From: Rolf Hempel <hempel@XXXXXXXXXXXXXXXXXXXXXXXXXXX>
Date: Tue, 23 Jan 2001 13:06:36 +0100 (MEZ)
To: mpi-core@XXXXXXXXXXX
Subject: Re: Buffer overlap with collective communications
Cc: hempel@XXXXXXXXXXXX
Reply-To: hempel@XXXXXXXXXXXXXXXXXXXXXXXXXXX
Sender: owner-mpi-core@XXXXXXXXXXXXX

Dear Rolf (the other one),

In "MPI, The Complete Reference", section 1.7.2 "Procedure
Specification", it says: "Unless specified otherwise, an argument of
type OUT or type INOUT cannot be aliased with any other argument passed
to an MPI procedure."

Since the recvbuf argument is of type OUT, and since I don't see where
aliasing is allowed explicitly, I conclude that it is forbidden to
aliase it with the sendbuf argument.

This said, I agree with (the other) Rolf that the descriptions of the
collective operations in terms of pt2pt operations are rather sloppy.
An example is the MPI_Gather function which is explained in terms of
MPI_Sends, executed by all processes (including root), and n MPI_Recvs
at root. Even if at root the buffers are not aliased, sending a message
to itself (with MPI_Send and MPI_Recv) is unsafe MPI programming, as
stated somewhere else in the standard.

Best Wishes,
  Rolf
------------------------------------------------------------------------
Rolf Hempel
Manager, C&C Research Laboratories, NEC Europe Ltd.
Rathausallee 10, 53757 Sankt Augustin, Germany
TEL.:  +49 (0) 2241 - 92 52 - 95,    FAX:  +49 (0) 2241 - 92 52 - 99
EMAIL: hempel@ccrl-nece.de
WWW:   http://www.ccrl-nece.de/~hempel/