MPI_GROUP_UNION(3)

💜 MPI_GROUP_UNION(3) — the Open MPI glow-up ✨

okay so bestie, welcome to the MPI_Group_union page 💅 this is the one that takes two groups and smushes them into one iconic combined group. It's from Open MPI's manual, section 3, and I'm gonna walk you through every single detail so you don't miss a thing 🫶

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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, and yes she's a handle 👑
• group2 — the second group, also a handle, also invited to the party 🎉

🌟 OUTPUT PARAMETERS 🌟

• newgroup — the union group (handle). Your brand new combined bestie group 💫
• ierror — Fortran only: error status (integer). Classic.

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🔥 DESCRIPTION 🔥

So the set-like operations are defined like this, bestie, listen up:

• union — all elements of the first group (group1), followed by all elements of the second group (group2) that are not already in the first. no duplicates allowed, we're not about that life 💅
• intersect — all elements of the first group that are also in the second group, ordered as in the 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 thing is important so I'm saying it in its own little box: 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 like, the way that neither union nor intersection are commutative 😭 but both are associative. Iconic math behavior honestly. fr fr.

Also the new group can be empty — like, equal to MPI_GROUP_EMPTY. Empty group is a valid vibe, no cap 💀

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😭 ERRORS 😭

Almost all MPI routines return an error value, bestie. C routines give it back as the return result of the function, and Fortran routines put it 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, and they're kinda the main characters of this section:

• 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. That's a real one, bestie — like the program might just be done for 🙈

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.

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👀 SEE ALSO 👀

• MPI_Group_free

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🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community.

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okay that's the whole page, you ate that 💅 stay slay, go union some groups, and remember: this Open MPI page is dated Sep 11, 2026 and the copyright belongs to The Open MPI Community, 2003-2026 🧚‍♀️💜 bye bestie! 🦋