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MPI_Wait.3.rst

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MPI_Wait

:ref:`MPI_Wait` |mdash| 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 the "Return Status" subsection of the "Point-to-Point Communication" chapter in the MPI Standard. The status object for a send operation may be queried by a call to :ref:`MPI_Test_cancelled` (see the "Probe and Cancel" section in the MPI Standard).

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 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 :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 the "Return Status" section in the "Point-to-Point Communication" chapter in the MPI Standard, 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 always 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`