According to Microsoft's documentation (https://docs.microsoft.com/en-us/windows/desktop/devnotes/rtlmovememory), the last parameter is of type SIZE_T.
As far as I know, size_t is 64 bits on 64-bit system so I believe LongPtr makes sense here.
As far as I know, size_t is 64 bits on 64-bit system so I believe LongPtr makes sense here.