Lifetime of a Variable in C Language

Storage classes define the scope, lifetime, and binding of variables. To fully define a variables., you need to specify both its type and storage class. A variable name identifies a physical location in computer memory where bits are allocated to store the variable's value.

Every object has a storage class that specifies the duration of its storage, which may be static, automatic, or dynamic, along with other features.

Storage class tells us the following factors ?

• Where is the variable stored: in memory or in the CPU register?
• What will be the initial value of a variable if it is not initialized?
• What is the scope of variable and where can it be accessed?
• What is the lifetime of a variable?

Lifetime

The lifetime of a variable specifies the duration for which the computer allocates memory to it, from allocation to deallocation.

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

• Automatic: A variable with an automatic lifetime is created every time its declaration is encountered and destroyed when its block is exited.
• Static: A variable is created when its declaration is executed for the first time and is destroyed when the execution terminates.
• Dynamic: The memory for variables is allocated and deallocated through memory management functions.

Storage Classes

There are four storage classes in C language ?

Example: 1

Here is a C program demonstrating the automatic storage class. This code illustrates variable shadowing, where each block declares a new i variable, hiding the outer one. The output is 3(innermost block), 2(middle block), and 1(outer block).

#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

We will get the generated output as follows ?

3 2 1

Example: 2

Following is the C program for the external storage class. This code illustrates the use of the extern keyword. The global i is 1, but within main, i is 3. The fun function prints the global i, resulting in the input 3 1.

#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