Pointer in C

Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps.
As you know every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator which denotes an address in memory. Consider the following example which will print the address of the variables defined:

#include <stdio.h>

int main ()
{
   int  var1;
   char var2[10];

   printf("Address of var1 variable: %x\n", &var1  );
   printf("Address of var2 variable: %x\n", &var2  );

   return 0;
}

When the above code is compiled and executed, it produces

result something as follows:

Address of var1 variable: bff5a400
Address of var2 variable: bff5a3f6

So you understood what is memory address and how to access it, so base of the concept is over. Now let us see what is a pointer.

What Are Pointers?

A pointer is a variable whose value is the address of another variable ie. direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid

C data type and var-name is the name of the pointer 

variable. The asterisk * you used to declare a pointer 

is the same asterisk that you use for multiplication. 

However, in this statement the asterisk is being used 

to designate a variable as a pointer. 

Following are the valid pointer declaration:

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or
otherwise, is the same, a long hexadecimal number that represents a memory address.
The only difference between pointers of different data types is the data type of the
variable or constant that the pointer points to.

How to use Pointers?

There are few important operations which we will do with the help of pointers very
frequently. (a) we define a pointer variables (b) assign the address of a variable to a
pointer and (c) finally access the value at the address available in the pointer variable.
This is done by using unary operator * that returns the value of the variable located
at the address specified by its operand. Following example makes use of these
operations:

#include <stdio.h>

int main ()
{
   int  var = 20;   /* actual variable declaration */
   int  *ip;        /* pointer variable declaration */

   ip = &var;  /* store address of var in pointer variable*/

   printf("Address of var variable: %x\n", &var  );

   /* address stored in pointer variable */
   printf("Address stored in ip variable: %x\n", ip );

   /* access the value using the pointer */
   printf("Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it 

produces result something as follows:

Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20

NULL Pointers in C

It is always a good practice to assign a NULL value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.
The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program:

#include <stdio.h>

int main ()
{
   int  *ptr = NULL;

   printf("The value of ptr is : %x\n", ptr  );
 
   return 0;
}

When the above code is compiled and executed, it produces following result:

The value of ptr is 0

On most of the operating systems, programs are not permitted to access memory
at address 0 because that memory is reserved by the operating system. However,
the memory address 0 has special significance; it signals that the pointer is not
intended to point to an accessible memory location. But by convention, if a pointer
contains the null (zero) value, it is assumed to point to nothing.
To check for a null pointer you can use an if statement as follows:

if(ptr)     /* succeeds if p is not null */
if(!ptr)    /* succeeds if p is null */