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.
