MPI_NEIGHBOR_ALLGATHERV_INIT(3)

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💅 MPI_NEIGHBOR_ALLGATHERV_INIT 💅

okay bestie, so this is the Open MPI manual page for section 3, and it's covering three whole icons: MPI_Neighbor_allgatherv, MPI_Ineighbor_allgatherv, and MPI_Neighbor_allgatherv_init. what do they do?? they gather and distribute data from and to all neighbors 🌈 and the gag is that each process is allowed to contribute a different amount of data. no uniformity required, she's flexible like that. periodt. 💖

✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

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

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

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

🌸 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 💖

  • sendbuf — starting address of send buffer (choice). this is where your lil data lives before it goes on its journey 🫶
  • 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. it's giving assigned seating fr 💅
  • recvtype — datatype of receive buffer elements (handle).
  • comm — communicator (handle).
  • info — Info (handle, persistent only).:

✨ OUTPUT PARAMETERS ✨

  • recvbuf — address of receive buffer (choice).
  • request — request (handle, non-blocking only).
  • ierror — Fortran only: error status (integer).

🌈 DESCRIPTION 🌈

MPI_Neighbor_allgatherv is literally the sibling of MPI_Neighbor_allgather, same energy — all processes gather data from all neighbors — except here each process can send a different amount of data. no matching set required, she's not like other girls. 💅

the block of data sent from the *j*th neighbor gets received by every neighbor and dropped into the *j*th block of the buffer. the neighbors and the buffer layout are determined by the topology of comm and by recvbuf.

ok so important one bestie: 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. no cap, they have to match. 😭

🧚 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. quick note though: general graph topologies should generally be replaced by the distributed graph topologies. it's giving deprecated-adjacent, she's had her moment. 💀

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. and 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. ghosted fr 👻

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. she's present but not participating, main character energy only. ✨

🎀 NOTES 🎀

the MPI_IN_PLACE option for sendbuf is not meaningful for this operation. like. don't even try it bestie. 🙈

🍓 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. yeah. she said what she said. 😭

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_allgather
  • MPI_Cart_create
  • MPI_Graph_create
  • MPI_Dist_graph_create

2003-2026, The Open MPI Community. all rights reserved, iconic behavior honestly. 💜

and that's the whole page bestie!! this was the MPI_NEIGHBOR_ALLGATHERV_INIT(3) page from the Open MPI manual, dated Sep 10, 2026 ✨ go gather and distribute your data, different amounts and all, you've got this 🦋💖

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