C – Storage Class Specifiers.

 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,

1. auto
2. extern
3. static
4. register

S.No.	Storage Specifier	Storage place	Initial / default value	Scope	Life
1	auto	CPU Memory	Garbage value	local	Within the function only.
2	extern	CPU memory	Zero	Global	Till the end of the main program. Variable definition might be anywhere in the C program
3	static	CPU memory	Zero	local	Retains the value of the variable between different function calls.
4	register	Register memory	Garbage value	local	Within the function

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.

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++;
}

Output:

0 0 0 0

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++;
}

Output:

0 1 2 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 ;

Output:

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

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;
}

 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