2. STORAGE CLASSES
The storage class determines the part of the
memory where the variables would be stored.
The storage class also determines the initial
value of the variable.
It is used to define the scope and lifetime of
variable.
There are two storage locations in computer,
1. CPU Registers
2.Memory.
A value stored in CPU register can always be
accessed faster than the one that is stored in
memory.
4. Automatic
Keywords : Auto
Storage : Memory
Default initial value : Garbage value
Scope : Local/Block scope
Life : Exists as long as
the control
remains in the
block
5. Example for Automatic
void main()
{
auto x= 20 ;
{
auto x = 60 ;
printf("n x: %d",x);
}
printf("nx: %d",x);
}
OUTPUT :
x: 60
x: 20
NOTE : Two variables are declared in different
blocks , so they are treated as different variables
6. Static
Keywords : Static
Storage : Memory
Default initial value : 0
Scope : Local to the block in
which
the variable is
defined.
Life : Value of the variable
persists
between different
function calls.
7. Difference between auto and static
class
/*auto class*/
#include<stdio.h>
void add();
int main()
{
add();
add();
add();
add();
return 0;
}
void add()
{
auto int i=1;
printf("n%d",i);
i=i+1;
OUTPUT:-
1
1
1
1
/*static class*/
#include<stdio.h>
void add();
int main()
{
add();
add();
add();
add();
return 0;
}
void add()
{
static int i=1;
printf("n%d",i);
i=i+1;
}
OUTPUT:-
1
2
3
4
8. Previous program explanation
If the storage class is static, then the statement
static int i = 1 is executed only once, irrespective
of how many times the same function is called.
If the storage class is auto,then the statement
auto int i=1 is executed each time where it
increments and re-initialize the value of i= 1.
The difference between them is
that static variable do not disappear when the
function is no longer active. There value persist. If
control comes back to the same function again ,
the static variables have the same values they
had last time around.
9. Register
Keywords : Register.
Storage : CPU register.
Default initial value : Garbage value
Scope : Local to the block in
which
the variable is
defined.
Life : Value of the variable
persists
till the control
remains within
the block in which
10. Why we need Register Variable
?
Whenever we declare any variable inside C Program
then memory will be randomly allocated at particular
memory location.
We have to keep track of that memory location. We
need to access value at that memory location using
ampersand operator/Address Operator i.e (&).
If we store same variable in the register memory then
we can access that memory location directly without
using the Address operator.
Register variable will be accessed faster than the
normal variable thus increasing the operation and
program execution. Generally we use register variable
as Counter.
Note : It is not applicable for arrays, structures or
pointers.
11. External
Keywords : Extern.
Storage : Memory.
Default initial value : 0
Scope : Global
Life : Exists as long as
variable is
running
Retains value within
the function
12. Example for Extern
int num = 75 ;
void display();
void main()
{
extern int num ;
printf("nNum : %d",num);
display();
}
void display()
{
extern int num ;
printf("nNum : %d",num);
}
OUTPUT:
Num : 75
Num : 75
13. Extern contd…
Note :Declaration within the function indicates
that the function uses external variable
Functions belonging to same source code ,
does not require declaration (no need to write
extern)
If variable is defined outside the source code ,
then declaration using extern keyword is
required
