MPI_ALLGATHERV(3)

📦

version 5.0.10 · 7 versions

💖 MPI_ALLGATHERV(3) — the Open MPI glow-up 💖

okay bestie, sit DOWN because today we're learning MPI_Allgatherv, MPI_Iallgatherv, and MPI_Allgatherv_init from the Open MPI manual, and honestly? they ate.

What it does: gathers data from literally every process and delivers it to every process. but here's the twist — each process may contribute a different amount of data. main character behavior fr. 👑

✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_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_Iallgatherv(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_Allgatherv_init(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_ALLGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF,
             RECVCOUNT, DISPLS, RECVTYPE, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT(*)
     INTEGER DISPLS(*), RECVTYPE, COMM, IERROR

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

MPI_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_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_Iallgatherv(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_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).
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 process.
displs — Integer array (of length group size). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from process i.
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_Allgatherv is basically MPI_Allgather's cooler sibling — same energy, all processes gather data from all other processes — except this one lets each process send a different amount of data. the block of data sent from the jth process is received by every process and placed in the jth block of the buffer recvbuf. no cap. ✨

The type signature associated with sendcount, sendtype, at process j must be equal to the type signature associated with recvcounts[j], recvtype at any other process.

The outcome is as if all processes executed calls to

MPI_Allgatherv(sendbuf, sendcount, sendtype, recvbuf, recvcount,
              displs,recvtype,root,comm);

// for root = 0 , ..., n-1.

The rules for correct usage of MPI_Allgatherv are easily found from the corresponding rules for MPI_Gatherv. iykyk 🫶

🔄 USE OF IN-PLACE OPTION 🔄

When the communicator is an intracommunicator, you can perform an all-gather operation in-place — meaning the output buffer is used as the input buffer. it's giving efficient, it's giving modest. 🌸

Use the variable MPI_IN_PLACE as the value of sendbuf. In this case, sendcount 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. Specifically, the outcome of a call to MPI_Allgatherv that used the in-place option is identical to the case in which all processes executed n calls to

MPI_Allgatherv ( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
                 recvcounts, displs, recvtype, root, comm );

// for root =0, ... , n-1.

⚠️ bestie listen — MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM. do not freestyle with it.

Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT. noted, logged, memorized. 📝

🧚 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, concatenated, and received by all the members of the second group. Then the data is gathered from all the members of the second group, concatenated, and received by all the members of the first.

The send buffer arguments in the one group must be consistent with the receive buffer arguments in the other group, and vice versa. The operation must exhibit symmetric, full-duplex behavior. it's a whole two-way situation, very balanced, very Libra of it 💫

💀 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. like, it might just be over. periodt.

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_Gatherv
MPI_Allgather

👑 Copyright 👑

2003-2026, The Open MPI Community

and that's the whole page bestie — ily, go write some gorgeous distributed code, and remember: the Open MPI crew dropped this on Sep 10, 2026, so shout them out accordingly 💖🦋✨ (MPI_ALLGATHERV(3) out 🫶)

📦 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