Storage Classes

C language supports four storage class specifiers: 

           (1) extern

           (2) static

           (3) register

           (4) auto 

These storage specifiers inform the compiler how to store the subsequent variable. 

           Syntax:

                          storage_specifier type var_name; 

1)extern:

• extern storage specifier is used for informing the compiler that the variable is defined as an extern, i.e., it is declared elsewhere.
• extern is only used to give a reference of a global variable.
• The variables that are declared as extern does not allocate any memory, and it is only a declaration.
• extern variable can only be initialized globally, i.e., it cannot be initialized within a method or any block.
• It can be declared multiple times but can be initialized only once.

2)Static:

• The variables declared as static preserves their value even after they are out of their scope.
• We can say that extern variables are initialized only once but exist till the termination of the program. 
• The scope of extern variables is local to the function to which they are defined.

3)Register:

• The register specifier is similar to auto except that it is used to request the compiler to keep the value of a variable in a CPU register, i.e., unlike standard variables, it is not stored in memory. 
• This makes the operations on a register variable faster than on a standard variable because the register variable was held in the CPU. It does not require memory access to determine or modify its value.

4)Auto:

• The variables declared as auto are allocated automatically at runtime in the memory.
• The visibility of the auto variables is restricted to the block in which it is defined.
• The keyword auto is rarely used in programs because the auto is the default storage class for the variables declared in the function or block.
• The automatic variables are initialized by default to garbage.