GCancellable

GCancellable — Thread-safe Operation Cancellation Stack

Signals

Types and Values

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.

[nullable]

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.

[nullable]

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.

[nullable]

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.

[nullable]

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.

[nullable]

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.

[nullable]

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