oracular (3) MPI_Request_free.openmpi.3.gz

NAME

       MPI_Request_free - Frees a communication request object.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Request_free(MPI_Request *request)

Fortran Syntax

       INCLUDE 'mpif.h'
       MPI_REQUEST_FREE(REQUEST, IERROR)
            INTEGER   REQUEST, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Request_free(request, ierror)
            TYPE(MPI_Request), INTENT(INOUT) :: request
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

C++ Syntax

       #include <mpi.h>
       void Request::Free()

INPUT/OUTPUT PARAMETER

       request
                       Communication request (handle).

DESCRIPTION

       This operation allows a request object to be deallocated without waiting for the associated communication
       to complete.

       MPI_Request_free marks the request object for deallocation and  sets  request  to  MPI_REQUEST_NULL.  Any
       ongoing  communication  that is associated with the request will be allowed to complete. The request will
       be deallocated only after its completion.

NOTES

       Once a request is freed by a call to MPI_Request_free, it is not possible to  check  for  the  successful
       completion  of  the associated communication with calls to MPI_Wait or MPI_Test. Also, if an error occurs
       subsequently during the communication, an error code cannot be returned to the user -- such an error must
       be  treated  as  fatal. Questions arise as to how one knows when the operations have completed when using
       MPI_Request_free. Depending on the program logic, there may be other ways in which the program knows that
       certain  operations  have  completed  and this makes usage of MPI_Request_free practical. For example, an
       active send request could be freed when the logic of the program is such that the receiver sends a  reply
       to  the  message  sent -- the arrival of the reply informs the sender that the send has completed and the
       send buffer can be reused. An active receive request should never be freed, as the receiver will have  no
       way to verify that the receive has completed and the receive buffer can be reused.

       Example:

           CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
           IF(rank.EQ.0) THEN
               DO i=1, n
                 CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
                 CALL MPI_REQUEST_FREE(req, ierr)
                 CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
                 CALL MPI_WAIT(req, status, ierr)
               END DO
           ELSE    ! rank.EQ.1
               CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
               CALL MPI_WAIT(req, status)
               DO I=1, n-1
                  CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
                  CALL MPI_REQUEST_FREE(req, ierr)
                  CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
                  CALL MPI_WAIT(req, status, ierr)
               END DO
               CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
               CALL MPI_WAIT(req, status)
           END IF

       This  routine  is  normally  used  to  free  persistent  requests  created  with  either MPI_Recv_init or
       MPI_Send_init and friends.  However, it can be used to free a request created with MPI_Irecv or MPI_Isend
       and friends; in that case the use can not use the test/wait routines on the request.

       It is permitted to free an active request.  However, once freed, you can not use the request in a wait or
       test routine (e.g., MPI_Wait ).

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; 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.

SEE ALSO

       MPI_Isend
       MPI_Irecv
       MPI_Issend
       MPI_Ibsend
       MPI_Irsend
       MPI_Recv_init
       MPI_Send_init
       MPI_Ssend_init
       MPI_Rsend_init
       MPI_Test
       MPI_Wait
       MPI_Waitall
       MPI_Waitany
       MPI_Waitsome
       MPI_Testall
       MPI_Testany
       MPI_Testsome