Understanding Storage Classes in C: Types and Usage

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.

These are also called storage class specifiers

These specifiers tell the compiler how to store the subsequent variable. The general form of a variable declaration that uses one is shown here:

storage_specifier type var_name;

Notice that the storage specifier precedes the rest of the variable declaration.

A variable’s storage class tells us:

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

There are four storage classes in C:

• Automatic storage class
• Register storage class
• Static storage class
• External storage class

Let us examine these storage classes one by one.

Automatic Storage Class

The features of a variable defined to have an automatic storage class are as under:

Features	Description
Storage	Memory
Default initial value	An unpredictable value, which is often called a garbage value
Scope	Local to the block in which the variable is defined
Life	Till the control remains within the block in which the variable is defined

Register Storage Class

The features of a variable defined to be of register storage class are as under:

Features	Description
Storage	CPU registers
Default initial value	Garbage value
Scope	Local to the block in which the variable is defined
Life	Till the control remains within the block in which the variable is defined

Static Storage Class

The features of a variable defined to have a static storage class are as under:

Features	Description
Storage	Memory
Default initial value	Zero
Scope	Local to the block in which the variable is defined
Life	Value of the variable persists between different function calls

External Storage Class

Features	Description
Storage	Memory
Default initial value	Zero
Scope	Global scope
Life scope	As long as the program’s execution doesn’t come to an end.