Newsgroups: comp.parallel.mpi
From: Ian Bush <I.J.Bush@dl.ac.uk>
Subject: MPI_gatherv
Organization: Daresbury Laboratory
Date: Thu, 14 May 1998 16:33:08 +0100
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Message-ID: <355B0EB4.D72967C8@dl.ac.uk>

Hi All,

    I'm hoping that some standards lawyer might be able to confirm
my understanding of something ...

    The basic question is can you have sendcount=0 in MPI_gatherv,
and if you can what corresponding values you should have for displs ?
As I read it IF sendcount=0 is allowed, and it is not explicitly
stated to be prohibited, then any old value for the relevant displs
will do, e.g. 0, because of the sentence

"The specification of counts, types and displacements should not cause 
any location on the root to be written more than once"

as if sendcount=0 for that processor no writing will occur.

    I'm asking because for various perverse reasons I want to gather
from a subset of the communicators processors. Yes, maybe I am being
a bit lazy, MPI_comm_split to make the communicator with the interesting
processors in it, gatherv then free the new communicator might be
better, 
but a new communicator for just one operation seems a little bit ugly to 
me, hence the question,

Ian