MPI_WAIT(3)

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💅 MPI_WAIT(3) — the Open MPI glow-up 🦋

okay so bestie, you're in the MPI_WAIT(3) part of the Open MPI manual and honestly? this one's a bop. it's the "pls just tell me when it's done" of MPI. let's get into it ✨

🎀 NAME 🎀

MPI_Wait — literally waits for an MPI send or receive to complete. that's it, that's the whole vibe. your main character blocking call 👑

✨ SYNTAX ✨

💖 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 — the Request handle. that's your lil ticket to the operation 🎫

🫶 OUTPUT PARAMETERS 🫶

  • status — the Status object (status). all the tea about the completed operation ☕
  • ierror — Fortran only, bestie: Error status (integer).

🔥 DESCRIPTION 🔥

so a call to MPI_Wait returns when the operation identified by request is complete. no cap. it just sits there until the thing is done.

if the communication object associated with this request was created by a nonblocking send or receive call, then guess what — the object is deallocated by the call to MPI_Wait, and the request handle is set to MPI_REQUEST_NULL. clean up after yourself, icon behavior 💅

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. and for a send operation, the status object may be queried by a call to MPI_Test_cancelled (see the “Probe and Cancel” section in the MPI Standard). iykyk.

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. literally free real estate, take it 🎀

and the sweetest part: 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, immediately done 🦋

🧚 NOTES 🧚

successful return of MPI_Wait after an 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. noted 📝

but heads up bestie — at this point, we can no longer cancel the send (for more information, see the “Probe and Cancel” section in the MPI Standard). if a matching receive is never posted, then the buffer cannot be freed. this runs somewhat counter to the stated goal of MPI_Cancel (always being able to free program space that was committed to the communication subsystem). so like... yeah. that's a real footnote, not me being dramatic 💀

example: simple usage of nonblocking operations and MPI_Wait. peek this:

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

💀 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.

Open MPI includes three predefined error handlers that can be used — stan list incoming ⭐:

  • MPI_ERRORS_ARE_FATAL — Causes the program to abort all connected MPI processes. yeah. everyone. 😭
  • 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. polite king 🫶

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, fr fr, don't count on it 🙈

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.

also, per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard: 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. 🎀

🌈 SEE ALSO 🌈

  • MPI_Comm_set_errhandler
  • MPI_File_set_errhandler
  • MPI_Test
  • MPI_Testall
  • MPI_Testany
  • MPI_Testsome
  • MPI_Waitall
  • MPI_Waitany
  • MPI_Waitsome
  • MPI_Win_set_errhandler

2003-2026, The Open MPI Community

and that's the whole page bestie, you ate 💅✨ this was MPI_WAIT(3) from Open MPI, dated Sep 11, 2026 — go forth and wait on your requests with confidence 🦋🍓

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