MPI_NEIGHBOR_ALLGATHER_INIT(3)			     Open MPI			    MPI_NEIGHBOR_ALLGATHER_INIT(3)

       MPI_Neighbor_allgather	<#mpi-neighbor-allgather>,   MPI_Ineighbor_allgather   <#mpi-ineighbor-allgather>,
       MPI_Neighbor_allgather <#mpi-neighbor-allgather> — Gathers and distributes data from and to all neighbors

SYNTAX
   C Syntax
	  #include <mpi.h>

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

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

	  int MPI_Neighbor_allgather_init(const void *sendbuf, int  sendcount,
	       MPI_Datatype sendtype, void *recvbuf, int recvcount,
		MPI_Datatype recvtype, MPI_Comm comm, MPI_Infoinfo, MPI_Request req)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_NEIGHBOR_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
		       RECVTYPE, COMM, IERROR)
	       <type>  SENDBUF (*), RECVBUF (*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
	       INTEGER IERROR

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

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

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Neighbor_allgather(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_allgather(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_allgather_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 in send buffer (integer).

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

       • recvbuf: Starting address of recv buffer (choice).

       • recvcount: Number of elements received from any process (integer).

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

       • comm: Communicator (handle).

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

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

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

DESCRIPTION
       MPI_Neighbor_allgather <#mpi-neighbor-allgather> is similar to MPI_Allgather <#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 comm.

       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.

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 operation.

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_allgatherv <#mpi-neighbor-allgatherv>

	  • MPI_Cart_create <#mpi-cart-create>

	  • MPI_Garph_create

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

	  • MPI_Gather <#mpi-gather>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026			    MPI_NEIGHBOR_ALLGATHER_INIT(3)
