MPI_IGATHER(3)

📦

version 5.0.10 · 4 versions

MPI_IGATHER(3) — Open MPI 💅✨

NAME 🎀

MPI_Gather, MPI_Igather, MPI_Gather_init — Gathers values from a group of processes.

SYNOPSIS 🦋

C Syntax

#include <mpi.h>

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

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

int MPI_Gather_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
    void *recvbuf, int recvcount, 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_GATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
        RECVTYPE, ROOT, COMM, IERROR)
    <type>  SENDBUF(*), RECVBUF(*)
    INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
    INTEGER COMM, IERROR

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

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

Fortran 2008 Syntax

USE mpi_f08

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

MPI_Igather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
        root, comm, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcount, root
    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_Gather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
        root, comm, info, request, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
    INTEGER, INTENT(IN) :: sendcount, recvcount, root
    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). It's giving “where my data lives”.
sendcount — Number of elements in send buffer (integer). Count ’em, bestie.
sendtype — Datatype of send buffer elements (handle).
recvcount — Number of elements for any single receive (integer, significant only at root).
recvtype — Datatype of recvbuffer elements (handle, significant only at root).
root — Rank of receiving process (integer).
comm — Communicator (handle).
info — Info (handle, persistent only).

OUTPUT PARAMETERS 💖

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

DESCRIPTION 🦋

Okay so picture this: every process in the group — yes, even the root — sends the contents of its send buffer to the root process. The root receives those messages and stores them in rank order. The whole outcome is as if each of the n processes in the group (root included) had run a call to:

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

and the root had run n calls to:

MPI_Recv(recfbuf + i * recvcount * extent(recvtype), recvcount,
         recvtype, i, ...)

where extent(recvtype) is the type extent you get from a call to MPI_Type_extent.

Another way to describe it: the n messages sent by the group get concatenated in rank order, and the root receives that resulting message as if by a call to MPI_Recv(recvbuf, recvcount * n, recvtype, ... ).

The receive buffer is ignored for all nonroot processes. Not their moment, honestly. 💅

General, derived datatypes are allowed for both sendtype and recvtype. The type signature of sendcount, sendtype on process i must be equal to the type signature of recvcount, recvtype at the root. That means the amount of data sent must equal the amount of data received, pairwise between each process and the root. Distinct type maps between sender and receiver are still allowed, though. Slay. ✨

All arguments to the function are significant on process root. On other processes, only sendbuf, sendcount, sendtype, root, comm are significant. The arguments root and comm must have identical values on all processes.

The specification of counts and types should not cause any location on the root to be written more than once.

Such a call is erroneous. Don’t do it. 💀

Note that the recvcount argument at the root indicates the number of items it receives from each process, not the total number of items it receives. So don’t get it twisted.

Example 1: Gather 100 ints from every process in group to root.

MPI_Comm comm;
int gsize, sendarray[100];
int root, *rbuf;
...

MPI_Comm_size( comm, &gsize);
rbuf = (int*)malloc(gsize* 100*sizeof(int));

MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);

Example 2: Previous example modified — only the root allocates memory for the receive buffer.

MPI_Comm comm;
int gsize, sendarray[100];
int root, myrank, *rbuf;
...

MPI_Comm_rank( comm, myrank);
if ( myrank == root) {
  MPI_Comm_size( comm, &gsize);
  rbuf = (int *)malloc(gsize * 100*sizeof(int));
}
MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);

Example 3: Do the same as the previous example, but use a derived datatype. Note that the type cannot be the entire set of gsize * 100 ints since type matching is defined pairwise between the root and each process in the gather.

MPI_Comm comm;
int gsize, sendarray[100];
int root, *rbuf;
MPI_Datatype rtype;
...

MPI_Comm_size( comm, &gsize);
MPI_Type_contiguous( 100, MPI_INT, &rtype);
MPI_Type_commit( &rtype );
rbuf = (int*)malloc(gsize* 100*sizeof(int));
MPI_Gather( sendarray, 100, MPI_INT, rbuf, 1, rtype, root, comm);

USE OF IN-PLACE OPTION 💅

When the communicator is an intracommunicator, you can do a gather operation in-place — meaning the output buffer is used as the input buffer. 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 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.

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.

💖 ERRORS 💖

okay bestie, real talk: almost every single MPI routine returns an error value. like, basically all of them. and the way you get that value depends on the language you're living in:

C routines — the error value is the return result of the function. it hands it right back to you. 🫶
Fortran routines — it shows up in the last argument. classic Fortran behaviour, no cap.

🎀 the error handler pipeline

so before that error value ever makes it back to you, MPI does a little ceremony: the current MPI error handler associated with the communication object gets called. and "communication object" here means like… a communicator, a window, or a file. you know, the whole squad.

if there's no communication object associated with the MPI call at all? then the call is considered attached to MPI_COMM_SELF and it'll call the associated MPI error handler. periodt.

and when MPI_COMM_SELF is not initialized — which happens:

before MPI_Init / MPI_Init_thread
after MPI_Finalize
or when you're using the Sessions Model exclusively

…then the error raises the initial error handler. 💀

that initial error handler isn't locked in forever though, bestie. you can change it by:

calling MPI_Comm_set_errhandler on MPI_COMM_SELF when you're using the World model, or
the mpi_initial_errhandler CLI argument to mpiexec, or
an info key to MPI_Comm_spawn / MPI_Comm_spawn_multiple.

and if literally no other appropriate error handler has been set? then:

MPI_ERRORS_RETURN gets called for MPI I/O functions
MPI_ERRORS_ABORT gets called for all other MPI functions

that's the default fallback, iykyk. ✨

👑 the three predefined error handlers

Open MPI comes with three of these bad boys, ready to go:

MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. like, full group chat kicked out. dramatic but effective. 💀
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, it 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, it aborts only the local process.
MPI_ERRORS_RETURN — just returns an error code to the application. chill. lowkey respectful. 🫶

🌸 DIY error handlers

not satisfied with the preset menu? iconic. 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

build your own, bestie. main character energy. 💅

⚠️ heads up fr fr: note that MPI does not guarantee that an MPI program can continue past an error. like, don't just assume you can bounce back and keep running. that's not a vibe, that's undefined territory. 😭

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

🌈 Copyright 🌈

2003-2026, The Open MPI Community

okay that's the whole page bestie, you made it to the end!! 🎉 this was MPI_IGATHER(3), dated Sep 10, 2026, brought to you by the Open MPI girlies. go forth and gather non-blockingly, you absolute legend. mwah. 💖✨🎀

📦 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