From: "Bronis R. de Supinski" <bronis@XXXXXXXX>
Date: January 22, 2008 4:32:16 PM CST
To: "mpi-21@mpi-forum.org" <mpi-21@XXXXXXXXXXXXX>
Subject: RE: [mpi-21] Proposal EH2 & MPI_INPLACE
Reply-To: mpi-21@XXXXXXXXXXXXX


Erez:

The issue at hand was not whether MPI_INPLACE should be const,
it's whether the send buffer should be const for operations that
use MPI_INPLACE for the receive buffer. The problem is that the
send buffer is modified.

Bronis

On Tue, 22 Jan 2008, Erez Haba wrote:

> 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@mpi-forum.org [mailto:owner-mpi-21@mpi- 
> forum.org] On Behalf Of Dries Kimpe
> Sent: Tuesday, January 22, 2008 12:40 PM
> To: mpi-21@mpi-forum.org
> 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
>
>