pub trait CancellableExt: 'static {
    fn cancel(&self);
    fn disconnect(&self, handler_id: c_ulong);
    fn fd(&self) -> i32;
    fn is_cancelled(&self) -> bool;
    fn pop_current(&self);
    fn push_current(&self);
    fn release_fd(&self);
    fn set_error_if_cancelled(&self) -> Result<(), Error>;
    fn connect_cancelled<F: Fn(&Self) + Send + Sync + 'static>(
        f: F
    ) -> SignalHandlerId; }
Expand description

Trait containing all Cancellable methods.



Required Methods

Will set self to cancelled, and will emit the signal::Cancellable::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 self.

If self is None, 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.

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 signal::Cancellable::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 signal::Cancellable::cancelled for details on how to use this.

If self is None or handler_id is 0 this function does nothing.


Handler id of the handler to be disconnected, or 0.

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 self 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 release_fd() to free up resources allocated for the returned file descriptor.

See also g_cancellable_make_pollfd().


A valid file descriptor. -1 if the file descriptor is not supported, or on errors.

Checks if a cancellable job has been cancelled.


true if self is cancelled, FALSE if called with None or if item is not cancelled.

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

Pushes self onto the cancellable stack. The current cancellable can then be received using Cancellable::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. File operations, so you rarely have to call this yourself.

Releases a resources previously allocated by 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 self is finalized. However, the self 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.

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


true if self was cancelled, false if it was not

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 disconnect() which protect against problems like this.

An example of how to us this:

⚠️ The following code is in C ⚠️

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

    // 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.