MPI_GROUP_INCL(3)

💅 MPI_GROUP_INCL(3) — the Open MPI glow-up

okay so bestie, we're in the Open MPI corner of the man pages today, and this one is called MPI_Group_incl. it produces a group by reordering an existing group and taking only listed members ✨ like a guest list but for processes, and honestly the way it works is so clean.

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🌸 SYNTAX 🌸

✨ C Syntax

#include <mpi.h>

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

✨ Fortran Syntax

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

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

✨ Fortran 2008 Syntax

USE mpi_f08

MPI_Group_incl(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, as a handle. your starting lineup 💖
• n — number of elements in array ranks (and also the size of newgroup), an integer. so it's doing double duty, main character behavior fr.
• ranks — the ranks of the processes in group that should appear in newgroup, as an array of integers. this is the VIP list 🫶

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🦋 OUTPUT PARAMETERS 🦋

• newgroup — the new group derived from the stuff above, in the order defined by ranks, as a handle. literally the whole point 💅
• ierror — Fortran only: the error status, an integer.

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⭐ DESCRIPTION ⭐

so MPI_Group_incl creates a group group_out that consists of the n processes in group with ranks rank[0], …, rank[n-1] 🌈. the process with rank i in group_out is the process with rank ranks[i] in group. yes, that's the reordering magic — you pick who's in and you decide the order, both at once, iconic.

⚠️ every one of the n elements of ranks must be a valid rank in group and all elements must be distinct, or else the program is erroneous. like, straight up not allowed. no cap. 💀

if n = 0, then group_out is MPI_GROUP_EMPTY (empty group era, we love a minimalist moment). this function can, for instance, be used to reorder the elements of a group — that's literally one of its use cases, tea.

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🙈 NOTE 🙈

this implementation does not currently check to ensure that there are no duplicates in the list of ranks. so like… it's on you bestie, be careful 🍓

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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. yeah. everyone. 😭
• 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. situational queen 👑
• MPI_ERRORS_RETURN — returns an error code to the application. polite. gives you the info back 🎀

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 just assume you can vibe through it.

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_compare

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💖 COPYRIGHT 💖

that's the whole page bestie, go build a group 🦋🌸✨

this little love letter was brought to you by the Open MPI crew — 2003-2026, The Open MPI Community — and it was dated Sep 11, 2026, which is when this page said its last words. MPI_GROUP_INCL(3) forever 💜🧚‍♀️