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?, you may ask.

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 ("").