What is a NULL pointer? How is it different from an unitialized pointer? How is a NULL pointer defined?

A null pointer simply means "I am not allocated yet!" and "I am not pointing to anything yet!".

The C language definition states that for every available pointer type, there is a special value which is called the null pointer. It is guaranteed to compare unequal to a pointer to any object or function.

A null pointer is very different from an uninitialized pointer. A null pointer does not point to any object or function; but an uninitialized pointer can point anywhere.

There is usually a null pointer for each type of a pointer, and the internal values of these null pointers for different pointer types may be different, its up to the compiler. The & operator will never yield a null pointer, nor will a successful call to malloc() (malloc() does return a null pointer when it fails).


execl("/bin/ls", "ls", "-l", (char *)0);


In this call to execl(), the last argument has been explicitly casted to force the 0 to be treated as a pointer.

Also, if ptr is a pointer then


if(ptr){}


and


if(!ptr){}


are perfectly valid.


How is NULL defined?

ANSI C allows the following definition


#define NULL ((void *)0)


NULL and 0 are interchangeable in pointer contexts.

Make sure you are able to distinguish between the following : the null pointer, the internal representation of a null pointer, the null pointer constant (i.e, 0), the NULL macro, the ASCII null character (NUL), the null string ("").



Examples:

What is the subtle error in the following code segment?
void fun(int n, int arr[])
{
int *p=0;
int i=0;
while(i++
p = &arr[i];
*p = 0;
}
Answer & Explanation:
If the body of the loop never executes p is assigned no address. So
p remains NULL where *p =0 may result in problem (may rise to
runtime error “NULL pointer assignment” and terminate the
program).