MPI\_NEIGHBOR\_ALLGATHERV\_INIT(3) Open MPI MPI\_NEIGHBOR\_ALLGATHERV\_INIT(3) MPI\_Neighbor\_allgatherv <#mpi-neighbor-allgatherv>, MPI\_Ineighbor\_allgatherv <#mpi-ineighbor-allgatherv>, MPI\_Neighbor\_allgatherv\_init — Gathers and distributes data from and to all neighbors. Each process may contribute a different amount of data. # [SYNTAX](#SYNTAX) ## [C Syntax](#C_Syntax) ``` #include int MPI_Neighbor_allgatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, MPI_Comm comm) int MPI_Ineighbor_allgatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, MPI_Comm comm, MPI_Request *request) int MPI_Neighbor_allgatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request) ``` ## [Fortran Syntax](#Fortran_Syntax) ``` USE MPI ! or the older form: INCLUDE 'mpif.h' MPI_NEIGHBOR_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, DISPLS, RECVTYPE, COMM, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*), INTEGER DISPLS(*), RECVTYPE, COMM, IERROR MPI_INEIGHBOR_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, DISPLS, RECVTYPE, COMM, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*), INTEGER DISPLS(*), RECVTYPE, COMM,REQUEST, IERROR MPI_NEIGHBOR_ALLGATHERV_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT, DISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR) SENDBUF(*), RECVBUF(*) INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*), INTEGER DISPLS(*), RECVTYPE, COMM,INFO,REQUEST, IERROR ``` ## [Fortran 2008 Syntax](#Fortran_2008_Syntax) ``` USE mpi_f08 MPI_Neighbor_allgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, comm, ierror) TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf TYPE(*), DIMENSION(..) :: recvbuf INTEGER, INTENT(IN) :: sendcount, recvcounts(*), displs(*) TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, OPTIONAL, INTENT(OUT) :: ierror MPI_Ineighbor_allgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, comm, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*) 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_allgatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs, recvtype, comm, info, request, ierror) TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf INTEGER, INTENT(IN) :: sendcount INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*) 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](#INPUT_PARAMETERS) - **sendbuf**: Starting address of send buffer (choice). - **sendcount**: Number of elements in send buffer (integer). - **sendtype**: Datatype of send buffer elements (handle). - **recvcount**: Integer array (of length group size) containing the number of elements that are received from each neighbor. - **displs**: Integer array (of length group size). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from neighbor i. - **recvtype**: Datatype of receive buffer elements (handle). - **comm**: Communicator (handle). - **info Info (handle, persistent only).**: # [OUTPUT PARAMETERS](#OUTPUT_PARAMETERS) - **recvbuf**: Address of receive buffer (choice). - **request**: Request (handle, non-blocking only). - **ierror**: Fortran only: Error status (integer). # [DESCRIPTION](#DESCRIPTION) MPI\_Neighbor\_allgatherv <#mpi-neighbor-allgatherv> is similar to MPI\_Neighbor\_allgather <#mpi-neighbor-allgather> in that all processes gather data from all neighbors, except that each process can send a different amount of data. The block of data sent from the jth neighbor is received by every neighbor and placed in the jth block of the buffer. The neighbors and buffer layout is determined by the topology of *comm*. *recvbuf.* The type signature associated with sendcount, sendtype, at process j must be equal to the type signature associated with the corresponding entry in *recvcounts* on neighboring processes. # [NEIGHBOR ORDERING](#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](#NOTES) The MPI\_IN\_PLACE option for *sendbuf* is not meaningful for this operation. # [ERRORS](#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\_allgather <#mpi-neighbor-allgather> - MPI\_Cart\_create <#mpi-cart-create> - MPI\_Graph\_create <#mpi-graph-create> - MPI\_Dist\_graph\_create <#mpi-dist-graph-create> # [Copyright](#Copyright) 2003-2026, The Open MPI Community September 11, 2026 --- Document: MPI_Neighbor_allgatherv_init(3) from openmpi-cuda-13.3 5.0.11-r0 (Chainguard OS) Source: https://github.com/open-mpi/ompi