2.9 STORAGE CLASSES

To fully define a variable, one needs to maintain not only its ‘type’ but also its ‘storage class’. A variable’s storage class tells us:

ü  Where the variable would be stored.

ü  What will be initial value of the variable

ü  What is the scope of the variable i.e. in which functions the value of the variable would be available.

ü  What is the life of variable i.e. how long would the variable exist.

There are four storage classes in C:

1.      Automatic storage class

2.      Register storage class

3.      Static storage class

4.      External storage class

Automatic Storage Class

Storage	Memory
Default Initial Value	Garbage
Scope	Local to the block, in which variable is defined
Life	Till the control remains within the block in which the variable is defined

Example:

(i)

#include<stdio.h>

void main()

{

            auto int i = 1;

            {

                        {

                                    {

                                                printf(“%d”,i):

                                    }

                                    printf(“%d”,i);

                        }

                        printf(“%d”,i);

            }

}

OUTPUT

1          1          1

(ii)

#include<stdio.h>

void main()

{

            auto int i, j = 1;

            printf(“%d%d”, i, j);

}

OUTPUT

216321            1

(iii)

#include<stdio.h>

void main()

{

            auto int i = 1;

            {

                        auto int i = 2;

{

                                    auto int i = 3;

                                    printf(“%d”,i):

                        }

                        printf(“%d”,i);

            }

            printf(“%d”,i);

}

 

OUTPUT

3          2          1

 

Register Storage Class

Storage	CPU Registers
Default Initial Value	Garbage
Scope	Local to the block, in which variable is defined
Life	Till the control remains within the block in which the variable is defined

NOTE: A value stored in CPU register can always be accessed faster than the one that is stored in memory.

 

Example:

 

#include<stdio.h>

void main()

{

            register int i;

            for(i = 1; i <= 5; i++)

                 printf(“%d”, i);

}

 

OUTPUT

1          2          3          4          5

 

Static Storage Class

Storage	Memory
Default Initial Value	Zero
Scope	Local to the block, in which variable is defined
Life	Value of the variable persists between different function calls.

Example:

AUTO #include<stdio.h> void increment(); void main() {             increment(); increment(); increment(); } void increment() { auto int i = 1;             printf(“%d”, i);             i++; }   OUTPUT 1          1          1

STATIC #include<stdio.h> void increment(); void main() {             increment(); increment(); increment(); } void increment() { static int i = 1;             printf(“%d”, i);             i++; }   OUTPUT 1          2          3

 

External Storage Class

Storage	Memory
Default Initial Value	Zero
Scope	Global
Life	As long as the program’s execution does not come to an end.

Example:

#include<stdio.h>

int i;

void increment();

void decrement();

void main()

{

            printf(“%d”, i);

increment();

increment();

decrement();

decrement();

}

void increment()

{

i = i + 1;

            printf(“%d”, i);

}

void decrement()

{

i = i - 1;

            printf(“%d”, i);

}

OUTPUT

0          1          2          1          0

 

WHICH TO USE WHEN

ü  Use static storage class only if you want the value of a variable to persist between different function calls.

ü  Use register storage class for only those variables that are being used very often in a program.

ü  Use extern storage class for only those variables that are being used by almost all the functions in the program. This would avoid unnecessary passing of these variables as arguments when making a function call.

ü  Most of the times, we use auto variables, because often it so happens that once we have used the variables in a function, we don’t mind losing them.