MPIX_COMM_SHRINK(3)

✨ NAME ✨

MPIX_Comm_shrink, MPIX_Comm_ishrink — bestie, this makes you a brand new communicator that has all the processes from the parent communicator that did not fail 💖

This is part of the User Level Fault Mitigation ULFM extension, iykyk.

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📜 SYNTAX 📜

C Syntax

#include <mpi.h>
#include <mpi-ext.h>

int MPIX_Comm_shrink(MPI_Comm comm, MPI_Comm *newcomm)

int MPIX_Comm_ishrink(MPI_Comm comm, MPI_Comm *newcomm, MPI_Request *request)

Fortran Syntax

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

MPIX_COMM_SHRINK(COMM, NEWCOMM, IERROR)
     INTEGER COMM, NEWCOMM, IERROR

MPIX_COMM_ISHRINK(COMM, NEWCOMM, REQUEST, IERROR)
     INTEGER COMM, NEWCOMM, REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
USE mpi_ext_f08

MPIX_Comm_shrink(comm, newcomm, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Comm), INTENT(OUT) :: newcomm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPIX_Comm_ishrink(comm, newcomm, request, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Comm), INTENT(OUT), ASYNCHRONOUS :: newcomm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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📥 INPUT PARAMETERS 📥

• comm — communicator (handle). That's the whole vibe, one handle 🎀

📤 OUTPUT PARAMETERS 📤

• newcomm — communicator (handle). Your shiny new one 💅
• request — request (handle, non-blocking only). Only shows up for ishrink, obviously.
• ierror — Fortran only: error status (integer).

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🌸 DESCRIPTION 🌸

Okay so this collective operation creates a new intra- or intercommunicator newcomm from the intra- or intercommunicator comm, respectively, by excluding the group of failed MPI processes as shrinkd upon during the operation. Bye losers, no cap. 🦋

The groups of newcomm must include every MPI process that returns from MPIX_Comm_shrink, and it must exclude every MPI process whose failure caused an operation on comm to raise an MPI error of class MPIX_ERR_PROC_FAILED or MPIX_ERR_PROC_FAILED_PENDING at a member of the groups of newcomm, before that member initiated the shrink operation. That's the whole rule, bestie, and it is not optional.

Said otherwise — this procedure is semantically equivalent to an MPI_Comm_split operation that would succeed despite failures, where members of the groups of newcomm participate with the same color and a key equal to their rank in comm. Literally the split that refuses to give up. Iconic. ✨

MPIX_Comm_ishrink is the non-blocking variant of MPIX_Comm_shrink. But listen: just like with MPI_Comm_idup, it is erroneous to use newcomm before request has completed. Do not touch it early. 🙈

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💀 WHEN THE COMMUNICATOR IS REVOKED OR CONTAINS FAILED PROCESSES 💀

This function never raises an error of classes MPIX_ERR_REVOKED or MPIX_ERR_PROC_FAILED. The defined semantics of MPIX_Comm_shrink and MPIX_Comm_ishrink are maintained when comm is revoked, or when the group of comm contains failed MPI processes.

In particular — and this is the part people get wrong — MPIX_Comm_shrink and MPIX_Comm_ishrink are collective operations, even when comm is revoked. Periodt.

Bestie, real talk: the implementation will strive to detect all failures during the shrink operation, but in certain circumpstances, the group of newcomm may still contain failed MPI processes. Their failure will be detected in subsequent MPI operations on newcomm. So it's giving "good faith effort", not "guaranteed clean". 😭

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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. Straight up 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 🫶

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, at all. Main character energy can't save you here.

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:

• MPIX_Comm_is_revoked

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👑 COPYRIGHT 👑

Copyright 2003-2026, The Open MPI Community

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And that's the whole page, bestie — brought to you by the Open MPI girlies, version stamped Sep 10, 2026, page MPIX_COMM_SHRINK(3). Go shrink something. Slay. 💖✨