.\" -*- nroff -*-
.\" 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
.\" $COPYRIGHT$
.TH MPI_Neighbor_allgather 3 "Jan 31, 2017" "2.0.2" "Open MPI"
.SH NAME
\fBMPI_Neighbor_allgather, MPI_Ineighbor_allgather\fP \- Gathers and distributes data from and to all neighbors

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Neighbor_allgather(const void\fI *sendbuf\fP, int \fI sendcount\fP,
	MPI_Datatype\fI sendtype\fP, void\fI *recvbuf\fP, int\fI recvcount\fP,
	 MPI_Datatype\fI recvtype\fP, MPI_Comm\fI comm\fP)

int MPI_Ineighbor_allgather(const void\fI *sendbuf\fP, int \fI sendcount\fP,
	MPI_Datatype\fI sendtype\fP, void\fI *recvbuf\fP, int\fI recvcount\fP,
	 MPI_Datatype\fI recvtype\fP, MPI_Comm\fI comm\fP, MPI_Request \fIreq\fP)

.fi
.SH Fortran Syntax
.nf
INCLUDE 'mpif.h'
MPI_NEIGHBOR_ALLGATHER(\fISENDBUF\fP,\fI SENDCOUNT\fP,\fI SENDTYPE\fP,\fI RECVBUF\fP,\fI RECVCOUNT\fP,\fI
		RECVTYPE\fP,\fI COMM\fP,\fI IERROR\fP)
	<type>	\fISENDBUF\fP (*), \fIRECVBUF\fP (*)
	INTEGER	\fISENDCOUNT\fP,\fI SENDTYPE\fP,\fI RECVCOUNT\fP,\fI RECVTYPE\fP,\fI COMM\fP,
	INTEGER	\fIIERROR\fP

MPI_INEIGHBOR_ALLGATHER(\fISENDBUF\fP,\fI SENDCOUNT\fP,\fI SENDTYPE\fP,\fI RECVBUF\fP,\fI RECVCOUNT\fP,\fI
		RECVTYPE\fP,\fI COMM\fP, \fPREQUEST\fI,\fI IERROR\fP)
	<type>	\fISENDBUF\fP (*), \fIRECVBUF\fP (*)
	INTEGER	\fISENDCOUNT\fP,\fI SENDTYPE\fP,\fI RECVCOUNT\fP,\fI RECVTYPE\fP,\fI COMM\fP,
	INTEGER	\fIREQUEST, IERROR\fP

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
sendbuf
Starting address of send buffer (choice).
.TP 1i
sendcount
Number of elements in send buffer (integer).
.TP 1i
sendtype
Datatype of send buffer elements (handle).
.TP 1i
recvbuf
Starting address of recv buffer (choice).
.TP 1i
recvcount
Number of elements received from any process (integer).
.TP 1i
recvtype
Datatype of receive buffer elements (handle).
.TP 1i
comm
Communicator (handle).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
recvbuf
Address of receive buffer (choice).
.TP 1i
request
Request (handle, non-blocking only).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Neighbor_allgather is similar to MPI_Allgather, except that only the neighboring processes receive the result, instead of all processes. The neighbors and buffer layout is determined by the topology of \fIcomm\fP.
.sp
The type signature associated with sendcount, sendtype at a process must be equal to the type signature associated with recvcount, recvtype at any other process.
.fi

.sp
.SH NEIGHBOR ORDERING
For a distributed graph topology, created with MPI_Dist_graph_create, the sequence of neighbors
in the send and receive buffers at each process is defined as the sequence returned by MPI_Dist_graph_neighbors
for destinations and sources, respectively. For a general graph topology, created with MPI_Graph_create, the order of
neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_Graph_neighbors.
Note that general graph topologies should generally be replaced by the distributed graph topologies.

For a Cartesian topology, created with MPI_Cart_create, the sequence of neighbors in the send and receive
buffers at each process is defined by order of the dimensions, first the neighbor in the negative direction
and then in the positive direction with displacement 1. The numbers of sources and destinations in the
communication routines are 2*ndims with ndims defined in MPI_Cart_create. If a neighbor does not exist, i.e., at
the border of a Cartesian topology in the case of a non-periodic virtual grid dimension (i.e.,
periods[...]==false), then this neighbor is defined to be MPI_PROC_NULL.

If a neighbor in any of the functions is MPI_PROC_NULL, then the neighborhood collective communication behaves
like a point-to-point communication with MPI_PROC_NULL in this direction. That is, the buffer is still part of
the sequence of neighbors but it is neither communicated nor updated.

.SH NOTES
.sp
The MPI_IN_PLACE option for \fIsendbuf\fP is not meaningful for this operation.


.SH ERRORS
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
.sp
MPI_Neighbor_allgatherv
MPI_Cart_create
MPI_Garph_create
MPI_Dist_graph_create
.br
MPI_Gather