all about storage class

Storage class

Storage class of a variable refer to the lifetime, scope and visibility of a variable where,

• Lifetime: time period for which the variable is active in memory.
• Scope: Accessibility of the variable within the program.
• Visibility: from which part  of the program the values of the variable is accessible.

Depending the storage class of a variable, a variable can be classified in four types.

1. Automatic/Local
2. External/global
3. Static
4. Register

1. Automatic/local variable:

A variable declared within a function is called an automatic/local variable.

• Lifetime: During the execution of the function containing it.
• Scope & visibility: Scope and visibility are limited within the function only.
• Priority of local variables is more than other variables.

2.Extern/Global variables:

A variable declared out side any function is an extern variable or global variable.

• Lifetime: Available in memory throughout the execution of the entire program.
• Scope & visibility: Visible thought the entire program after it’s definition.
• A global variable being visible throughout all the functions, is modifiable by the entire program.

External variable

If a global variable is accessed by a function before defining it, a compile time error is reported

                          Void function()

                          {

                                                Printf(“\n x=%d”,++x);

/* here a compile time error will be reported saying ‘undeclared symbol x’  */

                                }

                                int x;

                                int main()

                                {

                                                printf(“\n x=%d”,++x);

                                                function();

                                                printf(“\n x=%d”,x);

return 0;

                                }

To resolve the problem, we use a key word ‘extern’.

                                Void function()

                                {

                                                extern int x;

/* Note that : this is not a variable declaration, but a procedure to inform the compiler about the presence of external variable */

                                                Printf(“\n x=%d”,++x);

                                }

                                extern x;

                                int main()

                                {

                                                printf(“\n x=%d”,++x);

                                                function();

                                                printf(“\n x=%d”,x);

return 0;

                           }

    these type of variables are called External variables ..

3.Static variable:

Static variable are initially set to zero when it is declared.

• Lifetime: During the execution of the program containing it.

4.Register variables:

Register variables are stored in CPU registers. In a CPU the number of registers are limited. So, declaring register variables may produce error.