C Storage Classes

C Storage Classes

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 oftenly used in C programming,

  1. Automatic variables
  2. External variables
  3. Static variables
  4. 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.

void main()
    int detail;
    // or 
    auto int details;    //Both are same


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.


int number;     // global variable

void main()
    number = 10;
    printf("I am in main function. My value is %d\n", number);
    fun1();     //function calling, discussed in next topic
    fun2();     //function calling, discussed in next topic