MPI_NEIGHBOR_ALLTOALL(3)

📦

version 5.0.11 · 16 versions

💅 MPI_NEIGHBOR_ALLTOALL(3) 💅

okay bestie so this is an Open MPI page — welcome to the neighbor alltoall agenda 🌸 three functions, one page, and honestly they ate.

MPI_Neighbor_alltoall, MPI_Ineighbor_alltoall, MPI_Neighbor_alltoall_init — all processes send data to neighboring processes in a virtual topology communicator. that's it, that's the vibe check ✨

✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

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

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

int MPI_Neighbor_alltoall_init(const void *sendbuf, int sendcount,
     MPI_Datatype sendtype, void *recvbuf, int recvcount,
     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_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
     RECVTYPE, COMM, IERROR)

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

MPI_INEIGHBOR_ALLTOALL(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
     RECVTYPE, COMM, REQUEST, IERROR)

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

MPI_NEIGHBOR_ALLTOALL_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
     RECVTYPE, COMM, INFO, REQUEST, IERROR)

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

🧚 Fortran 2008 Syntax

USE mpi_f08
MPI_Neighbor_alltoall(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_alltoall(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_alltoall_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). where your data lives before it goes ✈️
sendcount — number of elements to send to each process (integer). each! not total!
sendtype — datatype of send buffer elements (handle).
recvcount — number of elements to receive from each process (integer).
recvtype — datatype of receive buffer elements (handle).
comm — communicator over which data is to be exchanged (handle).
info — Info (handle, persistent only). yeah that's the whole description, it's giving mysterious 🙈

🌈 OUTPUT PARAMETERS 🌈

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

👑 DESCRIPTION 👑

MPI_Neighbor_alltoall is a collective operation where every process sends and receives the same amount of data to each neighbor. no cap, everyone's talking, everyone's listening 🗣️👂

the way this works: each process does 2n independent point-to-point communications, where n is the number of neighbors in communicator comm. the neighbors and the buffer layout are determined by the topology of comm — so your topology is literally the main character here 💅

example of MPI_Neighbor_alltoall semantics for cartesian topologies:

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

MPI_Waitall (...);

so each process breaks up its local sendbuf into n blocks — each containing sendcount elements of type sendtype — and divides its recvbuf the same way according to recvcount and recvtype. process j sends the k-th block of its local sendbuf to neighbor k, and neighbor k places that data in the j-th block of its local recvbuf. 🎯

and the rule that will NOT be broken: the amount of data sent must be equal to the amount of data received, pairwise, between every pair of processes. mismatch = chaos, and not the cute kind 😭

🦋 NEIGHBOR ORDERING 🦋

distributed graph topology (made 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.

general graph topology (made 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. but lowkey — general graph topologies should generally be replaced by the distributed graph topologies. it's giving deprecated energy, they're just not that girl anymore 💀

Cartesian topology (made with MPI_Cart_create): the sequence of neighbors in the send and receive buffers at each process is defined by the order of the dimensions — first the neighbor in the negative direction, 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 — like at the border of a Cartesian topology when a virtual grid dimension is non-periodic (i.e. periods[…]==false) — then that neighbor is defined to be MPI_PROC_NULL. 🫥

and 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. translation: the buffer is still part of the sequence of neighbors, but it is neither communicated nor updated. it's there, it's invited, it's just not participating 🎀

⭐ NOTES ⭐

the MPI_IN_PLACE option for sendbuf is not meaningful for this function. don't try it bestie, it's not going to work out 🙈

all arguments on all processes are significant. the comm argument especially — it must describe the same communicator on all processes. and comm must be either a cartesian, graph, or dist graph communicator. no random comms allowed, that's the dress code 👑

there are two MPI library functions that are more general than MPI_Neighbor_alltoall:

MPI_Neighbor_alltoallv allows all-to-all communication to and from buffers that need not be contiguous; different processes may send and receive different amounts of data.
MPI_Neighbor_alltoallw expands MPI_Neighbor_alltoallv's functionality to allow the exchange of data with different datatypes.

so if you need that flexibility, go get it — this page is not jealous, it's supportive 🫶

💀 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. classic ✨

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 the 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. dramatic, but make it fatal 💀
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. ngl that's a real one, no promises 😭

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_alltoallv
MPI_Neighbor_alltoallw
MPI_Cart_create
MPI_Graph_create
MPI_Dist_graph_create
MPI_Dist_graph_create_adjacent

🍓 COPYRIGHT 🍓

2003-2026, The Open MPI Community

and that's the whole page bestie, you're officially neighbor-alltoall-literate 🎀✨ ily, go slay your topology. this was the Open MPI era, dated Sep 11, 2026, page MPI_NEIGHBOR_ALLTOALL(3) — periodt 💅🫶

📦 where this page comes from

arch rolling: openmpi 5.0.11-1 · BSD-3-Clause AND LicenseRef-MPICH
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: a language model's rewrite of the original, under its terms