## GCancellable

GCancellable — Thread-safe Operation Cancellation Stack

## Synopsis

    #include <gio/gio.h>

                        GCancellable;
    GCancellable *      g_cancellable_new                   (void);
    gboolean            g_cancellable_is_cancelled          (GCancellable *cancellable);
    gboolean            g_cancellable_set_error_if_cancelled
                                                            (GCancellable *cancellable,
                                                             GError **error);
    int                 g_cancellable_get_fd                (GCancellable *cancellable);
    void                g_cancellable_make_pollfd           (GCancellable *cancellable,
                                                             GPollFD *pollfd);
    GCancellable *      g_cancellable_get_current           (void);
    void                g_cancellable_pop_current           (GCancellable *cancellable);
    void                g_cancellable_push_current          (GCancellable *cancellable);
    void                g_cancellable_reset                 (GCancellable *cancellable);
    void                g_cancellable_cancel                (GCancellable *cancellable);

## Object Hierarchy

      GObject
       +----GCancellable

## Signals

      "cancelled"                                      : Run Last

## Description

GCancellable is a thread-safe operation cancellation stack used throughout GIO to allow for cancellation of synchronous and asynchronous operations.

## Details

### GCancellable

    typedef struct _GCancellable GCancellable;

Allows actions to be cancelled.

### g_cancellable_new ()

    GCancellable *      g_cancellable_new                   (void);

Creates a new GCancellable object.

Applications that want to start one or more operations that should be cancellable should create a GCancellable and pass it to the operations.

One GCancellable can be used in multiple consecutive operations, but not in multiple concurrent operations.

Returns : | a GCancellable.

### g_cancellable_is_cancelled ()

    gboolean            g_cancellable_is_cancelled          (GCancellable *cancellable);

Checks if a cancellable job has been cancelled.

`cancellable` : | a GCancellable or NULL.
Returns : | `TRUE` if `cancellable` is cancelled, FALSE if called with `NULL` or if item is not cancelled.

### g_cancellable_set_error_if_cancelled ()

    gboolean            g_cancellable_set_error_if_cancelled
                                                            (GCancellable *cancellable,
                                                             GError **error);

If the `cancellable` is cancelled, sets the error to notify that the operation was cancelled.

`cancellable` : | a GCancellable object.
`error` : | GError to append error state to.
Returns : | `TRUE` if `cancellable` was cancelled, `FALSE` if it was not.

### g_cancellable_get_fd ()

    int                 g_cancellable_get_fd                (GCancellable *cancellable);

Gets the file descriptor for a cancellable job. This can be used to implement cancellable operations on Unix systems. The returned fd will turn readable when `cancellable` is cancelled.

See also `g_cancellable_make_pollfd()`.

`cancellable` : | a GCancellable.
Returns : | A valid file descriptor. `-1` if the file descriptor is not supported, or on errors.

### g_cancellable_make_pollfd ()

    void                g_cancellable_make_pollfd           (GCancellable *cancellable,
                                                             GPollFD *pollfd);

Creates a GPollFD corresponding to `cancellable`; this can be passed to `g_poll()` and used to poll for cancellation.

`cancellable` : | a GCancellable.
`pollfd` : | a pointer to a GPollFD

### g_cancellable_get_current ()

    GCancellable *      g_cancellable_get_current           (void);

Gets the top cancellable from the stack.

Returns : | a GCancellable from the top of the stack, or `NULL` if the stack is empty.

### g_cancellable_pop_current ()

    void                g_cancellable_pop_current           (GCancellable *cancellable);

Pops `cancellable` off the cancellable stack (verifying that `cancellable` is on the top of the stack).

`cancellable` : | optional GCancellable object, `NULL` to ignore.

### g_cancellable_push_current ()

    void                g_cancellable_push_current          (GCancellable *cancellable);

Pushes `cancellable` onto the cancellable stack. The current cancllable can then be recieved using `g_cancellable_get_current()`.

This is useful when implementing cancellable operations in code that does not allow you to pass down the cancellable object.

This is typically called automatically by e.g. GFile operations, so you rarely have to call this yourself.

`cancellable` : | optional GCancellable object, `NULL` to ignore.

### g_cancellable_reset ()

    void                g_cancellable_reset                 (GCancellable *cancellable);

Resets `cancellable` to its uncancelled state.

`cancellable` : | a GCancellable object.

### g_cancellable_cancel ()

    void                g_cancellable_cancel                (GCancellable *cancellable);

Will set `cancellable` to cancelled, and will emit the "cancelled" signal. (However, see the warning about race conditions in the documentation for that signal if you are planning to connect to it.)

This function is thread-safe. In other words, you can safely call it from a thread other than the one running the operation that was passed the `cancellable`.

The convention within gio is that cancelling an asynchronous operation causes it to complete asynchronously. That is, if you cancel the operation from the same thread in which it is running, then the operation's GAsyncReadyCallback will not be invoked until the application returns to the main loop.

`cancellable` : | a GCancellable object.

## Signal Details

### The `"cancelled"` signal

    void                user_function                      (GCancellable *cancellable,
                                                            gpointer      user_data)        : Run Last

Emitted when the operation has been cancelled.

Can be used by implementations of cancellable operations. If the operation is cancelled from another thread, the signal will be emitted in the thread that cancelled the operation, not the thread that is running the operation.

Note that disconnecting from this signal (or any signal) in a multi-threaded program is prone to race conditions, and it is possible that a signal handler may be invoked even after a call to `g_signal_handler_disconnect()` for that handler has already returned. Therefore, code such as the following is wrong in a multi-threaded program:

        my_data = my_data_new (...);
        id = g_signal_connect (cancellable, "cancelled",
                               G_CALLBACK (cancelled_handler), my_data);
        /* cancellable operation here... */

        g_signal_handler_disconnect (cancellable, id);
        my_data_free (my_data);  /* WRONG! */
        /* if g_cancellable_cancel() is called from another
         * thread, cancelled_handler() may be running at this point,
         * so it's not safe to free my_data.
         */

The correct way to free data (or otherwise clean up temporary state) in this situation is to use `g_signal_connect_data()` (or `g_signal_connect_closure()`) to connect to the signal, and do the cleanup from a GClosureNotify, which will not be called until after the signal handler is both removed and not running:

    static void
    cancelled_disconnect_notify (gpointer my_data, GClosure *closure)
    {
      my_data_free (my_data);
    }

    ...

        my_data = my_data_new (...);
        id = g_signal_connect_data (cancellable, "cancelled",
                                    G_CALLBACK (cancelled_handler), my_data,
                                    cancelled_disconnect_notify, 0);

        /* cancellable operation here... */

        g_signal_handler_disconnect (cancellable, id);
        /* cancelled_disconnect_notify() may or may not have
         * already been called at this point, so the code has to treat
         * my_data as though it has been freed.
         */

`cancellable` : | a GCancellable.
`user_data` : | user data set when the signal handler was connected.