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Q&A What's the difference between null pointers and NULL?

To answer one specific part of the question: Does this have anything to do with some systems allowing a different representation of the null pointer other than zero? Sort of. A null pointer ...

posted 3y ago by Martin Bonner‭  ·  last activity 3y ago by Martin Bonner‭

Answer
#1: Initial revision by user avatar Martin Bonner‭ · 2020-12-08T19:11:07Z (over 3 years ago)
To answer one specific part of the question:

 > Does this have anything to do with some systems allowing a different representation of the null pointer other than zero?

Sort of.  A null pointer constant is zero or zero cast to a `void *`, but that doesn't mean that a null pointer is necessarily represented by "all bits zero".  If a null pointer is not "all bits zero", a compiler is obliged to recognize when it is being asked to assign a null pointer constant to a pointer variable, and assign the correct value.  Similarly for comparisons.

(I have actually used a machine where a null pointer was not "all bits zero" - Prime eventually gave up and added an instruction "is pointer C null" which meant "all bits zero or proper null pointer".)