[pxx]

(void *) 0 is a null pointer constant and is not necessarily an all zeros representation. This has been defined virtually forever.

https://c-faq.com/null/null2.html

https://c-faq.com/null/machexamp.html

Actual ways to do what you want to do are described in

https://c-faq.com/null/accessloc0.html

but technically speaking the pointer with a constant zero assigned to it _is_ a null pointer (which can be implemented as whatever bit pattern), independent of the preprocessor macro.