MPI_WIN_LOCK(3)

💅 MPI_Win_lock — Open MPI ✨

okay so bestie, gather round, because this one is literally the RMA bouncer. MPI_Win_lock starts an RMA access epoch and locks access to one specific rank. one rank. that's it. main character energy for a single target 🎀

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🌈 SYNTAX 🌈

✨ C Syntax ✨

#include <mpi.h>

int MPI_Win_lock(int lock_type, int rank, int assert, MPI_Win win)

✨ Fortran Syntax ✨

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

✨ Fortran 2008 Syntax ✨

USE mpi_f08
MPI_Win_lock(lock_type, rank, assert, win, ierror)
     INTEGER, INTENT(IN) :: lock_type, rank, assert
     TYPE(MPI_Win), INTENT(IN) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🫶 INPUT PARAMETERS 🫶

• lock_type — bestie, this is the whole vibe of the lock. it's either MPI_LOCK_EXCLUSIVE or MPI_LOCK_SHARED (and yeah, it's a state) 👑
• rank — the rank of the window you're locking, a nonnegative integer. like, whose window are we gatekeeping 💅
• assert — program assertion, an integer. more on this icon below ⭐
• win — the window object (handle). the actual window. the thing. 🪟

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🎀 OUTPUT PARAMETER 🎀

• ierror — Fortran only, error status as an integer. classic Fortran behavior, we love her 🙈

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🌸 DESCRIPTION 🌸

so MPI_Win_lock starts an RMA access epoch ✨ Locks make sure that only the windows created by specific processes can be accessed by those processes — and by literally no other processes — during that epoch. no gatecrashers, periodt.

Locks protect accesses to the locked target window that get effected by RMA calls issued between the lock and unlock call. They also protect local load/store accesses to a locked local window executed between the lock and unlock call. very protective, very guardian angel 🧚

Accesses protected by an exclusive lock will not be concurrent at the window site with other accesses to the same window that are lock protected. Accesses protected by a shared lock will not be concurrent at the window site with accesses protected by an exclusive lock to the same window. it's giving rules of the friend group, fr fr.

The assert argument is there to provide assertions on the context of the call, which can be used for various optimizations. (peep Section 6.4.4 of the MPI-2 Standard, iykyk.) A value of assert = 0 is always valid. The following assertion value is supported:

💜 MPI_MODE_NOCHECK 💜

No other processes will hold or attempt to acquire a conflicting lock while the caller holds the window lock. bold claim, bestie, but okay 🫶

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💀 NOTES 💀

okay so this one's a little scary, listen up 😭

In a client/server environment in which clients connect to a server and create windows that span both the client and the server, if a client or server that has obtained a lock on such a window and then terminates abnormally, the server or other clients may hang in a MPI_Win_lock call, failing to notice that the peer MPI job has terminated. like, ghosted so hard the whole function is just standing there 🙈

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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. 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. no cap, that's the whole chain of command 👑

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. so, everything. all of it. 💀
• 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, it 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, it aborts only the local process. situational drama, we stan 🔥
• MPI_ERRORS_RETURN — Returns an error code to the application. polite. respectful. gives you the info back 🙌

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 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. reading is fundamental, babes 📚

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🦋 SEE ALSO 🦋

• MPI_Win_unlock
• MPI_Win_lock_all

the whole lock/unlock cinematic universe, honestly obsessed 💖

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's the whole page, bestie! 🌈 written in the Open MPI era, dated Sep 11, 2026 — go forth and lock responsibly, mwah 💋✨