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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.



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