MPI_BARRIER(3)

💖 MPI_BARRIER(3) 💖

okay bestie, welcome to the Open MPI manual page for MPI_Barrier and MPI_Ibarrier — aka the whole "synchronization between MPI processes in a group" situation. we're doing a vibe check on everybody meeting up at the same spot before anyone moves on. slay.

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Barrier(MPI_Comm)
int MPI_Ibarrier(MPI_Comm comm, MPI_Request *request)
int MPI_Barrier_init(MPI_Comm comm, MPI_Info info, MPI_Request *request)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_BARRIER(COMM, IERROR)
    INTEGER COMM, IERROR
MPI_IBARRIER(COMM, REQUEST, IERROR)
    INTEGER COMM, REQUEST, IERROR
MPI_BARRIER_INIT(COMM, INFO, REQUEST, IERROR)
    INTEGER COMM, INFO, REQUEST, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Barrier(comm, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ibarrier(comm, request, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Request), INTENT (OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Barrier_init(comm, info, request, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Info), INTENT(IN) :: info
    TYPE(MPI_Request), INTENT (OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🌈 INPUT PARAMETER

• comm — the communicator, bestie. it's a handle.
• info — the info handle. this one's persistent only, so don't come at it with the plain blocking call, iykyk.

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⭐ OUTPUT PARAMETERS

• request — a request handle. non-blocking only — she's for the async girlies.
• ierror — Fortran only: the error status, as an integer.

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💅 DESCRIPTION

so here's the tea: an MPI barrier completes after all groups members have entered the barrier. that's it. that's the whole lore. everybody shows up, then everybody's free. no stragglers, no cutting the line. periodt.

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🌸 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

when the communicator is an inter-communicator, the barrier operation is performed across all processes in both groups. and the iconic detail: all processes in the first group may exit the barrier when all processes in the second group have entered the barrier. so group one can dip the second group two shows up. mutual respect, we love to see it 🫶

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😭 ERRORS

ngl this section is long but it's important, so no skipping. 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 the 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? yes. on purpose? also yes.
• 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. so like… don't count on it, fr fr.

⚠️ heads up bestie: MPI does not guarantee your program can keep going after an error. 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_Bcast

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👑 Copyright

2003-2026, The Open MPI Community

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alright that's the whole page, you ate 💅✨ this was the MPI_BARRIER(3) page from the Open MPI crew, dated Sep 10, 2026 — go forth and synchronize, main character 🌈🦋💜