MPI_GREQUEST_START(3)				     Open MPI				     MPI_GREQUEST_START(3)

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
	  USE MPI
	  ! or the older form: 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

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Grequest_start(query_fn, free_fn, cancel_fn, extra_state, request,
		  ierror)
	      PROCEDURE(MPI_Grequest_query_function) :: query_fn
	      PROCEDURE(MPI_Grequest_free_function) :: free_fn
	      PROCEDURE(MPI_Grequest_cancel_function) :: cancel_fn
	      INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: extra_state
	      TYPE(MPI_Request), INTENT(OUT) :: request
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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

       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);

       And in Fortran, it is

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

       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.

ERRORS
       Almost  all MPI routines return an error value; C routines as the return result of the function and Fortran
       routines in the last argument.

       Before the error value is returned, the current MPI error handler associated with the communication  object
       (e.g.,  communicator, window, file) is called.  If no communication object is associated with the MPI call,
       then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler.  When
       MPI_COMM_SELF  is  not  initialized  (i.e., before MPI_Init <#mpi-init>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#mpi-finalize>, or when using the Sessions Model  exclusively)  the	error  raises  the
       initial	error  handler.	 The  initial  error  handler can be changed by calling MPI_Comm_set_errhandler <#
       mpi-comm-set-errhandler> on MPI_COMM_SELF when using the World model,  or  the  mpi_initial_errhandler  CLI
       argument	  to   mpiexec	 or   info  key	 to  MPI_Comm_spawn  <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.  If no other appropriate error handler has been set, then  the	 MPI_ERRORS_RETURN
       error  handler  is  called  for	MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or  session.  When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       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 request``s 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.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				     MPI_GREQUEST_START(3)
