[Mpi-comments] Use of MPI_DATATYPE_NULL

[gilles at rist.or.jp]

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


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