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MPI_Wait.3.rst

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MPI_Wait

:ref:`MPI_Wait` - Waits for an MPI send or receive to complete.

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: Request (handle).

OUTPUT PARAMETERS

  • status: Status object (status).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

A call to :ref:`MPI_Wait` returns when the operation identified by request is complete. 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 :ref:`MPI_Wait` and the request handle is set to MPI_REQUEST_NULL.

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 Section 3.2.5 of the MPI-1 Standard, "Return Status." The status object for a send operation may be queried by a call to :ref:`MPI_Test_cancelled` (see Section 3.8 of the MPI-1 Standard, "Probe and Cancel").

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.

One is allowed to call :ref:`MPI_Wait` with a null or inactive request argument. In this case the operation returns immediately with empty status.

NOTES

Successful return of :ref:`MPI_Wait` after an :ref:`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 :ref:`MPI_Buffer_attach`. Note that, at this point, we can no longer cancel the send (for more information, see Section 3.8 of the MPI-1 Standard, "Probe and Cancel"). If a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of :ref:`MPI_Cancel` (always being able to free program space that was committed to the communication subsystem).

Example: Simple usage of nonblocking operations and :ref:`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

ERRORS

Note that per MPI-1 section 3.2.5, MPI errors on requests passed to :ref:`MPI_WAIT` do not set the status.MPI_ERROR field in the returned status. The error code is passed to the back-end error handler and may be passed back to the caller through the return value of :ref:`MPI_WAIT` if the back-end error handler returns it. The pre-defined MPI error handler MPI_ERRORS_RETURN exhibits this behavior, for example.

.. seealso::
   * :ref:`MPI_Comm_set_errhandler`
   * :ref:`MPI_File_set_errhandler`
   * :ref:`MPI_Test`
   * :ref:`MPI_Testall`
   * :ref:`MPI_Testany`
   * :ref:`MPI_Testsome`
   * :ref:`MPI_Waitall`
   * :ref:`MPI_Waitany`
   * :ref:`MPI_Waitsome`
   * :ref:`MPI_Win_set_errhandler`