NULL Pointer in C programming

Introduction

Definition : A NULL pointer is defined as the special pointer value that points to nowhere in the memory. If it is too early in the code to assign a value to the pointer, then it is better to assign NULL (i.e., \0 or 0) to the pointer.

For example , consider the following code:

#include<stdio.h>
int *p=NULL;
Here, the pointer variable p is a NULL pointer, this indicates that the pointer variable p does not point to any part of the memory. The value for NULL is defined in the header file "stdio.h". Instead of using NULL, we can also use '\0' or 0. The programmer can access the data using the pointer variable p if and only if it does not contain NULL. The error condition can be checked using the following statement:
if(p==NULL)
printf("p does not point to any memory\n");
else
{
printf("Access the value of p\n");
......................
}
Note : A pointer variable must be initialized. If it is too early to initialize a pointer variable, then it is better to initialize all pointer variables to NULL in the beginning of the code. This avoids unintentional use of an un-initialized pointer variable.

Example :  Consider the following statements:
int *x;
int y;
x=y;   /* Error*/
Note :  The value of data variable can not be assigned to a pointer variable. So, the statement x=y; result in an error . The correct statement is x=&y;