>>> How can a NULL pointer ever point to the same storage object as any
>>> other non-NULL pointer?
>> _Into_, not _to_.
> Same difference (at least for NULL).  ;-)

True.  I misread the question as asking, basically, how different
pointers can point into the same object - reinforced by the "other", I
misread the first NULL as non-NULL.

> As far as I can tell there's no one statement in the standard to the
> effect that "pointer arithmetic with the NULL pointer is invalid",

I think it follows because pointer arithmetic is valid only when you
stay within (or just beyond) the object the pointer started out
pointing into.  Since a null pointer does not point into any object, no
arithmetic on it can be valid.

Nevertheless, like a lot of other strictly nonportable hacks, the trick
given is a cool and useful hack.

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