MPI_Request_free(3)                Open MPI                MPI_Request_free(3)



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


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






1.4.1                            Jan 14, 2010              MPI_Request_free(3)
