Storage Classes in C programming data types

We have already said all that needs to be said about constants, but we are not finished with variables. To fully define a variable one needs to mention not only its ‘type’ but also its ‘storage class’. In other words, not only do all variables have a data type, they also have a ‘storage class’.
We have not mentioned storage classes yet, though we have written several programs in C. We were able to get away with this because storage classes have defaults. If we don’t specify the storage class of a variable in its declaration, the compiler will assume a storage class depending on the context in which the variable is used. Thus, variables have certain default storage classes.
From C compiler’s point of view, a variable name identifies some physical location within the computer where the string of bits representing the variable’s value is stored. There are basically two kinds of locations in a computer where such a value may be kept— Memory and CPU registers. It is the variable’s storage class that determines in which of these two locations the value is stored.
Moreover, a variable’s storage class tells us:

Moreover, a variable’s storage class tells us:

(a) Where the variable would be stored.
(b) What will be the initial value of the variable, if initial value is not specifically assigned.(i.e. the default initial value).
(c) What is the scope of the variable; i.e. in which functions the value of the variable would be available.
(d) What is the life of the variable; i.e. how long would the variable exist.

There are four storage classes in C:

(a) Automatic storage class
(b) Register storage class
(c) Static storage class
(d) External storage class
Let us examine these storage classes one by one.