[Mpi-comments] Use of MPI_DATATYPE_NULL

[George Bosilca]

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
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