MPI_COMM_SPLIT_TYPE(3)

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version 5.0.10 · 7 versions

💖 MPI_COMM_SPLIT_TYPE — the Open MPI glow-up 💖

okay bestie, welcome to the Open MPI manual, section 3 (library calls, the fun ones ✨). this one is all about splitting a communicator into friend groups based on vibes — well, based on a type, but you get it. no cap.

✨ SYNTAX ✨

C Syntax

#include <mpi.h>

int MPI_Comm_split_type(MPI_Comm comm, int split_type, int key,
     MPI_Info info, MPI_Comm *newcomm)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_SPLIT_TYPE(COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR)
     INTEGER COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Comm_split_type(comm, split_type, key, info, newcomm, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(IN) :: split_type, key
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Comm), INTENT(OUT) :: newcomm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🫶 INPUT PARAMETERS 🫶

comm — the communicator you're working with (a handle). your starting group chat, if you will.
split_type — which kind of processes get grouped together (integer). this is the whole personality of the call.
key — controls how ranks get assigned inside the new group (integer).
info — the info argument (handle).

💅 OUTPUT PARAMETERS 💅

newcomm — your shiny new communicator (handle). the group chat that got created 🎀
ierror — Fortran only: error status (integer).

🌸 DESCRIPTION 🌸

so what does it actually do. This function partitions the group associated with comm into disjoint subgroups, based on the type specified by split_type. and here's the slay: each subgroup contains all processes of the same type. like, sorting yourself into the right table at lunch, automatically. 💅

Within each subgroup, the processes are ranked in the order defined by the value of the argument key, with ties broken according to their rank in the old group. so key is giving main character ranking energy, and if two of y'all have the same key, it just falls back to whoever was ranked higher before.

A new communicator is created for each subgroup and returned in newcomm.

This is a collective call — every process has to show up, and all processes must provide the same split_type. but here's the tea: each process is permitted to provide different values for key. so you all agree on the group split, but you can each claim your own spot inside it. ✨

An exception to this rule is that a process may supply the type value MPI_UNDEFINED, in which case newcomm returns MPI_COMM_NULL. basically: "not me, I'm sitting this one out 🙈", and you get handed a null communicator instead.

🌈 SPLIT TYPES 🌈

MPI_COMM_TYPE_SHARED — This type splits the communicator into subcommunicators, each of which can create a shared memory region. 🧠✨
OMPI_COMM_TYPE_NODE — Synonym for MPI_COMM_TYPE_SHARED. same energy, different name, iconic.
OMPI_COMM_TYPE_HWTHREAD — This type splits the communicator into subcommunicators, each of which belongs to the same hardware thread.
OMPI_COMM_TYPE_CORE — This type splits the communicator into subcommunicators, each of which belongs to the same core/processing unit.
OMPI_COMM_TYPE_L1CACHE — This type splits the communicator into subcommunicators, each of which belongs to the same L1 cache.
OMPI_COMM_TYPE_L2CACHE — This type splits the communicator into subcommunicators, each of which belongs to the same L2 cache.
OMPI_COMM_TYPE_L3CACHE — This type splits the communicator into subcommunicators, each of which belongs to the same L3 cache.
OMPI_COMM_TYPE_SOCKET — This type splits the communicator into subcommunicators, each of which belongs to the same socket.
OMPI_COMM_TYPE_NUMA — This type splits the communicator into subcommunicators, each of which belongs to the same NUMA-node.
OMPI_COMM_TYPE_BOARD — This type splits the communicator into subcommunicators, each of which belongs to the same board.
OMPI_COMM_TYPE_HOST — This type splits the communicator into subcommunicators, each of which belongs to the same host.
OMPI_COMM_TYPE_CU — This type splits the communicator into subcommunicators, each of which belongs to the same computational unit.
OMPI_COMM_TYPE_CLUSTER — This type splits the communicator into subcommunicators, each of which belongs to the same cluster.

⭐ NOTES ⭐

okay important, listen up. The communicator keys denoted with an OMPI_ prefix instead of an MPI_ prefix are specific to Open MPI, and are not part of the MPI standard. so they're very much iykyk. 🫣

Their use should be protected by the OPEN_MPI C preprocessor macro. don't rawdog those constants, bestie, guard them.

💀 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, that's it, that's the vibe check. 😭

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_Comm_create
MPI_Intercomm_create
MPI_Comm_dup
MPI_Comm_free
MPI_Comm_split

👑 Copyright 👑

2003-2026, The Open MPI Community

and that's the whole page, bestie 🫶 this was Open MPI, page 3 of the manual, dated Sep 10, 2026 — MPI_COMM_SPLIT_TYPE(3), copyright 2003-2026, The Open MPI Community. go split some communicators, you're gonna eat 💅✨

📦 where this page comes from

ubuntu stonking: openmpi-doc 5.0.10-6 · other
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms