A simple rule is, “Always use `0′ or `NULL’ for null pointers, and always cast them when they are used as arguments in function calls.”
The fact that null pointers are represented both in source code, and internally to most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) suggests that the value might change later, or on some weird machine.
A “null pointer” is a language concept whose particular internal value does not matter. A null pointer is requested in source code with the character “0″. “NULL” is a preprocessor macro, which is always #defined as 0 (or (void *)0).
A constant 0 in a pointer context is converted into a null pointer at compile time. A “pointer context” is an initialization, assignment, or comparison with one side a variable or expression of pointer type, and (in ANSI standard C) a function argument which has a prototype in scope declaring a certain parameter as being of pointer type. In other contexts (function arguments without prototypes, or in the variable part of variadic function calls) a constant 0 with an appropriate explicit cast is required.
