## GCancellable

GCancellable — Thread-safe Operation Cancellation Stack

## Functions

GCancellable * | g_cancellable_new ()
gboolean | g_cancellable_is_cancelled ()
gboolean | g_cancellable_set_error_if_cancelled ()
int | g_cancellable_get_fd ()
gboolean | g_cancellable_make_pollfd ()
void | g_cancellable_release_fd ()
GSource * | g_cancellable_source_new ()
gboolean | (*GCancellableSourceFunc) ()
GCancellable * | g_cancellable_get_current ()
void | g_cancellable_pop_current ()
void | g_cancellable_push_current ()
void | g_cancellable_reset ()
gulong | g_cancellable_connect ()
void | g_cancellable_disconnect ()
void | g_cancellable_cancel ()

## Signals

void | cancelled | Run Last

## Types and Values

| GCancellable

## Object Hierarchy

        GObject
        ╰── GCancellable

## Includes

    #include <gio/gio.h>

## Description

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

## Functions

### 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 or 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.

#### Parameters

cancellable | a GCancellable or `NULL`. | [allow-none]

#### 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.

#### Parameters

cancellable | a GCancellable or `NULL`. | [allow-none]
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()`.

#### Parameters

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()`.

#### Parameters

cancellable | a GCancellable or `NULL`. | [allow-none]
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.

#### Parameters

cancellable | a GCancellable |

Since: 2.22

### g_cancellable_source_new ()

    GSource *
    g_cancellable_source_new (GCancellable *cancellable);

Creates a source that triggers if `cancellable` is cancelled and calls its callback of type GCancellableSourceFunc. This is primarily useful for attaching to another (non-cancellable) source with `g_source_add_child_source()` to add cancellability to it.

For convenience, you can call this with a `NULL` GCancellable, in which case the source will never trigger.

The new GSource will hold a reference to the GCancellable.

[skip]

#### Parameters

cancellable | a GCancellable, or `NULL`. | [allow-none]

#### Returns

the new GSource.

[transfer full]

Since: 2.28

### GCancellableSourceFunc ()

    gboolean
    (*GCancellableSourceFunc) (GCancellable *cancellable,
                               gpointer user_data);

This is the function type of the callback used for the GSource returned by `g_cancellable_source_new()`.

#### Parameters

cancellable | the GCancellable |
user_data | data passed in by the user. |

#### Returns

it should return `FALSE` if the source should be removed.

Since: 2.28

### 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.

[nullable][transfer none]

### 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).

#### Parameters

cancellable | a GCancellable object |

### g_cancellable_push_current ()

    void
    g_cancellable_push_current (GCancellable *cancellable);

Pushes `cancellable` onto the cancellable stack. The current cancellable can then be received 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.

#### Parameters

cancellable | a GCancellable object |

### g_cancellable_reset ()

    void
    g_cancellable_reset (GCancellable *cancellable);

Resets `cancellable` to its uncancelled state.

If cancellable is currently in use by any cancellable operation then the behavior of this function is undefined.

Note that it is generally not a good idea to reuse an existing cancellable for more operations after it has been cancelled once, as this function might tempt you to do. The recommended practice is to drop the reference to a cancellable after cancelling it, and let it die with the outstanding async operations. You should create a fresh cancellable for further async operations.

#### Parameters

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.

Since GLib 2.40, the lock protecting `cancellable` is not held when `callback` is invoked. This lifts a restriction in place for earlier GLib versions which now makes it easier to write cleanup code that unconditionally invokes e.g. `g_cancellable_cancel()`.

#### Parameters

cancellable | A GCancellable. |
callback | The GCallback to connect. |
data | Data to pass to `callback` . |
data_destroy_func | Free function for `data` or `NULL`. | [allow-none]

#### 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.

#### Parameters

cancellable | A GCancellable or `NULL`. | [allow-none]
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` .

If `cancellable` is `NULL`, this function returns immediately for convenience.

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.

#### Parameters

cancellable | a GCancellable object. | [nullable]

## Types and Values

### GCancellable

    typedef struct _GCancellable GCancellable;

Allows actions to be cancelled.

## Signal Details

### The `“cancelled”` signal

    void
    user_function (GCancellable *cancellable,
                   gpointer      user_data)

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 happens 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:

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    // Make sure we don't do unnecessary work if already cancelled
    if (g_cancellable_set_error_if_cancelled (cancellable, error))
      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.

#### Parameters

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

Flags: Run Last