# 17.2.259. MPI_Neighbor_allgather_init

MPI_Neighbor_allgather, MPI_Ineighbor_allgather, MPI_Neighbor_allgather — Gathers and distributes data from and to all neighbors

## 17.2.259.1. SYNTAX

### 17.2.259.1.1. 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)

### 17.2.259.1.2. 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

### 17.2.259.1.3. 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

## 17.2.259.2. 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).

## 17.2.259.3. OUTPUT PARAMETERS

- `recvbuf`: Address of receive buffer (choice).

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

- `ierror`: Fortran only: Error status (integer).

## 17.2.259.4. DESCRIPTION

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 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.

## 17.2.259.5. 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.

## 17.2.259.6. NOTES

The MPI_IN_PLACE option for sendbuf is not meaningful for this operation.

## 17.2.259.7. 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_thread, after 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 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_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 was called on that communicator. If called on a window or file, acts as if 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 then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

See the MPI man page for a full list of MPI error codes.

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

See also

- MPI_Neighbor_allgatherv

- MPI_Cart_create

- MPI_Garph_create

- MPI_Dist_graph_create

- MPI_Gather