MPI_COMM_COMPARE(3)

💖 MPI_COMM_COMPARE 💖

okay bestie, welcome to the Open MPI manual, page MPI_COMM_COMPARE(3). this one is a lil cutie: it's the function that looks at two communicators and tells you "are you two literally the same, or just twins, or cousins, or strangers?" 🫶

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✨ NAME ✨

MPI_Comm_compare — compares two communicators. that's it. that's the whole job. 💅

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🌸 SYNTAX 🌸

🎀 C Syntax

#include <mpi.h>

int MPI_Comm_compare(MPI_Comm comm1, MPI_Comm comm2, int *result)

🎀 Fortran Syntax

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

MPI_COMM_COMPARE(COMM1, COMM2, RESULT, IERROR)
    INTEGER COMM1, COMM2, RESULT, IERROR

🎀 Fortran 2008 Syntax

USE mpi_f08

MPI_Comm_compare(comm1, comm2, result, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm1, comm2
    INTEGER, INTENT(OUT) :: result
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🦋 INPUT PARAMETERS 🦋

• comm1 — Comm1 (handle). one of the two communicators in the lineup 💁‍♀️
• comm2 — Comm2 (handle). the other one. the vibes are about to be compared ✨

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🌈 OUTPUT PARAMETERS 🌈

• result — Result of comparison (integer). this is where the verdict lands, and it's gonna be one of the four constants below 👑
• ierror — Fortran only: Error status (integer).

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💜 DESCRIPTION 💜

okay so here's the tea. MPI_Comm_compare hands you back one of four answers, and each one means something very specific. no cap.

• MPI_IDENT results if and only if comm1 and comm2 are handles for the same object (identical groups and same contexts). like, actually the same communicator. not twins — the same person. 😭
• MPI_CONGRUENT results if the underlying groups are identical in constituents and rank order; these communicators differ only by context. same squad, same order, different group chat. 🫶
• MPI_SIMILAR results if the group members of both communicators are the same but the rank order differs. same people, swapped seats. iconic chaos energy. 🌀
• MPI_UNEQUAL results otherwise. yeah. we don't know each other like that. 💀

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🔥 ERRORS 🔥

ngl bestie, the error section is long but it's important, so we're doing all of it. no skimming. ✋

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. dramatic, but on purpose 💀
• 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. polite. respectful. we love her. 🫶

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, read that again. fr fr.

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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👑 Copyright 👑

2003-2026, The Open MPI Community

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and that's the whole page, bestie!! 🎀✨ this was MPI_COMM_COMPARE(3) from the Open MPI crew, dated Sep 10, 2026. go compare some communicators, you absolute main character. 💖🦋 periodt.