What is the difference between a null pointer and a null macro?

Answer 1

Using a NULL macro to make C portable

I'll assume that you're asking your question for C type language programming. A NULL pointer is a pointer that's guarnteed to point to nothing. This may be 0 in a UNIX/Linux system or some other address in another system. Using the NULL macro to set/initialize your pointers will make your programs more portable among systems than using something like the 0.

#include

char *c = 0; // initialize to NULL--not portable
char *p = NULL; // initialize to NULL as defined in stdio is portable

AddendumThe code:

char *c = 0;

actually is portable because the compiler converts 0's used in a pointer context (cast to a pointer) to the machine's representation of a NULL pointer, which may or may not be all 0 bits. The NULL macro itself might be defined as something like 0 or (void *)0, and both definitions are portable. As a corollary, the following code is also portable:

if (!c) {
// do something
}

because it is equivalent to:

if (c != 0) {
// do something
}

and the 0 above is converted to a NULL pointer because it is being compared with a pointer.

Answer 2

A null pointer is a pointer variable with the value 0. A null macro is a macro that defines the NULL symbol (typically 0L on a 32-bit system).