Storage Classes in C Programming

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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'.

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.

• Storage class in C decides the part of storage to allocate memory for a variable, it also determines the scope of a variable.

• All variables defined in a C program get some physical location in memory where variable's value is stored.

• Memory(RAM) and CPU registers are types of memory locations where a variable's value can be stored.

Types of Storage classes :

1. Automatic Storage Class.
2. Static Storage Class.
3. External Storage Class.
4. Register Storage Class.

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1. Automatic Storage Class:

A variable defined within a function or block with auto specifier belongs to automatic storage class.

All variables defined within a function or block by default belong to automatic storage class if no storage class is mentioned.

DETAILS

EXAMPLE

#include<stdio.h>
void fun( );
int main( ) {
 fun();
 fun();
 fun();
 return 0;
}
void fun() {
 auto int x=0;
 x=x+1;
 prinf("X = %d\n",x);
}

OUTPUT:

X = 1
X = 1
X = 1

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2. Static Storage Class:

When static specifier is applied to a local variable, the compiler creates permanent storage for it, much as it creates storage for a global variable but static local variable remains visible only to the function or block in which it is defined.

In simple terms, a static local variable is a local variable that retains its value between function calls.

DETAILS

EXAMPLE

#include<stdio.h>
void fun( );
int main( ) {
 fun();
 fun();
 fun();
 return 0;
}
void fun() {
 static int x=0;
 x=x+1;
 prinf("X = %d\n",x);
}

OUTPUT:

X = 1
X = 2
X = 3

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3. External Storage Class:

The extern specifier gives the declared variable external storage class.

The principal use of extern is to specify that a variable is declared with external linkage elsewhere in the program.

DETAILS

EXAMPLE

#include<stdio.h>
int main( ) {
 extern int x;
 printf("X = %d",x); 
 return 0;
}
int x = 40;

OUTPUT:

X = 40

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4. Register Storage Class:

A register declaration is equivalent to an auto declaration, but hints that the declared variable will be accessed frequently; therefore they are placed in CPU registers, not in memory.

DETAILS

EXAMPLE

#include<stdio.h>
int main( ) {
 register int x = 10;
 int sum = x+x+x+x+x;
 printf("X = %d",x); 
 return 0;
}

OUTPUT:

X = 50

Next topic is operators

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