MPI_GROUP_UNION(3)

💖 MPI_GROUP_UNION(3) — hey bestie, it's the Open MPI girlies 💖

okay so this is MPI_Group_union, from the Open MPI manual, and honestly? it's giving group chat merger energy. you've got two groups of processes and you want them in one big happy friend group. slay. let's get into it ✨

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🌸 NAME 🌸

MPI_Group_union — makes a whole new group by smashing two groups together 🫶

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✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

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

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GROUP_UNION(GROUP1, GROUP2, NEWGROUP, IERROR)
     INTEGER GROUP1, GROUP2, NEWGROUP, IERROR

🎀 Fortran 2008 Syntax

USE mpi_f08
MPI_Group_union(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, bestie. it's a handle 💅
• group2 — the second group, also a handle. yes we're doing two of them, no cap

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💜 OUTPUT PARAMETERS 💜

• newgroup — the union group, fresh out the group chat. handle energy 🔥
• ierror — Fortran only: error status, and it's an integer. periodt

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🦋 DESCRIPTION 🦋

so the set-like operations are defined like this, and i need you to focus:

• union — all elements of the first group (group1), followed by all elements of second group (group2) not in first. like, you invite everyone from group A, then you invite the rest of group B, but not the ones who were already there. iconic behavior honestly 👑
• intersect — all elements of the first group that are also in the second group, ordered as in first group. it's giving "who's in both group chats" ✨
• difference — all elements of the first group that are not in the second group, ordered as in the first group. group A minus group B, lowkey brutal but we respect it 💀

note that 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 the real tea 🍵: neither union nor intersection are commutative, but both are associative. so flip the arguments and you get a different order, bestie. do not come for me about it, that's just how it is 🌸

oh and the new group can be empty. like, equal to MPI_GROUP_EMPTY. yes, an empty friend group is valid 🙈

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💀 ERRORS 💀

almost all MPI routines return an error value, okay? the way that works: C routines give it back as the return result of the function, and Fortran routines put it in the last argument.

before that error value gets returned, the current MPI error handler tied to the communication object (like a communicator, window, or file — you know, the usual suspects) is called. if there's no communication object associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and it'll call the associated MPI error handler. and when MPI_COMM_SELF isn't initialized — meaning before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when you're 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 you're using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. and 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. whew 😭

Open MPI ships with three predefined error handlers you can use, and they're lowkey the whole lineup:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. absolutely nuclear 💀
• 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. multitasking, we love her 👑
• MPI_ERRORS_RETURN — returns an error code to the application. polite fr fr 🎀

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. so like… don't assume you're fine. you might not be 🫠

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_Group_free

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🩷 Copyright 🩷

2003-2026, The Open MPI Community

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and that's the whole page, bestie! this was MPI_GROUP_UNION(3) from the Open MPI crew, dated Sep 10, 2026 🌈 go union some groups, you absolute main character 💅✨