oracular (3) MPI_Wait.openmpi.3.gz

NAME

       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

C++ Syntax

       #include <mpi.h>
       void Request::Wait(Status& status)

       void Request::Wait()

INPUT PARAMETER

       request   Request (handle).

OUTPUT PARAMETERS

       status    Status object (status).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       A  call  to  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 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 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  MPI_Wait with a null or inactive request argument. In this case the operation
       returns immediately with empty status.

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. 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 MPI_Cancel (always being able to free program space that was  committed  to
       the communication subsystem).

       Example: Simple usage of nonblocking operations and  MPI_Wait.

           CALL MPI_COMM_RANK(comm, rank, ierr)
           IF(rank.EQ.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
               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 value of the function and Fortran
       routines in the last argument. C++ functions do not return errors. If the default error handler is set to
       MPI::ERRORS_THROW_EXCEPTIONS,  then  on  error  the  C++  exception  mechanism  will  be used to throw an
       MPI::Exception object.

       Before the error value is returned, the current MPI error handler  is  called.  By  default,  this  error
       handler  aborts  the  MPI  job,  except  for  I/O  function errors. The error handler may be changed with
       MPI_Comm_set_errhandler, MPI_File_set_errhandler, or MPI_Win_set_errhandler (depending on the type of MPI
       handle  that  generated the request); the predefined error handler MPI_ERRORS_RETURN may be used to cause
       error values to be returned. Note that MPI does not guarantee that an MPI program can  continue  past  an
       error.

       Note  that  per  MPI-1  section  3.2.5,  MPI  exceptions  on  requests  passed to 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 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