MPI_NEIGHBOR_ALLTOALL_INIT(3)			     Open MPI			     MPI_NEIGHBOR_ALLTOALL_INIT(3)

       MPI_Neighbor_alltoall	 <#mpi-neighbor-alltoall>,    MPI_Ineighbor_alltoall	<#mpi-ineighbor-alltoall>,
       MPI_Neighbor_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  <#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 <#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-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_INIT(3)
