MPI_Grequest_start(3)              Open MPI              MPI_Grequest_start(3)



NAME
       MPI_Grequest_start  - Starts a generalized request and returns a handle
       to it in request.


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Grequest_start(MPI_Grequest_query_function *query_fn,
            MPI_Grequest_free_function *free_fn,
            MPI_Grequest_cancel_function *cancel_fn, void *extra_state,
            MPI_Request *request)


Fortran Syntax (see FORTRAN 77 NOTES)
       INCLUDE 'mpif.h'
       MPI_GREQUEST_START(QUERY_FN, FREE_FN, CANCEL_FN, EXTRA_STATE,
            REQUEST, IERROR)
            INTEGER   REQUEST, IERROR
            EXTERNAL QUERY_FN, FREE_FN, CANCEL_FN
            INTEGER (KIND=MPI_ADDRESS_KIND) EXTRA_STATE


C++ Syntax
       #include <mpi.h>
       static MPI::Grequest
                    MPI::Grequest::Start(const MPI::Grequest::Query_function
                    query_fn, const MPI::Grequest::Free_function free_fn,
                    const MPI::Grequest::Cancel_function cancel_fn,
                    void *extra_state)


INPUT PARAMETERS
       query_fn  Callback function invoked when request status is queried
                 (function).

       free_fn   Callback function invoked when request is freed (function).

       cancel_fn Callback function invoked when request is canceled
                 (function).

       extra_state
                 Extra state.


OUTPUT PARAMETERS
       request   Generalized request (handle).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       MPI_Grequest_start starts a generalized request and returns a handle to
       it in request.

       The syntax and meaning of the callback functions are listed below. All
       callback functions are passed the extra_state argument that was
       associated with the request by the starting call MPI_Grequest_start.
       This can be used to maintain user-defined state for the request. In C,
       the query function is

          typedef int MPI_Grequest_query_function(void *extra_state,
                       MPI_Status *status);

       In Fortran, it is

          SUBROUTINE GREQUEST_QUERY_FUNCTION(EXTRA_STATE, STATUS, IERROR)
              INTEGER STATUS(MPI_STATUS_SIZE), IERROR
              INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE

       and in C++, it is

          typedef int MPI::Grequest::Query_function(void* extra_state,
                       MPI::Status& status);

       The query_fn function computes the status that should be returned for
       the generalized request. The status also includes information about
       successful/unsuccessful cancellation of the request (result to be
       returned by MPI_Test_cancelled).

       The query_fn function is invoked by the MPI_{Wait|Test}{any|some|all}
       call that completed the generalized request associated with this
       callback. The callback function is also invoked by calls to
       MPI_Request_get_status if the request is complete when the call occurs.
       In both cases, the callback is passed a reference to the corresponding
       status variable passed by the user to the MPI call. If the user
       provided MPI_STATUS_IGNORE or MPI_STATUSES_IGNORE to the MPI function
       that causes query_fn to be called, then MPI will pass a valid status
       object to query_fn, and this status will be ignored upon return of the
       callback function. Note that query_fn is invoked only after
       MPI_Grequest_complete is called on the request; it may be invoked
       several times for the same generalized request. Note also that a call
       to MPI_{Wait|Test}{some|all} may cause multiple invocations of query_fn
       callback functions, one for each generalized request that is completed
       by the MPI call. The order of these invocations is not specified by
       MPI.

       In C, the free function is

          typedef int MPI_Grequest_free_function(void *extra_state);

       In Fortran, it is

          SUBROUTINE GREQUEST_FREE_FUNCTION(EXTRA_STATE, IERROR)
              INTEGER IERROR
              INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE

       And in C++, it is

          typedef int MPI::Grequest::Free_function(void* extra_state);

       The free_fn callback function is invoked to clean up user-allocated
       resources when the generalized request is freed.

       The free_fn function is invoked by the MPI_{Wait|Test}{any|some|all}
       call that completed the generalized request associated with this
       callback. free_fn is invoked after the call to query_fn for the same
       request. However, if the MPI call completed multiple generalized
       requests, the order in which free_fn callback functions are invoked is
       not specified by MPI.

       The free_fn callback is also invoked for generalized requests that are
       freed by a call to MPI_Request_free (no call to
       MPI_{Wait|Test}{any|some|all} will occur for such a request). In this
       case, the callback function will be called either in the MPI call
       MPI_Request_free(request) or in the MPI call
       MPI_Grequest_complete(request), whichever happens last. In other words,
       in this case the actual freeing code is executed as soon as both calls
       (MPI_Request_free and MPI_Grequest_complete) have occurred. The request
       is not deallocated until after free_fn completes. Note that free_fn
       will be invoked only once per request by a correct program.

       In C, the cancel function is

          typedef int MPI_Grequest_cancel_function(void *extra_state, int complete);

       In Fortran, the cancel function is

          SUBROUTINE GREQUEST_CANCEL_FUNCTION(EXTRA_STATE, COMPLETE, IERROR)
              INTEGER IERROR
              INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE
              LOGICAL COMPLETE

       In C++, the cancel function is

          typedef in MPI::Grequest::Cancel_function(void* extra_state,
                      bool complete);

       The cancel_fn function is invoked to start the cancelation of a
       generalized request. It is called by MPI_Request_cancel(request). MPI
       passes to the callback function complete=true if MPI_Grequest_complete
       has already been called on the request, and complete=false otherwise.


FORTRAN 77 NOTES
       The MPI standard prescribes portable Fortran syntax for the EXTRA_STATE
       argument only for Fortran 90.  FORTRAN 77 users may use the non-
       portable syntax

            INTEGER*MPI_ADDRESS_KIND EXTRA_STATE

       where MPI_ADDRESS_KIND is a constant defined in mpif.h and gives the
       length of the declared integer in bytes.


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.

       All callback functions return an error code. The code is passed back
       and dealt with as appropriate for the error code by the MPI function
       that invoked the callback function. For example, if error codes are
       returned, then the error code returned by the callback function will be
       returned by the MPI function that invoked the callback function. In the
       case of a MPI_{Wait|Test}any call that invokes both query_fn and
       free_fn, the MPI call will return the error code returned by the last
       callback, namely free_fn. If one or more of the requests in a call to
       MPI_{Wait|Test}{some|all} has failed, then the MPI call will return
       MPI_ERR_IN_STATUS. In such a case, if the MPI call was passed an array
       of statuses, then MPI will return in each of the statuses that
       correspond to a completed generalized request the error code returned
       by the corresponding invocation of its free_fn callback function.
       However, if the MPI function was passed MPI_STATUSES_IGNORE, then the
       individual error codes returned by each callback function will be lost.

       See the MPI man page for a full list of MPI error codes.






1.4.1                            Jan 14, 2010            MPI_Grequest_start(3)
