## 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);
    gboolean            g_cancellable_make_pollfd           (GCancellable *cancellable,
                                                             GPollFD *pollfd);
    void                g_cancellable_release_fd            (GCancellable *cancellable);
    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);
    gulong              g_cancellable_connect               (GCancellable *cancellable,
                                                             GCallback callback,
                                                             gpointer data,
                                                             GDestroyNotify data_destroy_func);
    void                g_cancellable_disconnect            (GCancellable *cancellable,
                                                             gulong handler_id);
    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.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with `g_cancellable_reset()`.

After a successful return from this function, you should use `g_cancellable_release_fd()` to free up resources allocated for the returned file descriptor.

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 ()

    gboolean            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. This is useful both for unix systems without a native poll and for portability to windows.

When this function returns `TRUE`, you should use `g_cancellable_release_fd()` to free up resources allocated for the `pollfd`. After a `FALSE` return, do not call `g_cancellable_release_fd()`.

If this function returns `FALSE`, either no `cancellable` was given or resource limits prevent this function from allocating the necessary structures for polling. (On Linux, you will likely have reached the maximum number of file descriptors.) The suggested way to handle these cases is to ignore the `cancellable`.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with `g_cancellable_reset()`.

`cancellable` : | a GCancellable or `NULL`
`pollfd` : | a pointer to a GPollFD
Returns : | `TRUE` if `pollfd` was successfully initialized, `FALSE` on failure to prepare the cancellable.

Since 2.22

### g_cancellable_release_fd ()

    void                g_cancellable_release_fd            (GCancellable *cancellable);

Releases a resources previously allocated by `g_cancellable_get_fd()` or `g_cancellable_make_pollfd()`.

For compatibility reasons with older releases, calling this function is not strictly required, the resources will be automatically freed when the `cancellable` is finalized. However, the `cancellable` will block scarce file descriptors until it is finalized if this function is not called. This can cause the application to run out of file descriptors when many GCancellables are used at the same time.

`cancellable` : | a GCancellable

Since 2.22

### 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` : | a GCancellable object

### 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` : | a GCancellable object

### g_cancellable_reset ()

    void                g_cancellable_reset                 (GCancellable *cancellable);

Resets `cancellable` to its uncancelled state.

`cancellable` : | a GCancellable object.

### g_cancellable_connect ()

    gulong              g_cancellable_connect               (GCancellable *cancellable,
                                                             GCallback callback,
                                                             gpointer data,
                                                             GDestroyNotify data_destroy_func);

Convenience function to connect to the "cancelled" signal. Also handles the race condition that may happen if the cancellable is cancelled right before connecting.

`callback` is called at most once, either directly at the time of the connect if `cancellable` is already cancelled, or when `cancellable` is cancelled in some thread.

`data_destroy_func` will be called when the handler is disconnected, or immediately if the cancellable is already cancelled.

See "cancelled" for details on how to use this.

`cancellable` : | A GCancellable.
`callback` : | The GCallback to connect.
`data` : | Data to pass to `callback`.
`data_destroy_func` : | Free function for `data` or `NULL`.
Returns : | The id of the signal handler or 0 if `cancellable` has already been cancelled.

Since 2.22

### g_cancellable_disconnect ()

    void                g_cancellable_disconnect            (GCancellable *cancellable,
                                                             gulong handler_id);

Disconnects a handler from a cancellable instance similar to `g_signal_handler_disconnect()`. Additionally, in the event that a signal handler is currently running, this call will block until the handler has finished. Calling this function from a "cancelled" signal handler will therefore result in a deadlock.

This avoids a race condition where a thread cancels at the same time as the cancellable operation is finished and the signal handler is removed. See "cancelled" for details on how to use this.

If `cancellable` is `NULL` or `handler_id` is `0` this function does nothing.

`cancellable` : | A GCancellable or `NULL`.
`handler_id` : | Handler id of the handler to be disconnected, or `0`.

Since 2.22

### 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. For instance 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.

There is also a problem when cancellation happen right before connecting to the signal. If this happens the signal will unexpectedly not be emitted, and checking before connecting to the signal leaves a race condition where this is still happening.

In order to make it safe and easy to connect handlers there are two helper functions: `g_cancellable_connect()` and `g_cancellable_disconnect()` which protect against problems like this.

An example of how to us this:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20

    /* Make sure we don't do any unnecessary work if already cancelled */
    if (g_cancellable_set_error_if_cancelled (cancellable))
      return;
    /* Set up all the data needed to be able to
     * handle cancellation of the operation */
    my_data = my_data_new (...);

    id = 0;
    if (cancellable)
      id = g_cancellable_connect (cancellable,
                      G_CALLBACK (cancelled_handler)
                      data, NULL);

    /* cancellable operation here... */

    g_cancellable_disconnect (cancellable, id);

    /* cancelled_handler is never called after this, it
     * is now safe to free the data */
    my_data_free (my_data);

Note that the cancelled signal is emitted in the thread that the user cancelled from, which may be the main thread. So, the cancellable signal should not do something that can block.

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