MPI_ALLTOALLV(3)

📦

version 5.0.11 · 17 versions

💅 MPI_ALLTOALLV(3) 💅

okay bestie, this is the Open MPI manual page for MPI_Alltoallv, MPI_Ialltoallv, and MPI_Alltoallv_init — the collective where all processes send different amounts of data to, and receive different amounts of data from, all processes. It's giving full all-to-all flexibility, no cap. ✨

✨ SYNTAX ✨

💻 C Syntax 💻

          #include <mpi.h>

          int MPI_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_Ialltoallv(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_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_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_IALLTOALLV(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_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_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_Ialltoallv(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_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 the send buffer. This is where your outgoing data lives, bestie.
sendcounts — integer array where entry i specifies the number of elements to send to rank i.
sdispls — integer array where entry i specifies the displacement (offset from sendbuf, in units of sendtype) from which to send data to rank i.
sendtype — datatype of send buffer elements.
recvcounts — integer array where entry j specifies the number of elements to receive from rank j.
rdispls — integer array where entry j specifies the displacement (offset from recvbuf, in units of recvtype) to which data from rank j should be written.
recvtype — datatype of receive buffer elements.
comm — communicator over which data is to be exchanged.
info — info handle, persistent only. That's the whole note, periodt.

💖 OUTPUT PARAMETERS 💖

recvbuf — address of receive buffer. The landing zone, the destination, the main character energy.
request — request handle, non-blocking only.
ierror — Fortran only: error status.

👑 DESCRIPTION 👑

MPI_Alltoallv is a generalized collective operation where all processes send data to and receive data from all other processes. It adds flexibility to MPI_Alltoall by letting the user specify data to send and receive vector-style — via a displacement and element count. Obsessed.

The operation of this routine can be thought of as follows, where each process performs 2n (n being the number of processes in communicator comm) independent point-to-point communications (including communication with itself). Yes, including communication with itself. Self-love is important. 💅

          MPI_Comm_size(comm, &n);
          for (i = 0, i < n; i++)
              MPI_Send(sendbuf + sdispls[i] * extent(sendtype),
                  sendcounts[i], sendtype, i, ..., comm);
          for (i = 0, i < n; i++)
              MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
                  recvcounts[i], recvtype, i, ..., comm);

Process j sends the k-th block of its local sendbuf to process k, which places the data in the j-th block of its local recvbuf. Cute little block exchange, honestly.

When a pair of processes exchanges data, each may pass different element count and datatype arguments so long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive. Bytes match, vibes match, we move.

Note that 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. It's giving customized, per-rank communication. 🌈

🌈 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🌈

When the communicator is an inter-communicator, the gather operation occurs in two phases. The data is gathered from all the members of the first group and received by all the members of the second group. Then the data is gathered from all the members of the second group and received by all the members of the first. The operation exhibits a symmetric, full-duplex behavior. Two phases, full duplex, very symmetrical, very demure.

The first group defines the root process. The root process uses MPI_ROOT as the value of root. All other processes in the first group use MPI_PROC_NULL as the value of root. All processes in the second group use the rank of the root process in the first group as the value of root.

When the communicator is an intra-communicator, these groups are the same, and the operation occurs in a single phase. Simple like that.

🫶 USE OF IN-PLACE OPTION 🫶

When the communicator is an intracommunicator, you can perform an all-to-all operation in-place — the output buffer is used as the input buffer. Use the variable MPI_IN_PLACE as the value of sendbuf. In this case, sendcounts, sdispls, and sendtype are ignored. The input data of each process is assumed to be in the area where that process would receive its own contribution to the receive buffer. In-place? iconic. 🎀

🌸 NOTES 🌸

⚠️ The specification of counts and displacements should not cause any location to be written more than once. Protect your buffer, bestie.

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_Alltoallw, where these offsets are measured in bytes. Yes, bytes vs. datatype units — know the difference, iykyk.

😭 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. Dramatic? yes. Effective? also yes.
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. So like… don't just ignore it and hope for the best, bestie. 💀

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_Alltoall
MPI_Alltoallw

🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community

mwah 💋 — sign-off vibes: Open MPI · Sep 11, 2026 · MPI_ALLTOALLV(3) 🫶

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