[Mpi-forum] MPI_DATATYPE_NULL when count=0

[Jeff Hammond]

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
> <javascript:_e(%7B%7D,'cvml','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
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.open-2Dmpi.org_community_lists_users_2016_01_28255.php&d=BQMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=195OD1c98cxYlIPBUDzl00KueizThEKs34a9DvL37eY&m=tWmZZT774xAE-RjUpTuHeZ0-hHkwEoE5sQADPmobarg&s=7rkfIDjMiCphV1WD00EIRKeH44dIa97JAz6C3HTPF7M&e=>
> 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
> <javascript:_e(%7B%7D,'cvml','jeff.science at gmail.com');>
> http://jeffhammond.github.io/
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__jeffhammond.github.io_&d=BQMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=195OD1c98cxYlIPBUDzl00KueizThEKs34a9DvL37eY&m=tWmZZT774xAE-RjUpTuHeZ0-hHkwEoE5sQADPmobarg&s=l_bf2ha5FZxeiaQWAM3hSLdzVFrXjR7lP6X99U3j8Yo&e=>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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