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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 cancellation 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.