Storage_classes_and_Scope_rules.pptx

©LPU CSE101 C Programming
• Storage Classes and Scope Rules

Outline
• Storage Classes
– auto
– static
– extern
– register
• Scope Rules

Storage Classes
• Storage Classes are used to describe the features of a
variable. These features basically include the scope, visibility
and life-time which help us to trace the existence of a
particular variable during the runtime of a program.
• Storage class specifies four things-
I. Storage location: Where the variable will be stored?[ In
memory /or CPU register]
II. Scope: Block in which the variable is accessible.
III. Lifetime: How long would the variable exist?
IV. Default initial value: What will be the initial value of the
variable, if initial value is not specifically assigned ?

Storage Classes: Auto
• Automatic storage
– auto int x, y;
– It is the default storage class for a local variable
– This is the default storage class for all the variables
declared inside a function or a block
⮚ Storage − Memory(RAM).
⮚ Default initial value − An unpredictable value, which is
often called a garbage value.
⮚ Scope − Local to the block in which the variable is
defined.
⮚ Lifetime − Till the control remains within the block in
which the variable is defined.

Program example-auto storage class
#include<stdio.h>
void func1()
{
auto int a=10; // Local variable of func1()
printf("n a=%d",a);
}
void func2()
{
auto int a=20; //Local variable of func2()
printf("n a=%d",a);
}
int main()
{
auto int a=30;//Local variable of main()
func1();
func2();
printf("n a=%d",a);
return 0;
}

Storage Classes: Register
• register: tries to put variable into high-speed
registers.
– register int counter = 1;
⮚ Storage - CPU registers.
⮚ Default initial value - Garbage value.
⮚ Scope - Local to the block in which the variable is
defined.
⮚ Lifetime - Till the control remains within the block in
which the variable is defined.

More points in relation to register storage class
• This storage class declares register variables which have the same
functionality as that of the auto variables. The only difference is that the
compiler tries to store these variables in the register of the
microprocessor if a free register is available.
• This makes the use of register variables to be much faster than that of the
variables stored in the memory during the runtime of the program. If a
free register is not available, these are then stored in the memory only.
• Usually few variables which are to be accessed very frequently in a
program are declared with the register keyword which improves the
running time of the program. An important and interesting point to be
noted here is that we cannot obtain the address of a register variable
using pointers.

Program example-register storage class
#include<stdio.h>
int main()
{
register int i; // i will be used frequently so, it can be given register storage class
for(i=1;i<=20;i++)
{
printf("n%d",i);
}
return 0;
}

Storage Classes: Static
• Static storage
⮚ Default initial value − Zero.
⮚ Scope − Local to the block in which the variable is
defined.
⮚ Life time − variable will retain its value throughout the
program

More points in relation to static storage class
• Static variables have a property of preserving their value even
after they are out of their scope! Hence, static variables
preserve the value of their last use in their scope.
• So we can say that they are initialized only once and exist till
the termination of the program. Thus, no new memory is
allocated because they are not re-declared.
• Their scope is local to the function to which they were
defined. Global static variables can be accessed anywhere in
the program. By default, they are assigned the value 0 by the
compiler.

Program example-static storage class
#include<stdio.h>
void function();
int main()
{
function();
function();
function();
return 0;
}
void function()
{
int a=10;
static int b=10;
printf("n Value of a:%d, Value of
b:%d",a,b);
a++;
b++;
}
Output:
Value of a:10, Value of b:10

Storage Classes: extern
Extern storage class simply tells us that the variable is
defined elsewhere and not within the same block
where it is used. Basically, the value is assigned to it in
a different block and this can be overwritten/changed
in a different block as well
⮚ Default initial value − Zero.
⮚ Scope − Global.
⮚ Life − As long as the program’s execution doesn’t come
to an end.

More points in relation to extern storage
class
• extern variable is nothing but a global variable
initialized with a legal value where it is declared in
order to be used elsewhere. It can be accessed within
any function/block.
• Also, a normal global variable can be made extern as
well by placing the ‘extern’ keyword before its
declaration/definition in any function/block.
• This basically signifies that we are not initializing a new
variable but instead we are using/accessing the global
variable only. The main purpose of using extern
variables is that they can be accessed between two
different files which are part of a large program.

Program example 1-extern storage class
External variable in the same file
#include<stdio.h>
void first();
int main()
{
extern int x; /* declaration in main() */
printf("nx=%d",x); // x is used before its definition[Possible because of extern]
first();
printf("nx=%d",x);// Changes done by first are visible here
return 0;
}
void first()
{
extern int x; /* declaration in first() */
printf("nx=%d",x); // x is used again before its definition[Possible because of extern]
x=x+10;
}
int x=10; /* definition of external variable, here x is global variable */

Program example 2-extern storage class
External variable in different file
extern1.c file
#include<stdio.h>
#include"extern2.c"
//Global variable declared in extern1.c
int x=30;
int main()
{
print();
printf("%d",x);//Changes done by extern2.c
file are also reflected
}
extern2.c file
void print()
{
extern int x;//Taking reference of
global variable in different file or
Declaration
printf("%dn",x);
x=x+10;
}
//Output:
30
40

Summary

Scope Rules
• The scope of a variable is the portion of a
program where the variable has meaning
(where it exists).
• A global variable has global (unlimited) scope.
• A local variable’s scope is restricted to the
function that declares the variable.
• A block variable’s scope is restricted to the
block in which the variable is declared.

Local variables
• Parameters and variables declared inside the
definition of a function are local.
• They only exist inside the function body.
• Once the function returns, the variables no
longer exist!

Example-local variable
#include<stdio.h>
void function();
int main()
{
int a=1,b=2;
printf("n a is:%d,b is:%d",a,b);//a,b are local variables of main()
function();
return 0;
}
void function()
{
int c=3;
printf("n Value of c is:%d",c);// c is a local variable of function
}

Block Variables
• You can also declare variables that exist only
within the body of a compound statement (a
block):
{
int f;
…
…
}

Example-block scoped variable
#include<stdio.h>
int main()
{
int b=2;
{
int a=1; // a is block variable
printf("nValue of a is:%d",a);
}
printf("nValue of b is:%d",b);
return 0;
}

Global variables
• You can declare variables outside of any
function definition – these variables are
global variables.
• Any function can access/change global
variables.

Example-global variable
#include<stdio.h>
int a=1;// a is a global variable
void print();
int main()
{
print();
return 0;
}
void print()
{
}

More examples-Scope rules
#include<stdio.h>
void print();
int main()
{
int a=1;
printf("nValue of a is:%d",a);// It will
access local a
print();
return 0;
}
void print()
{
printf("nValue of a is:%d",a); //It will
access global a
}
Output:
Value of a is:1
Value of a is:10
Note:
When we have same named
local and global variables,
priority is always given to local
variable first, that is why in
main() function value of local a is
printed, whereas in function
definition, there is no local
variable so preference is given
to global version of a

#include<stdio.h>
void print();
int main()
{
print();
printf("nValue of a is:%d",a);// Change
done by print to global a is reflected here
return 0;
}
void print()
{
a=20;
}
Output:
Value of a is:10
Value of a is:10
Value of a is:20

#include<stdio.h>
int main()
{
int a=5;
{
int a=50;
{
int a=500;
printf("na:%d",a);
}
printf("na:%d",a);
}
printf("na:%d",a);
return 0;
}
Output:
a:500
a:50
a:5

Q1
What will be the output of the following
C code?
#include <stdio.h>
int x;
void m();
int main()
{
m();
printf("%d", x);
return 0;
}
void m()
{
x = 4;
}
A. 0
B. 4
C. Compile time error
D. Runtime error

Q2
What will be the output of the following C code?
#include <stdio.h>
int x = 5;
void m();
void n();
int main()
{
int x = 3;
m();
printf("%d", x);
return 0;
}
void m()
{
x = 8;
n();
}
void n()
{
printf("%d", x);
}
A. 8 3
B. 3 8
C. 8 5
D. 5 3

Q3
What will be the output of following
code?
#include <stdio.h>
int main()
{
int x=1;
{
x=2;
{
int x=3;
}
}
printf("%d",x);
return 0;
}
A. 1
B. 2
C. 3
D. Compile time error

Q4
In case of a conflict between the names of a
local and global variable what happens?
A. The global variable is given a priority.
B. The local variable is given a priority.
C. Which one will get a priority depends upon
which one is defined first.
D. The compiler reports an error.

Q5
What will be the storage class of variable i in the code
written below?
#include<stdio.h>
int main()
{
int i = 10;
printf("%d",i);
return 0;
}
A. Automatic storage class
B. Extern storage class
C. Static storage class
D. Register storage class

Q6
What will be the behaviour of following code?
#include<stdio.h>
int main()
{
register int a;
printf("nEnter value of a:");
scanf("%d",&a);
return 0;
}
A. Program will work normally
B. Compile time error
C. Runtime error
D. None of the above

Q7
#include<stdio.h>
int incr(int i)
{
static int count = 0;
count = count + i;
return (count);
}
int main()
{
int i,j;
for (i = 0; i <=2; i++)
j = incr(i);
printf("%d",j);
return 0;
}
A. 10
B. 4
C. 3
D. 2

Q8
#include<stdio.h>
void update();
int main()
{
update();
update();
return 0;
}
void update()
{
auto int a=1;
static int b=1;
a++;
b++;
printf("%d,%dn",a,b);
}
A. 2,2
2,3
B. 2,2
2,2
C. 1,1
1,2
D. 2,1
2,2

Q9
#include<stdio.h>
int main()
{
extern int a;
printf("%d",++a);
return 0;
}
int a;
A. 0
B. 1
C. -1
D. Compile time error

Storage_classes_and_Scope_rules.pptx

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