MPI_NEIGHBOR_ALLTOALLV(3)

📦

version 5.0.10 · 7 versions

👑 MPI_NEIGHBOR_ALLTOALLV 👑 — the Open MPI tea ☕

okay bestie, so this is an Open MPI manual page and it's about three functions that are basically siblings: MPI_Neighbor_alltoallv, MPI_Ineighbor_alltoallv, and MPI_Neighbor_alltoallv_init. The vibe? All processes send different amounts of data to, and receive different amounts of data from, all neighbors. 🦋 No cap.

✨ SYNTAX ✨

🌸 C Syntax 🌸

#include <mpi.h>

int MPI_Neighbor_alltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Ineighbor_alltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
     MPI_Request *request)

int MPI_Neighbor_alltoallv_init(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], 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_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, IERROR

MPI_INEIGHBOR_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, REQUEST, IERROR

MPI_NEIGHBOR_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, INFO, REQUEST, IERROR

🌸 Fortran 2008 Syntax 🌸

USE mpi_f08
MPI_Neighbor_alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
             recvcounts, rdispls, recvtype, comm, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
     rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ineighbor_alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
             recvcounts, rdispls, recvtype, comm, request, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
     recvcounts(*), rdispls(*)
     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_alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf,
             recvcounts, rdispls, recvtype, comm, info, request, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
     recvcounts(*), rdispls(*)
     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. ✨
sendcounts — Integer array, where entry i specifies the number of elements to send to neighbor i.
sdispls — Integer array, where entry i specifies the displacement (offset from sendbuf, in units of sendtype) from which to send data to neighbor i.
sendtype — Datatype of send buffer elements.
recvcounts — Integer array, where entry j specifies the number of elements to receive from neighbor j.
rdispls — Integer array, where entry j specifies the displacement (offset from recvbuf, in units of recvtype) to which data from neighbor j should be written.
recvtype — Datatype of receive buffer elements.
comm — Communicator over which data is to be exchanged.
info — Info (handle, persistent only).

🎀 OUTPUT PARAMETERS 🎀

recvbuf — Address of receive buffer.
request — Request (handle, non-blocking only).
ierror — Fortran only: Error status.

✨ DESCRIPTION ✨

So MPI_Neighbor_alltoallv is a generalized collective operation where every process sends data to and receives data from all its neighbors, simultaneously, no drama. It's giving MPI_Neighbor_alltoall but with extra flexibility 💅 — because it lets you specify the data to send and receive vector-style, via a displacement and an element count.

Think of it like this: each process does 2n independent point-to-point communications, where n is the number of neighbors in to topology of communicator comm. The neighbors and the buffer layout are decided by the topology of comm. And yes, both the send and the receive side get laid out that way. 🫶

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 + sdispls[i]  * extent(sendtype),
              sendcount, sendtype, r0, ..., comm, ...);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtype, r0, ..., comm, ...);
    ++i;
    MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
              sendcount, sendtype, r1, ..., comm, &req[i]);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtype, r1, ..., comm, ...);
    ++i;
}

Process j sends the k-th block of its local sendbuf to neighbor k, which places that data in the j-th block of its local recvbuf. 🔁

When a pair of processes exchanges data, each one may pass different element count and datatype arguments — totally fine — as long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive. Byte-for-byte matching or it's over. 😭

And note: process i may send a different amount of data to process j than it receives from process j. Also, a process may send entirely different amounts of data to different processes in the communicator. Everyone gets their own custom portion, main character energy for each neighbor. 👑

🦋 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.

fr fr, 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 that direction. That is, the buffer is still part of the sequence of neighbors, but it is neither communicated nor updated. It's just... there. Existing. Vibing. 💀

💅 NOTES 💅

The MPI_IN_PLACE option for sendbuf is not meaningful for this operation. Don't try it, bestie. 🙈
The specification of counts and displacements should not cause any location to be written more than once.
All arguments on all processes are significant. The comm argument, in particular, must describe the same communicator on all processes.
The offsets of sdispls and rdispls are measured in units of sendtype and recvtype, respectively. Compare this to MPI_Neighbor_alltoallw, where these offsets are measured in bytes. Different units, different rules, iykyk. 🎀

💀 ERRORS 💀

okay bestie, story time 📖 — almost all MPI routines hand you back an error value. For C routines it is literally the return result of the function, and for Fortran routines it shows up in the last argument, periodt.

Before that error value gets returned, the current MPI error handler associated with the communication object (think communicator, window, file, bestie) gets called. And if there's no communication object associated with the MPI call? Then the call is considered attached to MPI_COMM_SELF and it will call the associated MPI error handler instead. 🙈

When MPI_COMM_SELF is not initialized — i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when you're using the Sessions Model exclusively — the error raises the initial error handler. And you can change that initial error handler 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 the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then MPI_ERRORS_RETURN gets called for MPI I/O functions and MPI_ERRORS_ABORT gets called for all other MPI functions. slay, that's the chain of command 💅

Open MPI includes three predefined error handlers you can use:

MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. everybody out, immediately 💀
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, it 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, it aborts only the local process. so like, scope matters bestie ✨
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

🚨 okay real talk for a sec: note that MPI does not guarantee that an MPI program can continue past an error. so don't be out here acting like nothing happened, no cap.

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_alltoall
MPI_Neighbor_alltoallw
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create

📜 Copyright 📜

2003-2026, The Open MPI Community

aaaand that's the whole page, bestie, you made it to the end 🎀✨ this little love letter to MPI_NEIGHBOR_ALLTOALLV(3) is dated Sep 10, 2026, brought to you by the Open MPI crew — go forth and neighbor-alltoallv responsibly, you absolute icon 👑💖

📦 where this page comes from

ubuntu stonking: openmpi-doc 5.0.10-6 · other
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms