MPI_GROUP_INTERSECTION(3)

💖 MPI_GROUP_INTERSECTION 💖

okay so this is a page from the Open MPI manual, bestie — specifically MPI_GROUP_INTERSECTION(3), and it's giving "let's find out who's in BOTH groups" energy. stay with me, it's actually so cute once you get it 🫶

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✨ NAME ✨

MPI_Group_intersection — produces a group at the intersection of two existing groups. (yes literally just "the overlap", she's simple like that 💅)

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

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

🌸 Fortran Syntax

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

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

🌸 Fortran 2008 Syntax

USE mpi_f08

MPI_Group_intersection(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 — first group (handle). the OG, the main character 👑
• group2 — second group (handle). the one we're vibe-checking her against

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✨ OUTPUT PARAMETERS ✨

• newgroup — intersection group (handle). the new baby that comes out of it 🎀
• ierror — Fortran only: error status (integer).

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✨ DESCRIPTION ✨

so the set-like operations are defined like this, no cap:

• union — all elements of the first group (group1), followed by all elements of second group (group2) not in first.
• 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.

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. neither union nor intersection are commutative, but both are associative. read that again bestie, it matters — order of the outputs is NOT whatever you feel like, and swapping the two groups around is NOT the same thing. but if you're chaining them, grouping them differently is totally fine.

the new group can be empty, that is, equal to MPI_GROUP_EMPTY. (empty group rights!! she's valid, she's just built different 🙈)

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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_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. 🫠 like. it might just not.

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 — you ate that 💜 this was Open MPI's MPI_GROUP_INTERSECTION(3), dated Sep 10, 2026, and yeah that's the version stamp right there at the bottom of the original. go forth and intersect responsibly, periodt ✨🦋