The code is still wrong for another reason: if (uintptr_t)ptrOld < (uintptr_t)ptrNew, then the subtraction is calculated modulo some power of 2 yielding a large

The code is still wrong for another reason: if (uintptr_t)ptrOld < (uintptr_t)ptrNew, then the subtraction is calculated modulo some power of 2 yielding a large number. The result of converting this to a signed integral type like ptrdiff_t is implementation defined. In practice it works only if sizeof(ptrdiff_t) == sizeof(uintptr_t) AND the system uses two's complement for signed integers. Neither property is guaranteed by the standard. I think the correct way to find the byte offset is this: ptrdiff_t diff = (char*)ptrOld - (char*)ptrNew;