WHAT IS THE DIFFERENCE BETWEEN NULL AND ZERO?

NULL is a macro which is defined in C header files. The value of NULL macro is 0.
It is defined in C header files as below.
#define NULL (void *) 0;

• NULL is used for pointers only as it is defined as (void *) 0. It should not be used other than pointers. If NULL is assigned to a pointer, then pointer is pointing to nothing.
• 0 (zero) is a value.