[Mpi-comments] Use of MPI_DATATYPE_NULL

[Jim Edwards]

Right, so how do I formally request that the standard be updated to clarify
this issue?

On Thu, Dec 8, 2022 at 4:56 PM gilles--- via mpi-comments <
mpi-comments at lists.mpi-forum.org> wrote:

> I have a different recollection.
>
>
>
> The link is broken, here is one that works for me
>
> https://lists.mpi-forum.org/pipermail/mpi-forum/2016-January/003152.html
>
> TL;DR from Bill Gropp:
>
> "The standard says that the null handles are invalid for input unless explicitly permitted.  There’s no exemption for a datatype argument where the associated count is zero."
>
>
>
> IIRC there was no objection to updating the standard in order to allow
> this, but unless this has been completed, I am afraid MPI_DATATYPE_NULL is
> not a valid datatype here
>
>
>
> Gilles
>
> ----- Original Message -----
>
> You are correct. The MPI_DATATYPE_NULL is an acceptable datatype for cases
> where the target processor sends or receives no data. The standard
> uses MPI_DATATYPE_NULL in 2 instances (in example 5.23) to highlight no
> data movement, but fails to make a clear reference to the fact that this is
> an acceptable use.
>
>   George.
>
> On Thu, Dec 8, 2022 at 5:31 PM Jim Edwards via mpi-comments <
> mpi-comments at lists.mpi-forum.org> wrote:
>
>> There seems to be some ambiguity in the standard with respect to usage of
>> the MPI_DATATYPE_NULL variable.
>> It would seem to me that if an MPI routine such as MPI_Send or
>> MPI_ALLTOALLW has a data count argument of 0
>> then MPI_DATATYPE_NULL should be an acceptable value for the datatype.
>> Here is a discussion from several years
>> ago with openmpi developers.  It references a discussion in the standards
>> forum but I can no longer find it.
>>
>> http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php
>>
>> Would it be possibly to clarify this case in the next iteration of the
>> standard?
>>
>> --
>> Jim Edwards
>>
>> CESM Software Engineer
>> National Center for Atmospheric Research
>> Boulder, CO
>> _______________________________________________
>> mpi-comments mailing list
>> mpi-comments at lists.mpi-forum.org
>> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments
>
>
> _______________________________________________
> mpi-comments mailing list
> mpi-comments at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments
>


-- 
Jim Edwards

CESM Software Engineer
National Center for Atmospheric Research
Boulder, CO
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mpi-forum.org/pipermail/mpi-comments/attachments/20221208/0947109e/attachment.html>