[mpich-discuss] MPI_Reduce syntax

[Martin Siegert]

Hi,

the MPI standard
http://www.mpi-forum.org/docs/mpi22-report/node103.htm#Node103
says the following about MPI_Reduce:

"The routine is called by all group members using the same arguments for
 count, datatype, op, root and  comm. Thus, all processes provide input
 buffers and output buffers of the same length, with elements of the
 same type."

Thus, the following appears to be illegal:

buf = (double *)malloc(l*sizeof(double);
random(buf, l); /* fill buf with something */
if (myid == 0) {
   MPI_Reduce(MPI_IN_PLACE, buf, l, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
} else {
   MPI_Reduce(buf, NULL, l, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
}

because it violates the "same length" requirement of input and output
buffers. But why do I need to provide a valid output buffer for processes
other than the receiving one?

Will the code using NULL as the receive buffer for those processes
that do not receive anything work with mpich2?

Cheers,
Martin

-- 
Martin Siegert
Head, Research Computing
WestGrid Site Lead
IT Services                                phone: 778 782-4691
Simon Fraser University                    fax:   778 782-4242
Burnaby, British Columbia                  email: siegert at sfu.ca
Canada  V5A 1S6