[Mpi-forum] MPI Count proposal from today's meeting

[Supalov, Alexander]

Thanks. I agree that use of the MPI_UNDEFINED by the MPI_GET_COUNT may need additional review. Why do you think the ranks are "short int" entities?

-----Original Message-----
From: mpi-forum-bounces at lists.mpi-forum.org [mailto:mpi-forum-bounces at lists.mpi-forum.org] On Behalf Of N.M. Maclaren
Sent: Friday, June 18, 2010 9:30 AM
To: Main MPI Forum mailing list
Subject: Re: [Mpi-forum] MPI Count proposal from today's meeting

On Jun 17 2010, N.M. Maclaren wrote:
>On Jun 17 2010, Supalov, Alexander wrote:
>>
>>Thanks. Why signed?
>
>...
>
>    It is useful to be able to specify invalid or exceptional values as
>out-of-band ones, and that isn't possible for any positive integer.  The
>standard states that MPI_UNDEFINED is negative in one place, and that
>can be returned from MPI_Get_count.

Sorry about following up to myself, but I though about it later, and
this is a mathematically solid proof that either MPI_Count must be
signed or the specification of MPI_Get_count (at least) needs changing.

MPI_UNDEFINED has to be a value that will fit in a 'short' integer, as
it is heavily used for undefined ranks.  But ALL non-negative 'short'
integer values are also valid counts when using 'long' integers for
those, and it has to be something that can't be a valid count.  Q.E.D.

Regards,
Nick Maclaren.

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