What Is Null Pointer in C Language?

It is always a good practice to assign a NULL value to a Pointer variable in case you do not have an 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 at 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. However, 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 */

Posted in: C faq