Storage classes

• A variable in C can have any one of the four storage classes.

1. Automatic variables.2. External variables.3. Static variables.4. Register variables.

AUTOMATIC VARIABLES (LOCAL/INTERNAL)• Automatic variables are declared inside a function in

which they are to be utilized.• They are created when a function is called and destroyed

automatically when the function is exited.• Eg:main()

{int number;

}• We may also use the keyword auto to declare automatic

variables explicitly.

EXTERNAL VARIABLES• Variables that are both alive and active throughout

the entire program are known as external variables. They are also known as global variables.extern float length = 7.5;void main(){}function1( ){}The keyword extern can be used for explicit declarations of external variables.

STATIC VARIABLES• As the name suggests, the value of a static

variable persists until the end of the program. A variable can be declared static using the keyword static.

• Eg:• 1) static int x;• 2) static int y;

REGISTER VARIABLES• We can tell the compiler that a variable should be

kept in one of the machine’s registers, instead of keeping in the memory.

• Since a register access is much faster than a memory access, keeping the frequently accessed variables in the register will lead to faster execution of programs.

• This is done as follows:register int count;

POINTERS • A pointer is a variable that represents the location of a

data item, such as a variable or an array element.

• A pointer is a memory address.

• In the simplest term pointer is a nearly integer variable which stores a memory address of a computer which may contain other variable or even another pointer.

Concept of Address and Pointers• Memory can be

conceptualized as a linear set of data locations.

• Variables reference the contents of a locations

• Pointers have a value of the address of a given location

Contents1

Contents11

Contents16

ADDR1ADDR2ADDR3ADDR4ADDR5ADDR6

***

ADDR11

**

ADDR16

Pointer Variable• Declaring a pointer variable is quite similar to

declaring an normal variable all you have to do is to insert a star '*' operator before it.

• General form of pointer declaration is -type* name;

For Exampleint * variable1;

• Say you declare a variable named foo.int foo;

• Now let's declare another variable of pointer type

int *foo_ptr = &foo;foo_ptr is declared as a pointer to int. We have initialized it to point to foo.

Use of & and *

• When is & used?• When is * used?

• & -- "address operator" which gives or produces the memory address of a data variable.

• * -- "dereferencing operator" which provides the contents in the memory location specified by a pointer

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

int count = 10, x;int *int_pointer;int_pointer = &count;x = *int_pointer;printf ("count = %d, x = %d\n", count, x);

}

Using Pointers in Expressions#include <stdio.h>void main (){ int i1, i2;

int *p1;i1 = 5;p1 = &i1;i2 = *p1 + 20;

printf ("i1 = %d, i2 = %d, *p1 = %d ", i1, i2, *p1);}

Pointer arithmetic• Pointers can be added and subtracted.

• However pointer arithmetic is quite meaningless unless performed on arrays.

• Addition and subtraction are mainly for moving forward and backward in an array.

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

int ArrayA[3]={1,2,3};int *ptr;ptr=ArrayA;printf("address: %p - array value:%d “,ptr,*ptr);ptr++;printf("address: %p – array value:%d”,ptr,*ptr);

}

Pointers and Functions

• Pointers can be used to pass addresses of variables to called functions, thus allowing the called function to alter the values stored there.

• There are two mechanism to call a function1) Pass by Value2) Pass By Reference

Pass by Value#include <stdio.h>

void main ( )

{

int a = 5, b = 6;

printf("a=%d b=%d\n",a,b) ;

swap (a, b) ;

printf("a=%d b=%d\n",a,b) ;

}

void swap(int a, int b)

{

int temp;

temp= a; a= b; b = temp ;

printf ("a=%d b=%d\n", a, b);

}

Results:a=5 b=6a=6 b=5a=5 b=6

#include <stdio.h>

void main ( )

{

int a = 5, b = 6;

printf("a=%d b=%d\n",a,b) ;

swap (&a, &b) ;

printf("a=%d b=%d\n",a,b) ;

}

void swap(int *a, int *b)

{

int temp;

temp=*a; *a=*b; *b = temp ;

printf ("a=%d b=%d\n", *a, *b);

}

Pass by Reference

Results:a=5 b=6a=6 b=5a=6 b=5