Isn't another problem of "ptrdiff_t diff = (uintptr_t)ptrOld - (uintptr_t)ptrNew;" that you're assigning an unsigned value to a signed variable?

If ptrNew is larger than ptrOld this can give a very large value via wrapping around, so large that it will probably be bigger than the max value of ptrdiff_t, making the assignment a form of signed overflow.