# 18.2.182. MPI_Grequest_start

MPI_Grequest_start — Starts a generalized request and returns a handle to it in `request`.

## 18.2.182.1. SYNTAX

### 18.2.182.1.1. 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)

### 18.2.182.1.2. 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

### 18.2.182.1.3. 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

## 18.2.182.2. 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.

## 18.2.182.3. OUTPUT PARAMETERS

- `request` : Generalized request (handle).

- `ierror` : Fortran only: Error status (integer).

## 18.2.182.4. 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.

## 18.2.182.5. 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/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 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 by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

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-3.1 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.