MPI_GATHERV_INIT(3)

💖 MPI_Gatherv_init 💖

okay so bestie, welcome to Open MPI's corner of the manual, and today we're talking about MPI_Gatherv_init. She's the persistent-planning cousin of the gather crew, and she's a lot (affectionate). This is part 1 of 3, so buckle up 🎀

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🌸 NAME 🌸

MPI_Gatherv, MPI_Igatherv, MPI_Gatherv_init — gathers varying amounts of data from all processes to the root process.

bestie note: yes, varying. Not everyone sends the same amount. That's the whole point of the v. She's flexible like that 💅

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✨ SYNTAX ✨

🦋 C Syntax 🦋

#include <mpi.h>

int MPI_Gatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
    void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype,
    int root, MPI_Comm comm)

int MPI_Igatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
    void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype,
    int root, MPI_Comm comm, MPI_Request *request)

int MPI_Gatherv_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
    void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype,
    int root, MPI_Comm comm, MPI_Info info, MPI_Request *request)

🌈 Fortran Syntax 🌈

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
        DISPLS, RECVTYPE, ROOT, COMM, IERROR)
    <type>  SENDBUF(*), RECVBUF(*)
    INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
    INTEGER RECVTYPE, ROOT, COMM, IERROR

MPI_IGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
        DISPLS, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
    <type>  SENDBUF(*), RECVBUF(*)
    INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
    INTEGER RECVTYPE, ROOT, COMM, REQUEST, IERROR

MPI_GATHERV_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
        DISPLS, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
    <type>  SENDBUF(*), RECVBUF(*)
    INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
    INTEGER RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR

🧚 Fortran 2008 Syntax 🧚

USE mpi_f08

MPI_Gatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
        recvtype, root, comm, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
    TYPE(*), DIMENSION(..) :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcounts(*), displs(*), root
    TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Igatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
        recvtype, root, comm, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, root
    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_Gatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
        recvtype, root, comm, info, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, root
    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

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💌 INPUT PARAMETERS 💌

• sendbuf — starting address of send buffer (choice).
• sendcount — number of elements in send buffer (integer).
• sendtype — datatype of send buffer elements (handle).
• recvcounts — an integer array, of length group size, containing the number of elements that are received from each process (significant only at root) 👑.
• displs — an integer array of length group size. Entry i specifies the displacement relative to recvbuf at which to place the incoming data from process i (significant only at root) 📍.
• recvtype — datatype of recv buffer elements (significant only at root) (handle).
• root — rank of receiving process (integer). This is the main character of the operation, no cap 👑.
• comm — communicator (handle).
• info — info (handle, persistent only).

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🫶 OUTPUT PARAMETERS 🫶

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

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okay bestie that's the setup, and trust, the rest is coming 💖

✨ DESCRIPTION ✨

okay bestie, so MPI_Gatherv is literally just MPI_Gather but with ✨range✨. It extends the functionality of MPI_Gather by letting you pull a different count of data from every process, because recvcounts is a whole array now, not one little number. 💅 And it also gives you way more freedom about where on the root all that data lands, thanks to the new argument displs.

The outcome is as if every process — including the root itself — sends a message to the root,

MPI_Send(sendbuf, sendcount, sendtype, root, ...);

and then the root runs n receives,

MPI_Recv(recvbuf + disp[i] * extent(recvtype), recvcounts[i],
         recvtype, i, ...);

Messages get placed in the root's receive buffer in rank order, meaning the data from process j goes into the jth portion of recvbuf on process root. And that *j*th portion starts at offset displs[j] elements (counted in terms of recvtype) into recvbuf. 🎯

The receive buffer is completely ignored for all nonroot processes. Like, they literally do not care. 🙈

The type signature implied by sendcount, sendtype on process i must be equal to the type signature implied by recvcounts[i], recvtype at the root. Which basically means the amount of data sent has to equal the amount of data received, pairwise, between each process and the root. Different type maps between sender and receiver are still allowed though — see Example 2 below for the receipts. 💖

On process root, all arguments are significant. On the other processes, only sendbuf, sendcount, sendtype, root, comm matter. Oh and root and comm must have identical values on all processes, no cap.

⚠️ The specification of counts, types, and displacements should not cause any location on the root to be written more than once. Such a call is erroneous. bestie don't do it. 💀

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Example 1: Now have each process send 100 ints to root, but place each set (of 100) stride ints apart at the receiving end. Use MPI_Gatherv and the displs argument to achieve this effect. Assume stride >= 100.

MPI_Comm comm;
int gsize, sendarray[100];
int root, *rbuf, stride;
int *displs, i, rcounts;
...

MPI_Comm_size(comm, &gsize);
rbuf = (int)malloc(gsize * stride * sizeof(int));
displs = (int)malloc(gsize * sizeof(int));
rcounts = (int )malloc(gsize * sizeof(int));

for (i=0; i<gsize; ++i) {
  displs[i] = i * stride;
  rcounts[i] = 100;
}
MPI_Gatherv(sendarray, 100, MPI_INT, rbuf, rcounts, displs, MPI_INT,
            root, comm);

🚨 Note that the program is erroneous if stride < 100.

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Example 2: Same as Example 1 on the receiving side, but send the 100 ints from the 0th column of a 100 150 int array, in C.

MPI_Comm comm;
int gsize, sendarray[100][150];
int root, *rbuf, stride;
MPI_Datatype stype;
int displs,i, rcounts;
...

MPI_Comm_size(comm, &gsize);
rbuf = (int )malloc(gsize * stride * sizeof(int));
displs = (int)malloc(gsize * sizeof(int));
rcounts = (int )malloc(gsize * sizeof(int));

for (i=0; i<gsize; ++i) {
  displs[i] = i * stride;
  rcounts[i] = 100;
}

// Create datatype for 1 column of array
MPI_Type_vector(100, 1, 150, MPI_INT, &stype);
MPI_Type_commit( &stype );
MPI_Gatherv(sendarray, 1, stype, rbuf, rcounts, displs, MPI_INT,
            root, comm);

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Example 3: Process i sends (100-i) ints from the *i*th column of a 100 x 150 int array, in C. It is received into a buffer with stride, as in the previous two examples.

MPI_Comm comm;
int gsize, sendarray[100][150], *sptr;
int root, *rbuf, stride, myrank;
MPI_Datatype stype;
int displs, i, rcounts;
...

MPI_Comm_size(comm, &gsize);
MPI_Comm_rank( comm, &myrank );
rbuf = (int)malloc(gsize * stride * sizeof(int));
displs = (int)malloc(gsize * sizeof(int));
rcounts = (int )malloc(gsize * sizeof(int));

for (i=0; i<gsize; ++i) {
  displs[i] = i * stride;
  rcounts[i] = 100-i; // note change from previous example
}

// Create datatype for the column we are sending
MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
MPI_Type_commit( &stype );
// sptr is the address of start of "myrank" column
sptr = &sendarray[0][myrank];
MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
            root, comm);

💡 Note that a different amount of data is received from each process.

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Example 4: Same as Example 3, but done in a different way at the sending end. We create a datatype that causes the correct striding at the sending end so that we read a column of a C array.

MPI_Comm comm;
int gsize, sendarray[100][150], *sptr;
int root, *rbuf, stride, myrank, disp[2], blocklen[2];
MPI_Datatype stype, type[2];
int displs, i, rcounts;
...

MPI_Comm_size(comm, &gsize);
MPI_Comm_rank(comm, &myrank );
rbuf = (int )alloc(gsize * stride * sizeof(int));
displs = (int )malloc(gsize * sizeof(int));
rcounts = (int)malloc(gsize * sizeof(int));

for (i=0; i<gsize; ++i) {
  displs[i] = i* stride;
  rcounts[i] = 100-i;
}
// Create datatype for one int, with extent of entire row
disp[0] = 0;
disp[1] = 150 * sizeof(int);
type[0] = MPI_INT;
type[1] = MPI_UB;
blocklen[0] = 1;
blocklen[1] = 1;

MPI_Type_struct( 2, blocklen, disp, type, &stype );
MPI_Type_commit(&stype );
sptr = &sendarray[0][myrank];
MPI_Gatherv(sptr, 100-myrank, stype, rbuf, rcounts, displs, MPI_INT,
            root, comm);

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Example 5: Same as Example 3 at sending side, but at receiving side we make the stride between received blocks vary from block to block.

MPI_Comm comm;
int gsize, sendarray[100][150], *sptr;
int root, *rbuf, *stride, myrank, bufsize;
MPI_Datatype stype;
int *displs, i, *rcounts, offset;
...

MPI_Comm_size( comm, &gsize);
MPI_Comm_rank( comm, &myrank );
de = (int )malloc(gsize * sizeof(int));
...
// stride[i] for i = 0 to gsize-1 is set somehow

// set up displs and rcounts vectors first
displs = (int)malloc(gsize * sizeof(int));
rcounts = (int )malloc(gsize * sizeof(int));
offset = 0;

for (i=0; i<gsize; ++i) {
  displs[i] = offset;
  offset += stride[i];
  rcounts[i] = 100-i;
}

// the required buffer size for rbuf is now easily obtained
bufsize = displs[gsize-1]+rcounts[gsize-1];
rbuf = (int )malloc(bufsize * sizeof(int));
// Create datatype for the column we are sending
MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
MPI_Type_commit( &stype );
sptr = &sendarray[0][myrank];
MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
            root, comm);

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Example 6: Process i sends num ints from the *i*th column of a 100 x 150 int array, in C. The complicating factor is that the various values of num are not known to root, so a separate gather must first be run to find these out. The data is placed contiguously at the receiving end.

MPI_Comm comm;
int gsize, sendarray[100][150], *sptr;
int root, *rbuf, stride, myrank, disp[2], blocklen[2];
MPI_Datatype stype,types[2];
int *displs, i, *rcounts, num;
...

MPI_Comm_size( comm, &gsize);
MPI_Comm_rank( comm, &myrank );

// First, gather nums to root
rcounts = (int )malloc(gsize * sizeof(int));
MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, root, comm);
// root now has correct rcounts, using these we set
// displs[] so that data is placed contiguously (or concatenated) at receive end

displs = (int)malloc(gsize * sizeof(int));
displs[0] = 0;
for (i=1; i<gsize; ++i) {
  displs[i] = displs[i-1]+rcounts[i-1];
}

// And, create receive buffer
rbuf = (int *)malloc(gsize * (displs[gsize-1]+rcounts[gsize-1]) * sizeof(int));
// Create datatype for one int, with extent of entire row
disp[0] = 0;
disp[1] = 150 * sizeof(int);
type[0] = MPI_INT;
type[1] = MPI_UB;
blocklen[0] = 1;
blocklen[1] = 1;
MPI_Type_struct(2, blocklen, disp, type, &stype );
MPI_Type_commit( &stype );
sptr = &sendarray[0][myrank];
MPI_Gatherv(sptr, num, stype, rbuf, rcounts, displs, MPI_INT, root, comm);

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🫶 USE OF IN-PLACE OPTION 🫶

The in-place option operates in the exact same way as it does for MPI_Gather. When the communicator is an intracommunicator, you can perform a gather operation in-place (meaning the output buffer doubles as the input buffer, iconic tbh). Just use the variable MPI_IN_PLACE as the value of the root process sendbuf. In that case, sendcount and sendtype are ignored, and the contribution of the root process to the gathered vector is assumed to already be sitting in the correct place in the receive buffer. ✨

💡 Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

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. periodt.

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🌈 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🌈

When the communicator is an inter-communicator, the root process in the first group gathers data from all the processes in the second group. The first group defines the root process. That process uses MPI_ROOT as the value of its root argument. The remaining processes use MPI_PROC_NULL as the value of their root argument. And all processes in the second group use the rank of that root process in the first group as the value of their root argument. Also — the send buffer argument of the processes in the first group must be consistent with the receive buffer argument of the root process in the second group. 🦋

💥 ERRORS 💥

okay so bestie, almost every single MPI routine returns an error value ✨ In C it's like, the actual return value of the function, and in Fortran it's chillin' in the last argument. iykyk 🫶

But before that error value even gets back to you? The current MPI error handler that's attached to the communication object — think communicator, window, file — gets called first. The way that it works:

• if there's no communication object tied to the MPI call, the call is considered attached to MPI_COMM_SELF, and it calls that associated MPI error handler 💅
• when MPI_COMM_SELF is not initialized (so like, 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 instead
• the initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when you're using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or an info key to MPI_Comm_spawn / MPI_Comm_spawn_multiple 🌈
• and 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 😭

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🌸 Open MPI ships three predefined error handlers 🌸

obsessed with these honestly:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes 💀 periodt.
• MPI_ERRORS_ABORT — an error handler that can be invoked on a communicator, window, file, or session. when it's called on a communicator, it acts as if MPI_Abort was called on that communicator. if it's 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. and if it's called on a session, it aborts only the local process ✨
• MPI_ERRORS_RETURN — returns an error code to the application 🫶

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🎀 and you can build your own, bestie 🎀

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

main character energy fr fr 👑

⚠️ note that MPI does not guarantee that an MPI program can continue past an error. like. not even a little. so don't be out here assuming you can just catch it and keep going 🙈

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_Gather

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👑 Copyright 👑

2003-2026, The Open MPI Community

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okay bestie that's the whole page, we ate and left absolutely no crumbs 💖✨ this was MPI_GATHERV_INIT(3) hmm wait no, it's the MPI_GATHERV_INIT(3) page dated Sep 10, 2026 — go forth and gather, but like, variably 🦋💅🌸