Answer Posted / debasis roy

There are 4 storage classes in c
1. Auto
2. Static
3. Extern
4. Register

1. Auto :
A variable,defined normally without any storage specification is treated as automatic variable.
Example :

#include<stdio.h>
void main()
{
int num; //declaration of a variable num
num=7; //assign an integer to num
printf("%d",num);
}

output
7

2.Static :
A variable is declared as static when it needs to remember it's previous value.
To declare a variable as static an explicit mention of the word 'static' is necessary.
It can be used in recursion , a function that need to remember it's previous call's change etc.

Example:
#include<stdio.h>
void see_the_change(int);
void main()
{
see_the change(5);
see_the_change(60);
see_the_change(70);
}
void see_the_change(int vulejaoa)
{
static int capture;
cature=vulejaoa;
capture++;
printf("%d ",capture);
}

output :
6 7 8

3.Extern :
when a variable needs to be reused by different functions extern variables are used.
If we define something in a function and declare it externally then we can use it in that particular scope of funcion block.

Example :

#include<stdio.h>
int m;
void in_a();
void in_b();
void main()
{
extern int m;
m=10;
printf("%d",m);
in_a();
in_b();
}
void in_a()
extern int m;
m=19;
printf("%d",m);
}
void in_b()
{
extern int m;
m=52;
printf("%d",m);
}

4.Register :
Normally one should avoid this type as if your processor lacks more number of registers then system would have been slow down.
But if you have a processor having sufficient number of registers and you need to run it in less time, i.e without wasting any time, then you should go for a register variable.
As register variable declaration stores number stored in registers like A,B,C,D,E or H-L pair and eventually those are stored in cache memory, program can access it very fast. If all registers are used then the numbers will be stored in main memory.

Example :

#include<stdio.h>
void main()
{
register int m;
m=12;
printf("%d",m);
}

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