MPI_COMM_CREATE_FROM_GROUP(3)

💖 MPI_COMM_CREATE_FROM_GROUP 💖

hiii bestie, you've landed in the Open MPI manual, section 3, which is literally where all the library functions live. Pull up a chair, we're making communicators from scratch today. 🫶

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✨ NAME ✨

MPI_Comm_create_from_group — Creates a new communicator from a group and stringtag

lowkey the whole plot in one line, no cap.

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🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_Comm_create_from_group(MPI_Group group, const char *stringtag, MPI_Info info, MPI_Errhandler errhandler, MPI_Comm *newcomm)

Fortran Syntax

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

MPI_COMM_CREATE_FROM_GROUP(GROUP, STRINGTAG, INFO, ERRHANDLER, NEWCOMM, IERROR)
    INTEGER GROUP, INFO, ERRHANDLER, NEWCOMM, IERROR
    CHARACTER*(*) STRINGTAG

Fortran 2008 Syntax

USE mpi_f08

MPI_Comm_create_from_group(group, stringtag, info, errhandler, newcomm, ierror)
    TYPE(MPI_Group), INTENT(IN) :: group
    CHARACTER(LEN=*), INTENT(IN) :: stringtag
    TYPE(MPI_Info), INTENT(IN) :: info
    TYPE(MPI_Errhandler), INTENT(IN) :: errhandler
    TYPE(MPI_Comm), INTENT(OUT) :: newcomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

it's giving glow-up, three dialects, all serving. ✨

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🎀 INPUT PARAMETERS 🎀

• group — Group (handler)
• stringtag — Unique identifier for this operation (string)
• info — info object (handler)
• errhandler — error handler to be attached to the new intra-communicator (handle)

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🦋 OUTPUT PARAMETERS 🦋

• newcomm — New communicator (handle).
• ierror — Fortran only: Error status (integer).

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💅 DESCRIPTION 💅

So MPI_Comm_create_from_group is basically the sibling of MPI_Comm_create_group, except the set of MPI processes involved in the creation of the new intra-communicator is specified by a group argument, rather than the group associated with a pre-existing communicator. that's the whole twist, bestie.

If a non-empty group is specified, then all MPI processes in that group must call the function, and each of these MPI processes must provide the same arguments — including a group that contains the same members with the same ordering, and an identical stringtag value. everybody has to show up in the same outfit, fr fr. 👯

In the event that MPI_GROUP_EMPTY is supplied as the group argument, then the call is a local operation and MPI_COMM_NULL is returned as newcomm. solo mission, no group chat needed. 🙈

The stringtag argument is analogous to the tag used for MPI_Comm_create_group. If multiple threads at a given MPI process perform concurrent MPI_Comm_create_from_group operations, the user must distinguish these operations by providing different stringtag arguments — otherwise the vibes collide and nobody knows who's who. 😭

The stringtag shall not exceed MPI_MAX_STRINGTAG_LEN characters in length. For C, this includes space for a null terminating character. yes bestie, the \0 counts. it always counts.

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🍓 NOTES 🍓

The errhandler argument specifies an error handler to be attached to the new intracommunicator.

The info argument provides hints and assertions, possibly MPI implementation dependent, which indicate desired characteristics and guide communicator creation.

MPI_MAX_STRINGTAG_LEN shall have a value of at least 63.

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👑 ERRORS 👑

Important In Open MPI, this function only works properly when the MPI job is launched via mpirun / mpiexec. Using this function in other run-time environments is currently unsupported, and may result in undefined behavior (e.g., the MPI job may hang).

like, genuinely, do not skip that one. that's the "your job will just sit there staring at you" warning. 💀

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. 🫠

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_Comm_create_group

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🌈 Copyright 🌈

2003-2026, The Open MPI Community

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and that's the page, bestie!! 🎀✨ you did it, you read the whole thing, main character energy fr. this was section MPI_COMM_CREATE_FROM_GROUP(3) from Open MPI, dated Sep 10, 2026, brought to you by The Open MPI Community (2003-2026) — mwah, go make some communicators. 💖🦋🌸