MPI_GROUP_DIFFERENCE(3)

💖 NAME 💖

MPI_Group_difference — bestie, this one makes a group out of the difference of two groups. Two in, the leftovers out. periodt. ✨

This page lives in the Open MPI manual, section 3 (that's the (3) up top, iykyk). 🫶

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🌸 SYNTAX 🌸

🎀 C Syntax

#include <mpi.h>

int MPI_Group_difference(MPI_Group group1, MPI_Group group2,
    MPI_Group *newgroup)

🎀 Fortran Syntax

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

MPI_GROUP_DIFFERENCE(GROUP1, GROUP2, NEWGROUP, IERROR)
    INTEGER GROUP1, GROUP2, NEWGROUP, IERROR

🎀 Fortran 2008 Syntax

USE mpi_f08

MPI_Group_difference(group1, group2, newgroup, ierror)
    TYPE(MPI_Group), INTENT(IN) :: group1, group2
    TYPE(MPI_Group), INTENT(OUT) :: newgroup
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🧚 INPUT PARAMETERS 🧚

• group1 — the first group, as a handle. your main character group. 👑
• group2 — the second group, also a handle. the one we're subtracting, no hard feelings. 💅

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🦋 OUTPUT PARAMETERS 🦋

• newgroup — the difference group (a handle). this is the slay that comes out the other side. ✨
• ierror — Fortran only: error status as an integer. 🍓

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🌈 DESCRIPTION 🌈

okay so the set-like operations are defined like this:

• union — all elements of the first group (group1), followed by all elements of second group (group2) that are not in the first group
• intersect — all elements of the first group that are also in the second group, ordered as in first group
• difference — all elements of the first group that are not in the second group, ordered as in the first group

ngl the ordering rules matter here, so listen up bestie: for these operations the order of processes in the output group is determined primarily by order in the first group (if possible) and then, if necessary, by order in the second group. 🫶

And here's the tea: neither union nor intersection are commutative, but both are associative. fr fr. 💀

The new group can be empty, that is, equal to MPI_GROUP_EMPTY. empty group is still a valid group, she's just booked and busy elsewhere. 🙈

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💜 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>/MPI_Init_thread <# mpi-init-thread>, after MPI_Finalize <#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 <#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>/MPI_Comm_spawn_multiple <# 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. it's giving no survivors. 😭
• 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 <#mpi-abort> was called on that communicator. If called on a window or file, acts as if MPI_Abort <#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. polite of her honestly. 🎀

MPI applications can also implement their own error handlers by calling:

• MPI_Comm_create_errhandler <#mpi-comm-create-errhandler> then MPI_Comm_set_errhandler <#mpi-comm-set-errhandler>
• MPI_File_create_errhandler <#mpi-file-create-errhandler> then MPI_File_set_errhandler <#mpi-file-set-errhandler>
• MPI_Session_create_errhandler <#mpi-session-create-errhandler> then MPI_Session_set_errhandler <#mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>
• MPI_Win_create_errhandler <#mpi-win-create-errhandler> then MPI_Win_set_errhandler <#mpi-win-set-errhandler>

⚠️ Note that MPI does not guarantee that an MPI program can continue past an error. read that again, bestie. do not assume she survives. 🫣

See the MPI man page <#open-mpi> for a full list of MPI error codes <# open-mpi-errors>.

See the Error Handling section of the MPI-3.1 standard for more information.

See also: MPI_Group_free <#mpi-group-free> 🌟

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👑 Copyright 👑

2003-2026, The Open MPI Community

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okay that's the whole thing, you ate that 💅 and just so you know exactly which version of reality you're living in: this was the Sep 10, 2026 edition of MPI_GROUP_DIFFERENCE(3), brought to you by Open MPI and the lovely Open MPI Community, copyright 2003-2026. slay, go diff some groups bestie 🦋✨🎀💖