MPI_GROUP_EXCL(3)

💖 MPI_Group_excl 💖

okay bestie, welcome to the Open MPI manual, section 3 🫶 ngl this one is kinda iconic so let's get into it ✨

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🎀 NAME 🎀

MPI_Group_excl — produces a group by reordering an existing group and taking only unlisted members. it's giving "un-invite everyone on this list, keep the rest" energy, and honestly? slay.

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🦋 SYNTAX 🦋

C Syntax

#include <mpi.h>

int MPI_Group_excl(MPI_Group group, int n, const int ranks[],
    MPI_Group *newgroup)

Fortran Syntax

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

MPI_GROUP_EXCL(GROUP, N, RANKS, NEWGROUP, IERROR)
    INTEGER GROUP, N, RANKS(*), NEWGROUP, IERROR

Fortran 2008 Syntax

USE mpi_f08

MPI_Group_excl(group, n, ranks, newgroup, ierror)
    TYPE(MPI_Group), INTENT(IN) :: group
    INTEGER, INTENT(IN) :: n, ranks(n)
    TYPE(MPI_Group), INTENT(OUT) :: newgroup
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💅 INPUT PARAMETERS 💅

• group — the group handle. this is your starting lineup, your main character group, the one we're about to curate.
• n — the number of elements in the array ranks (an integer, obvi). basically "how many names are on the list".
• ranks — an array of integer ranks in group that are NOT going to appear in newgroup. the exclusion list. the block list. iykyk.

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🌟 OUTPUT PARAMETERS 🌟

• newgroup — the new group derived from above (and it preserves the order defined by group) — a handle. this is the glow-up group that comes out the other side ✨
• ierror — Fortran only: error status (integer).

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🌸 DESCRIPTION 🌸

so the way that MPI_Group_excl works: it creates a group of processes called newgroup that you get by deleting from group those processes with ranks ranks[0], … ranks[n-1]. literally just yeet those specific ranks out of the group, bestie.

the ordering of processes in newgroup is identical to the ordering in group — no reshuffling, no chaos, we keep the vibe consistent, periodt.

each of the n elements of ranks must be a valid rank in group, and all elements must be distinct; otherwise, the call is erroneous 😭 no duplicates allowed, that's the rule.

and if n = 0? then newgroup is identical to group. excluding nobody = same group. a no-op. we love a chill case 💖

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🧚 NOTE 🧚

currently, each of the ranks you exclude must be a valid rank in the group and all elements must be distinct, or the function is erroneous. this restriction is per the draft. (yes, it's a whole thing, don't fight it 🙈)

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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 the 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. (brutal, but okay 💀)
• 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. the chill one 🫶

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. fr fr, plan accordingly.

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_range_excl

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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okay that's the whole page, bestie, you ate that 🌈💜 this was the Open MPI MPI_Group_excl(3) page, dated Sep 10, 2026 — no cap, go forth and exclude those ranks with confidence ✨🎀🦋