Different storage classes in C Language

Problem

What are different storage classes in C Language? Explain them with programs?

Solution

A storage class is defined as the scope and life-time of variables or a functions that is present within a C Program.

Storage classes

Following are the storage classes in C −

auto
extern
static
register

Automatic variables/Local variables

Keyword − auto
These are also called local variables
Scope −
- Scope of a local variable is available within the block in which they are declared.
- These variables are declared inside a block
Default value − garbage value

Example

#include<stdio.h>
void main (){
   auto int i=1;{
      auto int i=2;{
         auto int i=3;
         printf ("%d",i);
      }
      printf("%d", i);
   }
   printf("%d", i);
}

Output

3 2 1

Global Variables/External variables

Keyword − extern
These variables are declared outside the block and so they are also called global variables
Scope − Scope of a global variable is available throughout the program.
Default value − zero

Example

#include<stdio.h>
extern int i =1; /* this ‘i’ is available throughout program */
main (){
   int i = 3; /* this ‘i' available only in main */
   printf ("%d", i);
   fun ();
}
fun (){
   printf ("%d", i);
}

Output

Static variables

Keyword − static
Scope − Scope of a static variable is that it retains its value throughout the program and in between function calls.
Static variables are initialized only once.
Default value − zero

Example

#include<stdio.h>
main (){
   inc ();
   inc ();
   inc ();
}
inc (){
   static int i =1;
   printf ("%d", i);
   i++;
}

Output

1    2    3

Register variables

Keyword − register
Register variable values are stored in CPU registers rather than in memory where normal variables are stored.
Registers are temporary storage units in CPU.

Example

#include<stdio.h>
main (){
   register int i;
   for (i=1; i< =5; i++)
      printf ("%d",i);
}

Output

1 2 3 4 5