NULL Pointers in C

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 */

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