MPI_IALLGATHER(3)

📦

version 5.0.11 · 16 versions

💅 MPI_IALLGATHER(3) 💅

okay so bestie, you've stumbled into the Open MPI corner of the man pages and honestly? you're gonna slay this. this is the whole MPI_Allgather, MPI_Iallgather, and MPI_Allgather_init situation — gathering data from all the processes and then handing it right back to all the processes. everyone eats, nobody's left on read 🍓

✨ NAME ✨

MPI_Allgather, MPI_Iallgather, MPI_Allgather_init — gathers data from all processes and distributes it to all processes. the way that everyone gets a copy?? iconic. 👑

🛠️ SYNTAX 🛠️

💻 C Syntax

#include <mpi.h>

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

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

int MPI_Allgather_init(const void *sendbuf, int  sendcount,
      MPI_Datatype sendtype, void *recvbuf, int recvcount,
      MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

🧚 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, COMM, IERROR)
     <type>  SENDBUF (*), RECVBUF (*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
     INTEGER IERROR

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

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

yeah that trailing comma after COMM, in the first block is actually in there. we don't question it, we just vibe 💀

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     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_Allgather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                     comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount
     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). this is where your lil contribution lives 💌
sendcount — number of elements in send buffer (integer).
sendtype — datatype of send buffer elements (handle).
recvbuf — starting address of recv buffer (choice).
recvcount — number of elements received from any process (integer).
recvtype — datatype of receive buffer elements (handle).
comm — communicator (handle). the group chat itself 🫶
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 🌸

so MPI_Allgather is basically MPI_Gather's more generous cousin. like, MPI_Gather sends the result to just the root, but MPI_Allgather?? every single process gets the result. everyone contributes, everyone receives. no gatekeeping, no "sorry you're not the root" energy. periodt 💅

the type signature associated with sendcount, sendtype at a process must be equal to the type signature associated with recvcount, recvtype at any other process. (match your signatures bestie, it's giving compatibility.)

the outcome of a call to MPI_Allgather is as if all processes executed n calls to

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

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

the rules for correct usage of MPI_Allgather are easily found from the corresponding rules for MPI_Gather. (copy-paste the rules, basically — but make it all-gather flavored ✨)

Example: the all-gather version of Example 1 in MPI_Gather. using MPI_Allgather, we will gather 100 ints from every process in the group to every process.

MPI_Comm comm;
int gsize,sendarray[100];
int *rbuf;
...
MPI_Comm_size( comm, &gsize);
rbuf = (int *)malloc(gsize*100*sizeof(int));
MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);

after the call, every process has the group-wide concatenation of the sets of data. literally everyone's holding the whole picture. main character energy for the entire communicator 👑

🪞 USE OF IN-PLACE OPTION 🪞

when the communicator is an intracommunicator, you can do an all-gather operation in-place — meaning the output buffer is used as the input buffer. so efficient, we love a sustainable queen 🌈

use the variable MPI_IN_PLACE as the value of sendbuf. in this case, sendcount and sendtype are ignored (yes, ignored, don't @ them). 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_Allgather that used the in-place option is identical to the case in which all processes executed n calls to

MPI_Allgather( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
               recvcount, recvtype, root, comm )

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

note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM. (special lil guy, handle with care 🙈)

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. not me stressing about intent annotations 😭 — but fr, INOUT, got it?

🌍 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🌍

when the communicator is an inter-communicator, the gather operation happens in two phases, like a two-part season finale:

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, however, need not be symmetric — the number of items sent by the processes in the first group need not be equal to the number of items sent by the processes in the second group. no pressure to match, it's not that kind of relationship 💜

you can move data in only one direction by giving sendcount a value of 0 for communication in the reverse direction. one-way street, adorable 🚗

💀 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. 😭 the ultimate shutdown.
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. the chill one, hands you the receipt 🧾

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. 💀 read that again. does not guarantee. so like… don't count on it 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_Allgatherv
MPI_Gather

🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community

and that's the whole page, no cap 🫶 go forth and all-gather, you gorgeous distributed little icon. this was the Open MPI MPI_IALLGATHER(3) page, dated Sep 11, 2026 — mwah 💋✨

📦 where this page comes from

arch rolling: openmpi 5.0.11-1 · BSD-3-Clause AND LicenseRef-MPICH
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-06: a language model's rewrite of the original, under its terms