[Mpi-forum] MPI_DATATYPE_NULL when count=0

[Jeff Hammond]

That is precisely what I want: when count=0, the buffer and the datatype
arguments are ignored.  There may be a good reason for MPI_DATATYPE_NULL to
be invalid when it is actually relevant, but when count=0, the datatype
argument is not relevant, because there are zero of them.

We already state explicitly that arguments are ignored for MPI_IN_PLACE and
MPI_NO_OP:

* The “in place” option for intracommunicators is specified by passing
MPI_IN_PLACE as the value of recvbuf at the root. In such a case, recvcount
and recvtype are ignored..."

* "When MPI_NO_OP is specified as the operation, the origin_addr,
origin_count, and origin_datatype arguments are ignored."

I see no reason why count=0 is different from these cases.

Jeff

On Wed, Jan 13, 2016 at 5:47 AM, William Gropp <wgropp at illinois.edu> wrote:
>
> True, MPI_BOTTOM need not be null, but it might be, so the standard
doesn’t prohibit it.
>
> A carefully worded exception to the general rule for NULL handles would
be ok.  I would still make it narrow; for example, just for the use of
MPI_DATATYPE_NULL in communication operations with a zero count.
>
> 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 11:22 PM, Fab Tillier <ftillier at microsoft.com> wrote:
>
> Don’t datatypes with absolute addresses rely on MPI_BOTTOM, and a
portable program can’t assume that MPI_BOTTOM == NULL?
>
> I think from an ease of use perspective, if I have a count of zero,
having to pick an arbitrary committed datatype is non-intuitive.  If count
is zero, there is no buffer or datatype, and there should be no requirement
such non-existent datatype be committed.
>
> Cheers,
> -Fab
>
> From: mpi-forum [mailto:mpi-forum-bounces at lists.mpi-forum.org] On Behalf
Of William Gropp
> Sent: Wednesday, 13 January 2016 4:52 p.m.
> To: Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
> Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
>
> 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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--
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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