From: Erez Haba <erezh@XXXXXXXXXXXXX>
Date: January 22, 2008 2:58:59 PM CST
To: "mpi-21@mpi-forum.org" <mpi-21@XXXXXXXXXXXXX>
Subject: RE: [mpi-21] Proposal EH2 & MPI_INPLACE
Reply-To: mpi-21@XXXXXXXXXXXXX

I don't see the problem, would you care to elaborate?

As far as I know the users specifies MPI_IN_PLACE as the sendbuf and the real read/write buff to the recvbuff. Thus the implementation read/writes to the recvbuf and not to the MPI_IN_PLACE value.

As a complement to the const proposal we should actually define MPI_IN_PLACE and a pointer to const (e.g., '#define MPI_IN_PLACE  (const void*)-1'), which will help people avoid making the mistake of specifying MPI_IN_PLACE to the recv buffer.

Thanks,
.Erez

-----Original Message-----
From: owner-mpi-21@XXXXXXXXXXXXX
Sent: Tuesday, January 22, 2008 12:40 PM
To: mpi-21@XXXXXXXXXXXXX
Subject: [mpi-21] Proposal EH2 & MPI_INPLACE

There is a problem with adding const to the send buffer of some of the
collective operations:

Because of MPI_IN_PLACE, in some cases, the send buffer will not be
read-only but read-write.

Example:
MPI_Allreduce

Affected:
All collectives that support MPI_IN_PLACE --AND-- modify the data in the send
buffer.

Solution:
Drop the const from the send buffer argument of the affected functions

  Dries