[Mpi-forum] MPI_DATATYPE_NULL when count=0

William Gropp wgropp at illinois.edu
Tue Jan 12 21:44:12 CST 2016

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.


William Gropp
Director, Parallel Computing Institute
Thomas M. Siebel Chair in Computer Science
Chief Scientist, NCSA
University of Illinois Urbana-Champaign

On Jan 12, 2016, at 9:26 PM, Jeff Hammond <jeff.science at gmail.com> wrote:

> Is it allowed to communicate messages with (buffer=NULL,count=0,type=MPI_DATATYPE_NULL)?
> George thinks MPI_DATATYPE_NULL cannot be used even when count=0, which I think is ridiculous, particularly when one makes the analogy to buffer=NULL.
> See http://www.open-mpi.org/community/lists/users/2016/01/28255.php for the thread in question.
> There is example code in MPI that uses this behavior.  George and I agree that example text is non-normative, but I cannot imagine how this example came to exist without the Forum believing it to be valid.
> Jeff
> -- 
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
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