Storage class in c

Storage class tells about the storage location ,life time and initial value of a variable/function.

There are four types of storage class in c

1.auto storage class

2.register storage class 

3.static storage class 

4.extern storage class 

let's understand with the help of example

1. auto storage class 

 example declaration-   auto int a ;
       or simply -    int a ;

A)Storage location -> Storage location of this variable is main memory i.e. if we declare the variable in auto storage mode then that variable will store into RAM 

 B)Default value -> Default value of this type of variable is GARBAGE , i.e.until we don't assign any value to this variable it will contain GARBAGE value.

    

 C)Scope ->      local to the block in which it's defined. i.e. we can use this variable in only the block in which it's defined.

 D)LifeTime -> Till the control remains in block .

 understand with example !!

main() 

 { 

int a;

printf("%d",a);// garbage value(initial value)

a=10;

printf("%d",a); // 10 

{ 

int  b=10; 

printf("%d",b);

}

printf("%d",b); // error b is not in its scope


}

if we ignore the third printf
 //printf("%d",b);     then code will work smoothly and first printf the GARBAGE value because initial value of auto class is GARBAGE  ,
in next time 10 will be printed, but in next printf there will be an error 
because the variable b is declared in another block and we are printing the value of b in another block.
that's why we say the scope of auto variable is block in which that is defined .

2.Resister storage class 

  A)Storage location ->Storage location of this variable is CPU register,they are stored in CPU register.but sometimes if the CPU register are not free they will get stored in main memory.

  B)Default value -> default value of this type of variable is GARBAGE .initial value to this variable is GARBAGE.

 C)Scope  ->  Scope of this variable is local to the block in which it is defined .
 

 D)LifeTime ->Till control remains within the block 

ex- register int a ; 

3.Static storage class 

  A) Storage location -> Storage location of this type of variable is main memory .

  B) Default value -> Default value of this variable is ZERO , initially the variable will contain ZERO.

  C) scope -> Local to the block in which the variable is defined.

  D)Life Time -> Till the value of variable persists between different function calls.

Declaration method -
 static int a ;

program to understand the term 

int func(); 

#include<stdio.h>
void main() 
 {
    int i ;

 int a=10;

   for(i=0;i<10;i++)
       { 
          func();
       }
 }
int func()
  {
static int c=10;
    printf("%d\n",c);c++;
return (0);
  }

            

Output:

10 11 12 13 14 15 16 17 18 19

  

Here you can see the value of c is persisting in function calling 

 4. External storage class

  A)Storage -> Storage class of this type of variable is Main memory ,variable of this type is stored in main memory. 

  B) Default value -> Default value of this type variable is ZERO 

  C)Scope  -> Global ( we can access this variable from any where any block . it's accessible for all block of program.

D)Life Time -> As long as the program execution doesn't comes too an end . 

method of declaration

extern int a ;

remember one thing about extern variable 
extern int a ; 
// it's a declaration so we  have to define it later to use in program

like :

extern int a ; #include<stdio.h> int main() { int a=10; printf("%d",a); return 0; }

Output:

10

if we use it wihout definition like below

extern int a ; #include<stdio.h> int main() { //int a=10; printf("%d",a); return 0; }

it will be error ! 
Output:

In function `main': undefined reference to `a'