MPI_NEIGHBOR_ALLTOALL(3)			     Open MPI				  MPI_NEIGHBOR_ALLTOALL(3)

MPI_Neighbor_alltoall,	MPI_Ineighbor_alltoall	<#mpi-ineighbor-alltoall>,  MPI_Neighbor_alltoall  - All processes
send data to neighboring processes in a virtual topology communicator

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Neighbor_alltoall(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm)

	  int MPI_Ineighbor_alltoall(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request)

	  int MPI_Neighbor_alltoall_init(const void *sendbuf, int sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
	       MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_NEIGHBOR_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
	       INTEGER COMM, IERROR

	  MPI_INEIGHBOR_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
	       INTEGER COMM, REQUEST, IERROR

	  MPI_NEIGHBOR_ALLTOALL_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
	       RECVTYPE, COMM, INFO, REQUEST, IERROR)

	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE
	       INTEGER COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Neighbor_alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount,
		       recvtype, comm, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ineighbor_alltoall(sendbuf, sendcount, sendtype, recvbuf, recvcount,
		       recvtype, comm, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Neighbor_alltoall_init(sendbuf, sendcount, sendtype, recvbuf, recvcount,
		       recvtype, comm, info, request, ierror)

	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Info), INTENT(IN) :: info
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • sendbuf: Starting address of send buffer (choice).

       • sendcount: Number of elements to send to each process (integer).

       • sendtype: Datatype of send buffer elements (handle).

       • recvcount: Number of elements to receive from each process (integer).

       • recvtype: Datatype of receive buffer elements (handle).

       • comm: Communicator over which data is to be exchanged (handle).

       • info: Info (handle, persistent only).

OUTPUT PARAMETERS
       • recvbuf: Starting address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Neighbor_alltoall is a collective operation in which all processes send and receive the same amount	of
       data  to	 each  neighbor.  The  operation of this routine can be represented as follows, where each process
       performs	 2n  (n	 being	the  number  of	 neighbors  in	communicator  comm)   independent   point-to-point
       communications. The neighbors and buffer layout are determined by the topology of comm.

       Example of MPI_Neighbor_alltoall semantics for cartesian topologies:

	  MPI_Cart_get(comm, maxdims, dims, periods, coords);
	  for (dim = 0, i = 0 ; dim < dims ; ++dim) {
	      MPI_Cart_shift(comm, dim, 1, &r0, &r1);
	      MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
			sendcount, sendtype, r0, ..., comm, ...);
	      MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
			recvcount, recvtype, r0, ..., comm, ...);
	      ++i;
	      MPI_Isend(sendbuf + i * sendcount * extent(sendtype),
			sendcount, sendtype, r1, ..., comm, &req[i]);
	      MPI_Irecv(recvbuf + i * recvcount * extent(recvtype),
			recvcount, recvtype, r1, ..., comm, ...);
	      ++i;
	  }

	  MPI_Waitall (...);

       Each  process  breaks  up  its  local  sendbuf  into  n blocks - each containing sendcount elements of type
       sendtype - and divides its recvbuf similarly according to recvcount and recvtype. Process j sends the  k-th
       block of its local sendbuf to neighbor k, which places the data in the j-th block of its local recvbuf. The
       amount  of  data	 sent  must  be	 equal	to  the	 amount	 of data received, pairwise, between every pair of
       processes.

NEIGHBOR ORDERING
       For a distributed graph topology, created with MPI_Dist_graph_create <#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	 <#mpi-dist-graph-neighbors>  for  destinations	 and  sources, respectively. For a
       general graph topology, created with MPI_Graph_create <#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 <#
       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 <#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	<#
       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.

NOTES
       The MPI_IN_PLACE option for sendbuf is not meaningful for this function.

       All arguments on all processes are significant. The comm argument, in particular, must  describe	 the  same
       communicator on all processes. comm must be either a cartesian, graph, or dist graph communicator.

       There   are   two   MPI	 library   functions   that   are   more   general   than   MPI_Neighbor_alltoall.
       MPI_Neighbor_alltoallv <#mpi-neighbor-alltoallv> allows all-to-all communication to and from  buffers  that
       need   not   be	 contiguous;  different	 processes  may	 send  and  receive  different	amounts	 of  data.
       MPI_Neighbor_alltoallw <#mpi-neighbor-alltoallw> expands MPI_Neighbor_alltoallv <#mpi-neighbor-alltoallv>’s
       functionality to allow the exchange of data with different datatypes.

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Neighbor_alltoallv <#mpi-neighbor-alltoallv>

	  • MPI_Neighbor_alltoallw <#mpi-neighbor-alltoallw>

	  • MPI_Cart_create <#mpi-cart-create>

	  • MPI_Graph_create <#mpi-graph-create>

	  • MPI_Dist_graph_create <#mpi-dist-graph-create>

	  • MPI_Dist_graph_create_adjacent <#mpi-dist-graph-create-adjacent>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				  MPI_NEIGHBOR_ALLTOALL(3)
