openmpi/ompi/mpi/man/man3/MPI_Neighbor_alltoallw.3in

.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.TH MPI_Neighbor_alltoallw 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#"

.SH NAME
\fBMPI_Neighbor_alltoallw\fP \- All processes send data of different types to, and receive data of different types from, all processes

.SH SYNTAX
.ft R

.SH C Syntax
.nf
#include <mpi.h>
int MPI_Neighbor_alltoallw(void *\fIsendbuf\fP, int *\fIsendcounts\fP,
	int *\fIsdispls\fP, MPI_Datatype *\fIsendtypes\fP,
	void *\fIrecvbuf\fP, int *\fIrecvcounts\fP,
	int *\fIrdispls\fP, MPI_Datatype *\fIrecvtypes\fP, MPI_Comm \fIcomm\fP)

.fi
.SH Fortran Syntax
.nf
INCLUDE 'mpif.h'
MPI_ALLTOALLW(\fISENDBUF, SENDCOUNTS, SDISPLS, SENDTYPES,
	RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPES, COMM, IERROR\fP)

	<type>	\fISENDBUF(*), RECVBUF(*)\fP
	INTEGER	\fISENDCOUNTS(*), SDISPLS(*), SENDTYPES(*)\fP
	INTEGER	\fIRECVCOUNTS(*), RDISPLS(*), RECVTYPES(*)\fP
	INTEGER	\fICOMM, IERROR\fP

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1.2i
sendbuf
Starting address of send buffer.
.TP 1.2i
sendcounts
Integer array, where entry i specifies the number of elements to send
to neighbor i.
.TP 1.2i
sdispls
Integer array, where entry i specifies the displacement (in bytes,
offset from \fIsendbuf\fP) from which to send data to neighbor i.
.TP 1.2i
sendtypes
Datatype array, where entry i specifies the datatype to use when
sending data to neighbor i.
.TP 1.2i
recvcounts
Integer array, where entry j specifies the number of elements to
receive from neighbor j.
.TP 1.2i
rdispls
Integer array, where entry j specifies the displacement (in bytes,
offset from \fIrecvbuf\fP) to which data from neighbor j should
be written.
.TP 1.2i
recvtypes
Datatype array, where entry j specifies the datatype to use when
receiving data from neighbor j.
.TP 1.2i
comm
Communicator over which data is to be exchanged.

.SH OUTPUT PARAMETERS
.ft R
.TP 1.2i
recvbuf
Address of receive buffer.
.ft R
.TP 1.2i
IERROR
Fortran only: Error status.

.SH DESCRIPTION
.ft R
MPI_Neighbor_alltoallw is a generalized collective operation in which all
processes send data to and receive data from all neighbors. It
adds flexibility to MPI_Neighbor_alltoallv by allowing the user to specify the
datatype of individual data blocks (in addition to displacement and
element count). Its operation can be thought of in the following way,
where each process performs 2n (n being the number of neighbors in
the topology of communicator \fIcomm\fP) independent point-to-point communications.
The neighbors and buffer layout are determined by the topology of \fIcomm\fP.
.sp
.nf
        MPI_Cart_get(\fIcomm\fP, maxdims, dims, periods, coords);
        for (dim = 0, i = 0 ; dim < dims ; ++dim) {
            MPI_Cart_shift(\fIcomm\fP, dim, 1, &r0, &r1);
            MPI_Isend(\fIsendbuf\fP + \fIsdispls\fP[i]  * extent(\fIsendtype\fP),
                      \fIsendcount\fP, \fIsendtypes\fP[i], r0, ..., \fIcomm\fP, ...);
            MPI_Irecv(\fIrecvbuf\fP + \fIrdispls\fP[i] * extent(\fIrecvtype\fP),
                      \fIrecvcount\fP, \fIrecvtypes\fP[i], r0, ..., \fIcomm\fP, ...);
            ++i;
            MPI_Isend(\fIsendbuf\fP + \fIsdispls\fP[i] * extent(\fIsendtype\fP),
                      \fIsendcount\fP, \fIsendtypes\fP[i], r1, ..., \fIcomm\fP, &req[i]);
            MPI_Irecv(\fIrecvbuf\fP + \fIrdispls\fP[i] * extent(\fIrecvtype\fP),
                      \fIrecvcount\fP, \fIrecvtypes\fP[i], r1, ..., \fIcomm\fP, ...);
            ++i;
        }

        MPI_Wait_all (...);

	MPI_Comm_size(\fIcomm\fP, &n);
	for (i = 0, i < n; i++)
	    MPI_Send(\fIsendbuf\fP + \fIsdispls\fP[i], \fIsendcounts\fP[i],
	        \fIsendtypes\fP[i], i, ..., \fIcomm\fP);
	for (i = 0, i < n; i++)
	    MPI_Recv(\fIrecvbuf\fP + \fIrdispls\fP[i], \fIrecvcounts\fP[i],
	        \fIrecvtypes\fP[i], i, ..., \fIcomm\fP);
.fi
.sp
Process j sends the k-th block of its local \fIsendbuf\fP to neighbor
k, which places the data in the j-th block of its local
\fIrecvbuf\fP.
.sp
When a pair of processes exchanges data, each may pass different
element count and datatype arguments so long as the sender specifies
the same amount of data to send (in bytes) as the receiver expects
to receive.
.sp
Note that process i may send a different amount of data to process j
than it receives from process j. Also, a process may send entirely
different amounts and types of data to different processes in the
communicator.

.sp
.SH NOTES
.ft R
The MPI_IN_PLACE option for \fIsendbuf\fP is not meaningful for this operation
.sp
The specification of counts, types, and displacements should not cause
any location to be written more than once.
.sp
All arguments on all processes are significant. The \fIcomm\fP argument,
in particular, must describe the same communicator on all processes.
.sp
The offsets of \fIsdispls\fP and \fIrdispls\fP are measured in bytes.
Compare this to MPI_Neighbor_alltoallv, where these offsets are measured in units
of \fIsendtype\fP and \fIrecvtype\fP, respectively.

.SH ERRORS
.ft R
Almost all MPI routines return an error value; C routines as
the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.nf
MPI_Neighbor_alltoall
MPI_Neighbor_alltoallv
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create