It's kind of implied by the parameters that they point to the same object; they're named "pos" and "end" after all. But I agree, a count variable is nicer.

The subtraction also has a (very theoretical) problem that its result must fit in ptrdiff_t (signed), and it would produce undefined behavior when you have a huge object whose size doesn't fit in ptrdiff_t (but does in size_t, by definition).