[Mpi-forum] MPI_DATATYPE_NULL when count=0

[William Gropp]

Yes, NULL buffers are valid - they aren’t an MPI object.  There is no prohibition about passing them; only if they are used to form an invalid address (note that a datatype with absolute addresses relies on NULL as the buffer address).

Bill

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 9:49 PM, Jeff Hammond <jeff.science at gmail.com> wrote:

> Who wrote Example 4.23 of MPI 3.1?
> 
> I propose to add the exception rather than delete that example. I do not want to have to use real datatypes when sending nothing. 
> 
> Is NULL a valid buffer handle? Can I pass null pointers when count=0?
> 
> Jeff 
> 
> On Tuesday, January 12, 2016, William Gropp <wgropp at illinois.edu> wrote:
> 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.
> 
> Bill
> 
> 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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> 
> 
> 
> -- 
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
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