MPI_INTERCOMM_MERGE(3)

💖 MPI_INTERCOMM_MERGE(3) — the Open MPI glow-up 💖

okay bestie, gather round, because today we're talking about MPI_INTERCOMM_MERGE from the Open MPI manual (third section, the library calls, you know the vibe). This is the one that takes your awkward two-group intercommunicator situation and turns it into one big happy intracommunicator. it's giving ✨blended family✨ and honestly? iconic.

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🌸 NAME 🌸

MPI_Intercomm_merge — Creates an intracommunicator from an intercommunicator.

like. literally what it says on the tin. no cap. 💅

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✨ SYNTAX ✨

🎀 C Syntax

#include <mpi.h>

int MPI_Intercomm_merge(MPI_Comm intercomm, int high,
     MPI_Comm *newintracomm)

🦋 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_INTERCOMM_MERGE(INTERCOMM, HIGH, NEWINTRACOMM, IERROR)
     INTEGER INTERCOMM, NEWINTRACOMM, IERROR
     LOGICAL HIGH

🌈 Fortran 2008 Syntax

USE mpi_f08
MPI_Intercomm_merge(intercomm, high, newintracomm, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: intercomm
     LOGICAL, INTENT(IN) :: high
     TYPE(MPI_Comm), INTENT(OUT) :: newintracomm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

three syntaxes, three aesthetics, all valid, we love options 💜

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🍓 INPUT PARAMETERS 🍓

• intercomm — the intercommunicator you're working with (type indicator). your starting material, bestie.
• high — this is the ordering knob. it's used to order the groups of the two intracommunicators within comm when creating the new communicator (type indicator). yes it's just a flag. yes it decides fates. 🫶

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👑 OUTPUT PARAMETERS 👑

• newintracomm — the created intracommunicator (type indicator). your brand new baby. 🎀
• ierror — Fortran only: Error status (integer). Fortran girlies get an extra status report, as they deserve.

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💫 DESCRIPTION 💫

okay so here's the tea 🍵 — this function creates an intracommunicator from the union of the two groups that are associated with intercomm.

And now the part where you must listen, because it's a rule and not a suggestion:

All processes should provide the same high value within each of the two groups.

so like, group A all agrees on one value, group B all agrees on one value. that's the assignment.

• If processes in one group provide the value high = false and processes in the other group provide the value high = true, then the union orders the “low” group before the “high” group. so the low group goes first. line order. queue. you get it. 🚶♀️
• If all processes provide the same high argument, then the order of the union is arbitrary. not me — that's just what it says. don't go assuming, bestie. 🙈

also, this call is blocking and collective within the union of the two groups. so everybody shows up, everybody waits, no skipping. periodt. 💅

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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. (iconic asymmetry, we stan consistency 💀)

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. blunt. dramatic. 💀
• 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. it just tells you. 🫶

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 bank on it, bestie. 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_Intercomm_create
• MPI_Comm_free

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🌟 Copyright 🌟

2003-2026, The Open MPI Community

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and that's the whole page, no crumbs left 💖 this guide was served to you by the Open MPI girlies, page dated Sep 10, 2026 — MPI_INTERCOMM_MERGE(3), now go merge those communicators and slay ✨👑🌸