Storage classes in C

In C language, each variable has a storage class which decides the following things:

scope i.e where the value of the variable would be available inside a program.

default initial value i.e if we do not explicitly initialize that variable, what will be its default initial value.

lifetime of that variable i.e for how long will that variable exist.

The following storage classes are most often used in C programming,

  • Automatic variables
  • External variables
  • Static variables
  • Register variables

Automatic variables: auto

Scope: Variable defined with auto storage class are local to the function block inside which they are defined.

Default Initial Value: Any random value i.e garbage value.

Lifetime: Till the end of the function/method block where the variable is defined.

A variable declared inside a function without any storage class specification, is by default an automatic variable. They are created when a function is called and are destroyed automatically when the function’s execution is completed. Automatic variables can also be called local variables because they are local to a function. By default they are assigned garbage value by the compiler.


External or Global variable

Scope: Global i.e everywhere in the program. These variables are not bound by any function, they are available everywhere.

Default initial value: 0(zero).

Lifetime: Till the program doesn’t finish its execution, you can access global variables.

A variable that is declared outside any function is a Global Variable. Global variables remain available throughout the program execution. By default, initial value of the Global variable is 0(zero). One important thing to remember about global variable is that their values can be changed by any function in the program.

extern keyword

The extern keyword is used with a variable to inform the compiler that this variable is declared somewhere else. The extern declaration does not allocate storage for variables.

Static variables

Scope: Local to the block in which the variable is defined

Default initial value: 0(Zero).

Lifetime: Till the whole program doesn’t finish its execution.

A static variable tells the compiler to persist/save the variable until the end of program. Instead of creating and destroying a variable every time when it comes into and goes out of scope, static variable is initialized only once and remains into existence till the end of the program. A static variable can either be internal or external depending upon the place of declaration. Scope of internal static variable remains inside the function in which it is defined. External static variables remain restricted to scope of file in which they are declared.

They are assigned 0 (zero) as default value by the compile