value of the variable between different function calls.
register	Storage place: Register memory Initial/default value: Garbage value Scope: local Life: Within the function only.

Note:

For faster access of a variable, it is better to go for register specifiers rather than auto specifiers.
Because, register variables are stored in register memory whereas auto variables are stored in main CPU memory.
Only few variables can be stored in register memory. So, we can use variables as register that are used very often in a C program.

1. Example program for auto variable in C:

The scope of this auto variable is within the function only. It is equivalent to local variable. All local variables are auto variables by default.

#include<stdio.h>
void increment(void);
int main()
{
   increment();
   increment();
   increment();
   increment();
   return 0;
}

void increment(void)
{
   auto int i = 0 ;
   printf ( "%d ", i ) ;
   i++;
}

#include<stdio.h>

void increment(void);

int main()

{

increment();

return 0;

}

void increment(void)

{

auto int i = 0 ;

printf ( "%d ", i ) ;

i++;

}

Output:

0 0 0 0

2. Example program for static variable in C:

Static variables retain the value of the variable between different function calls.

//C static example
#include<stdio.h>
void increment(void);
int main()
{
increment();
increment();
increment();
increment();
return 0;
}
void increment(void)
{
static int i = 0 ;
printf ( "%d ", i ) ;
i++;
}

//C static example

#include<stdio.h>

void increment(void);

int main()

{

increment();

return 0;

}

void increment(void)

{

static int i = 0 ;

printf ( "%d ", i ) ;

i++;

}

Output:

0 1 2 3

3. Example program for extern variable in C:

The scope of this extern variable is throughout the main program. It is equivalent to global variable. Definition for extern variable might be anywhere in the C program.

#include<stdio.h>

int x = 10 ;
int main( )
{
extern int y;
printf("The value of x is %d \n",x);
printf("The value of y is %d",y);
return 0;
}
int y=50;

#include<stdio.h>

int x = 10 ;

int main( )

{

extern int y;

printf("The value of x is %d \n",x);

printf("The value of y is %d",y);

return 0;

}

int y=50;

Output:

The value of x is 10
The value of y is 50

4. Example program for register variable in C:

Register variables are also local variables, but stored in register memory. Whereas, auto variables are stored in main CPU memory.
Register variables will be accessed very faster than the normal variables since they are stored in register memory rather than main memory.
But, only limited variables can be used as register since register size is very low. (16 bits, 32 bits or 64 bits)

#include <stdio.h>
int main()
{
   register int i;
   int arr[5];// declaring array
   arr[0] = 10;// Initializing array
   arr[1] = 20;
   arr[2] = 30;
   arr[3] = 40;
   arr[4] = 50;
   for (i=0;i<5;i++)
   {
      // Accessing each variable
      printf("value of arr[%d] is %d \n", i, arr[i]);
   }
   return 0;
}

#include <stdio.h>

int main()

{

int arr[5];// declaring array

arr[0] = 10;// Initializing array

arr[1] = 20;

arr[2] = 30;

arr[3] = 40;

arr[4] = 50;

for (i=0;i<5;i++)

{

// Accessing each variable

printf("value of arr[%d] is %d \n", i, arr[i]);

}

return 0;

}

Output:

value of arr[0] is 10
value of arr[1] is 20
value of arr[2] is 30
value of arr[3] is 40
value of arr[4] is 50

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