>>> When subtracting pointers they both have to point into the same
>>> array.
>> Isn't it into the same _object_?
> How can a NULL pointer ever point to the same storage object as any
> other non-NULL pointer?

_Into_, not _to_.

{
	struct foo { ... int bar; ... int baz; ... };
	int *a;
	int *b;
	struct foo f;
	...
	a = &f.bar;
	b = &f.baz;

Now, is b-a a legal subtraction?

How about if s/int/char/ throughout (char * is special)?

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