MPI_WIN_START(3)

💖 MPI_Win_start 💖

okay bestie, welcome to the Open MPI manual, page MPI_WIN_START(3) — yes it's a manpage but make it fashion. this one is all about kicking off an RMA access epoch and honestly? it's giving main character energy. let's go 🦋

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✨ NAME ✨

MPI_Win_start — starts an RMA access epoch for win 💅

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🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_Win_start(MPI_Group group, int assert, MPI_Win win)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WIN_START(GROUP, ASSERT, WIN, IERROR)
     INTEGER GROUP, ASSERT, WIN, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Win_start(group, assert, win, ierror)
     TYPE(MPI_Group), INTENT(IN) :: group
     INTEGER, INTENT(IN) :: assert
     TYPE(MPI_Win), INTENT(IN) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎀 INPUT PARAMETERS 🎀

• group — the group of target processes (handle), i.e. the little squad of processes we're gonna be reaching into 👯‍♀️
• assert — program assertion (integer). yes it's just an int, she's simple like that ✨
• win — window object (handle). our beloved window 🪟

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🫶 OUTPUT PARAMETERS 🫶

• ierror — Fortran only: error status (integer). the classic "did it work tho" number 💀

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🔥 DESCRIPTION 🔥

okay so. MPI_Win_start is a one-sided MPI communication synchronization call that starts an RMA access epoch for win. periodt. 🎯

the way this works is: any RMA calls you issue on win during this epoch must access only windows at processes inside group. and here's the tea — each process in group has to issue a matching call to MPI_Win_post. no cap, it's a whole handshake situation 🤝

MPI_Win_start is allowed to block until the corresponding MPI_Win_post calls have been executed, but it is not required to. so like, she might wait for you, she might not — she's mysterious like that 😌

the assert argument is there to provide assertions about the context of the call that may be used for various optimizations (peep Section 6.4.4 of the MPI-2 Standard if you wanna be academic about it 📚). a value of assert = 0 is always valid. the following assertion value is supported:

MPI_MODE_NOCHECK

• when this value is passed in to this call, the library assumes that the post call on the target has been called and it is not necessary for the library to check to see if such a call has been made. lowkey a shortcut, highkey a trust fall 🙈

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😭 ERRORS 😭

almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines put it in the last argument. classic.

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. she said everybody 💀
• 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. so many vibes, one handler ✨
• MPI_ERRORS_RETURN — returns an error code to the application. polite. respectful. we stan 🙏

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… be careful bestie 🫠

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_Win_post
• MPI_Win_complete

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👑 Copyright 👑

2003-2026, The Open MPI Community

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and that's the whole page, no notes. this was MPI_WIN_START(3) from the Open MPI manual, dated Sep 10, 2026 — go forth and start those epochs, you gorgeous synchronized icon 🧚✨💖