MPI_IGATHERV(3)

📦

version 5.0.10 · 4 versions

👑 MPI_IGATHERV(3) — the Open MPI manual, section 3, bestie edition 💅

okay so this is the page for MPI_Gatherv, MPI_Igatherv and MPI_Gatherv_init — three icons, one family, all living in the Open MPI manual. if you came here looking for how to scoop up varying amounts of data from every process and dump it all on the root process, you are SO in the right place. no cap. 🎀

✨ NAME ✨

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

like, every process sends a different number of elements, and the root catches them all. it's giving "everyone brings a different amount of snacks to the party" energy. 🍓

✨ 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

✨ INPUT PARAMETERS ✨

okay bestie, here's the roster of everything you gotta hand over. 💜

sendbuf — starting address of the send buffer (choice). this is your lil package, bestie 📦
sendcount — number of elements in the send buffer (integer).
sendtype — datatype of the 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 — like, everyone else can just vibe, only root cares 👑
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. also significant only at root — it's giving "assigned seating but only the host has the seating chart" energy 🪑
recvtype — datatype of the recv buffer elements (significant only at root) (handle).
root — rank of the receiving process (integer). the main character of this operation, fr fr ✨
comm — communicator (handle). the group chat everyone's in 💬
info — info (handle, persistent only).

✨ OUTPUT PARAMETERS ✨

and here's what comes back out — the good stuff 🎁

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

lowkey obsessed with how the root is the only one holding the receive buffer, the counts AND the displacements. main character energy, no cap. 💅

✨ DESCRIPTION ✨

okay so bestie, MPI_Gatherv is literally MPI_Gather but with ✨range✨. It extends the functionality of MPI_Gather by letting the count of data coming from each process be different — because recvcounts is now a whole array instead of one sad little number 🎀. And it also gives you more flexibility about where the data lands on the root, thanks to the new displs argument. It's giving choose-your-own-adventure, but for buffer offsets. 💅

The outcome is as if every process — root included — 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 receive buffer of the root process in rank order, meaning: the data sent from process j goes into the jth portion of the receive buffer recvbuf on process root. And that jth portion of recvbuf starts at offset displs[j] elements (counting in terms of recvtype) into recvbuf. simple as that, no cap 🫶

The receive buffer is ignored for all nonroot processes. yes literally irrelevant, mind your business bestie 💀

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 means the amount of data sent must equal the amount of data received, pairwise, between each process and the root. ✨But✨ — distinct type maps between sender and receiver are still allowed, as illustrated in Example 2 below. iconic behavior tbh.

All arguments to the function are significant on process root, while on other processes only sendbuf, sendcount, sendtype, root, comm are significant. The arguments root and comm must have identical values on all processes. (yes queen, consistency.)

🚨 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. 🚨

🌸 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. 😭 don't do that.

🦋 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);

🎀 Example 3

Process i sends (100-i) ints from the ith 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. the range era has arrived 😌

💜 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);

⭐ 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. chaotic? yes. allowed? also yes. 💅

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);

🍓 Example 6

Process i sends num ints from the ith 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);

🌈 USE OF IN-PLACE OPTION 🌈

The in-place option operates in the same way as it does for MPI_Gather 💫. When the communicator is an intracommunicator, you can perform a gather operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the root process sendbuf. In this case, sendcount and sendtype are ignored 😶, and the contribution of the root process to the gathered vector is assumed to already be in the correct place in the receive buffer. it's giving self-sufficient queen 👑

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. 📝 don't forget bestie.

🔥 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. All processes in the second group use the rank of that root process in the first group as the value of their root argument. 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. 🌟 group dynamics but make it MPI.

💀 ERRORS 💀

okay bestie, error handling time, and this is the part where you have to pay attention because MPI is lowkey chaotic about it.

Like, almost every MPI routine returns an error value. In C that's the return value of the function itself; in Fortran it's that last argument you keep forgetting to pass. 🙈

Before that error value even comes back to you, the current MPI error handler tied to the communication object — think communicator, window, file — gets called. That's just how it works.

If there's no communication object associated with the MPI call? Then the call is considered attached to MPI_COMM_SELF and it calls the error handler associated with that. The way that MPI_COMM_SELF isn't always initialized though — specifically before MPI_Init / MPI_Init_thread, after MPI_Finalize, or if you're running the Sessions Model exclusively — the error raises the initial error handler instead.

And you can change that initial error handler! You do it by calling MPI_Comm_set_errhandler on MPI_COMM_SELF (when using the World model), or with the mpi_initial_errhandler CLI argument to mpiexec, or with the info key to MPI_Comm_spawn / MPI_Comm_spawn_multiple. 🌈

If no other appropriate error handler has been set, then:

MPI_ERRORS_RETURN is called for MPI I/O functions 💅
MPI_ERRORS_ABORT is called for all other MPI functions 😭

✨ the three predefined error handlers Open MPI ships with ✨

MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. it's giving main character ending everyone's arc. 💀
MPI_ERRORS_ABORT — an error handler you can invoke 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 you call it 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 you call it on a session, it aborts only the local process.
MPI_ERRORS_RETURN — returns an error code to the application. polite. respectful. 🫶

🎀 making your own error handlers 🎀

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. no cap, fr fr. it might just be over.

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.

📜 Copyright 📜

2003-2026, The Open MPI Community

and that's the whole page bestie, you read all three parts, I'm so proud, genuinely obsessed with you rn 🦋✨ this one's the Open MPI MPI_IGATHERV(3) manual page, dated Sep 10, 2026, and it lives at the bottom of the page as its lil footer. go forth and gather, variable-style 💖🌸🎀

📦 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