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Subject: Re: [OMPI users] MPI_Gatherv and 0 size?
From: jody (jody.xha_at_[hidden])
Date: 2009-02-23 16:59:41


Yes, Eugene fomulates my question in a clear way.

I have indeed been successfully using NULL for all those arguments
in MPI_Gatherv (for instance) where it is explicitly stated that
the argument in question is ignored.
So i have been wondering whether a buffer argument is also ignored
when the bufcount is 0.

Jody

On Mon, Feb 23, 2009 at 9:55 PM, Eugene Loh <Eugene.Loh_at_[hidden]> wrote:
> I think the question is about passing NULL as a buffer pointer.  E.g.,
>
> MPI_Send(NULL, 0, mytype,dst, tag,comm);
>
> vs
>
> MPI_Send(&dummy,0,mytype,dst,tag,comm);
>
> George Bosilca wrote:
>
>> The only restriction the MPI standard impose on the sendcounts and
>>  recvcounts arrays is that the values be non-negative, so zero is a  legal
>> value.
>>
>> I don't really understand your second question. Where do you want to  use
>> NULL ? In general NULL is not accepted in MPI as an argument for  any
>> function ...
>>
>> On Feb 23, 2009, at 02:55 , jody wrote:
>>
>>> I have an application in which various processes create different
>>> amounts of data (including no data at all).
>>> I have found that MPI_Scatterv and MPI_Gatherv do work with arrays
>>> sendcounts or receivecounts containing zeros.
>>>
>>> Since i didn't find the case of 0 sizes in the descriptions
>>> of these functions in the MPI reference, i wondered whether
>>> the use of zeroes is legal or if i was simply lucky that it worked.
>>>
>>> An other point: currently i use 1-sized buffers in the
>>> case of a 0-sized data transfer.
>>> Now if 0-sized data transfer is legal, would it be ok to
>>> pass NULL for the buffer?
>>
>
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