17.2.181. MPI_Grequest_start

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)

! or the older form: INCLUDE 'mpif.h'


USE mpi_f08

MPI_Grequest_start(query_fn, free_fn, cancel_fn, extra_state, request,
    PROCEDURE(MPI_Grequest_query_function) :: query_fn
    PROCEDURE(MPI_Grequest_free_function) :: free_fn
    PROCEDURE(MPI_Grequest_cancel_function) :: cancel_fn
    TYPE(MPI_Request), INTENT(OUT) :: request

  • 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


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


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


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


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 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_THREAD, after 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 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[_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.

In the sessions model, the error handler can be set during MPI_Session_init.

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 was called on that communicator. If
                      called on a window or file, acts as if 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.

Custom MPI error handlers can be created by calling: MPI_Comm_create_errhandler(3) MPI_File_create_errhandler(3) MPI_Session_create_errhandler(3) MPI_Win_create_errhandler(3)

Predefined and custom error handlers can be set by calling: MPI_Comm_set_errhandler(3) MPI_File_set_errhandler(3) MPI_Session_set_errhandler(3) MPI_Win_set_errhandler(3)

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

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

See the Error Handling section of the MPI-3 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 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.