Explain Lifetime of a variable in C language.

Storage classes specify the scope, lifetime and binding of variables.

To fully define a variable, one needs to mention not only its ‘type’ but also its storage class.

A variable name identifies some physical location within computer memory, where a collection of bits are allocated for storing values of variable.

Storage class tells us the following factors −

• Where the variable is stored (in memory or cpu register)?
• What will be the initial value of variable, if nothing is initialized?
• What is the scope of variable (where it can be accessed)?
• What is the life of a variable?

Lifetime

The lifetime of a variable defines the duration for which the computer allocates memory for it (the duration between allocation and deallocation of memory).

In C language, a variable can have automatic, static or dynamic lifetime.

• Automatic − A variable with automatic lifetime are created. Every time, their declaration is encountered and destroyed. Also, their blocks are exited.
• Static − A variable is created when the declaration is executed for the first time. It is destroyed when the execution stops/terminates.
• Dynamic − The variables memory is allocated and deallocated through memory management functions.

Storage Classes

There are four storage classes in C language −

Example

Following is the C program for automatic storage class −

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

Output

When the above program is executed, it produces the following output −

3 2 1

Example

Following is the C program for external storage class −

#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

When the above program is executed, it produces the following output −

3 1