[Mpi3-bwcompat] MPI_Count mixing with int...

[Fab Tillier]

Quincey Koziol wrote on Thu, 15 Jul 2010 at 00:50:14

> Hi Fab,
> 	Sorry for the delay in replying...
> 
> On Jul 9, 2010, at 2:22 PM, Fab Tillier wrote:
> 
>> Quincey Koziol wrote on Fri, 9 Jul 2010 at 12:05:52
>> 
>>> 	I still think aliasing MPI_Size and MPI_Count will cause problems
>>> down the line, since the two concepts are different.  I'd be much
>>> happier with separating them now rather than having to change things
>>> that are switched from int to MPI_Size now to MPI_Count later.
>> 
>> Having MPI_Size and MPI_Count different types causes
> MPI_Pack/MPI_Send and MPI_Recv/MPI_Unpack to have problems, since the
> count and sizes are passed between the two functions and should be
> interchangeable, though.
> 
> 	Well, there probably is something wrong with the definition of
> MPI_Pack and MPI_Unpack then.  OK, I've gone off and re-read the
> definition of MPI_Pack in the standard and it looks OK to me.  What is
> the problem with using MPI_Count for the 'incount' parameter and
> MPI_Size for the 'outsize' parameter?

When you go to MPI_Send a packed buffer, you would pass the 'outsize' as the count parameter, and MPI_PACKED as the datatype.  If MPI_Size has a larger range than MPI_Count, you could run into problems.  The converse is true of MPI_Recv and MPI_Unpack - if the MPI_Count type is larger than the MPI_Size type, you may not be able to unpack (you could receive more than what you can express in the call to MPI_Unpack.)

>  (I think I would suggest
> changing 'inposition' from a 'int *' to a 'void *' while we're mucking
> about in here, also)

The inposition is an offset from buf, it's an inout parameter thus being passed by reference so that MPI_Pack can update the position.

-Fab