[Mpi-forum] MPI_DATATYPE_NULL when count=0
wgropp at illinois.edu
Tue Jan 12 21:51:48 CST 2016
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).
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?
> 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.
> 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 Hammond
>> jeff.science at gmail.com
>> mpi-forum mailing list
>> mpi-forum at lists.mpi-forum.org
> Jeff Hammond
> jeff.science at gmail.com
> mpi-forum mailing list
> mpi-forum at lists.mpi-forum.org
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