MPI_Grequest_start(3OpenMPI)											      MPI_Grequest_start(3OpenMPI)

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_com-
       plete  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 call-
       back. 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  deal-
       located 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  other-
       wise.

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++ func-
       tions  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.

Open MPI 1.2							  September 2006				      MPI_Grequest_start(3OpenMPI)