MPI_WAIT(3)

📦

version 5.0.10 · 7 versions

💖 MPI_WAIT 💖

okay bestie, gather round 🫶 this is MPI_WAIT(3), straight outta the Open MPI manual (that's section 3, aka library calls, iykyk). It's the function that Waits for an MPI send or receive to complete. literally the "girl just hold on a sec" of parallel programming ✨

✨ SYNTAX ✨

no cap, there's a flavor for everyone here 🌈

💻 C Syntax 💻

#include <mpi.h>

int MPI_Wait(MPI_Request *request, MPI_Status *status)

🧚 Fortran Syntax 🧚

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WAIT(REQUEST, STATUS, IERROR)
     INTEGER REQUEST, STATUS(MPI_STATUS_SIZE), IERROR

🦋 Fortran 2008 Syntax 🦋

USE mpi_f08
MPI_Wait(request, status, ierror)
     TYPE(MPI_Request), INTENT(INOUT) :: request
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🫶 INPUT PARAMETER 🫶

request: Request (handle). yeah it's just a handle, the little ticket that says "this operation? mine." 🎟️

🌸 OUTPUT PARAMETERS 🌸

status: Status object (status). the receipts of what actually went down 📬
ierror: Fortran only: Error status (integer). Fortran girlies get their errors right here, no return value for you 💅

💅 DESCRIPTION 💅

So like, the way that this works: a call to MPI_Wait returns when the operation identified by request is complete. That's it, that's the vibe. ✨

And if the communication object associated with this request was created by a nonblocking send or receive call, then the object is deallocated by the call to MPI_Wait and the request handle is set to MPI_REQUEST_NULL. so it doesn't just finish — it cleans up after itself. main character behavior, honestly 👑

The call returns, in status, information on the completed operation. The content of the status object for a receive operation can be accessed as described in the "Return Status" subsection of the "Point-to-Point Communication" chapter in the MPI Standard https://www.mpi-forum.org/docs/. The status object for a send operation may be queried by a call to MPI_Test_cancelled <#mpi-test-cancelled> (see the "Probe and Cancel" section in the MPI Standard https://www.mpi-forum.org/docs/). 🔍

If your application does not need to examine the status field, you can save resources by using the predefined constant MPI_STATUS_IGNORE as a special value for the status argument. not me just... ignoring my status... resource-efficient queen 🎀

One is allowed to call MPI_Wait with a null or inactive request argument. In this case the operation returns immediately with empty status. zero drama, returns instantly, iconic 🌸

🎀 NOTES 🎀

Successful return of MPI_Wait after an MPI_Ibsend <#mpi-ibsend> implies that the user send buffer can be reused i.e., data has been sent out or copied into a buffer attached with MPI_Buffer_attach <#mpi-buffer-attach>. Note that, at this point, we can no longer cancel the send (for more information, see the "Probe and Cancel" section in the MPI Standard https://www.mpi-forum.org/docs/). 🚫

okay this next bit is lowkey a whole tragedy: if a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of MPI_Cancel <#mpi-cancel> (always being able to free program space that was committed to the communication subsystem). 😭 so sad, so real, so unavoidable.

Example: Simple usage of nonblocking operations and MPI_Wait. 🍓

CALL MPI_COMM_RANK(comm, rank, ierr)
IF(rank == 0) THEN
    CALL MPI_ISEND(a(1), 10, MPI_REAL, 1, tag, comm, request, ierr)
    **** do some computation ****
    CALL MPI_WAIT(request, status, ierr)
ELSE IF (rank == 1) THEN
    CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr)
    **** do some computation ****
    CALL MPI_WAIT(request, status, ierr)
END IF

look at that. rank 0 sends, rank 1 receives, everybody waits. balance. harmony. friendship 🫶

💀 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>/MPI_Init_thread <#mpi-init-thread>, after MPI_Finalize <#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 <#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>/MPI_Comm_spawn_multiple <#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. 💀 the nuclear option, fr fr
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 <#mpi-abort> was called on that communicator. If called on a window or file, acts as if MPI_Abort <#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, hands you the code and walks away 🫡

MPI applications can also implement their own error handlers by calling:

MPI_Comm_create_errhandler <#mpi-comm-create-errhandler> then MPI_Comm_set_errhandler <#mpi-comm-set-errhandler>
MPI_File_create_errhandler <#mpi-file-create-errhandler> then MPI_File_set_errhandler <#mpi-file-set-errhandler>
MPI_Session_create_errhandler <#mpi-session-create-errhandler> then MPI_Session_set_errhandler <#mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>
MPI_Win_create_errhandler <#mpi-win-create-errhandler> then MPI_Win_set_errhandler <#mpi-win-set-errhandler>

build your own handler, be your own hero ✨

Note that MPI does not guarantee that an MPI program can continue past an error. 🚨

See the MPI man page MPI <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

See the Error Handling section of the MPI-3.1 standard for more information.

Note that per the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard https://www.mpi-forum.org/docs/, MPI errors on requests passed to MPI_Wait do not set the status.MPI_ERROR field in the returned status. The error code is always passed to the back-end error handler and may be passed back to the caller through the return value of MPI_Wait if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example. 👀 so if you're out here checking status.MPI_ERROR after a MPI_Wait... bestie, check the return value instead.

👑 Copyright 👑

2003-2026, The Open MPI Community

aaaand that's the whole page, bestie 🌈 handing you off with a kiss: this was Open MPI, MPI_WAIT(3), dated Sep 10, 2026. go forth and wait responsibly ✨💖🦋

📦 where this page comes from

ubuntu stonking: openmpi-doc 5.0.10-6 · other
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms