How do I interrupt xcb_wait_for_event?

Question

In a separate thread (std::thread), I have an event loop that waits on xcb_wait_for_event. When the program exits, I'd like to shut things down nicely by interrupting (I have a solution that sets a thread-local variable, and checkpoints in the loop throw an exception), and then joining my event thread into the main thread. The issue is xcb_wait_for_event; I need a way to return from it early, or I need an alternative to the function.

Can anyone suggest a solution? Thanks for your help!

Answer 1

A neater way would be to do something like this (the code snippet is extracted from some code I am currently working on):

void QXcbEventQueue::sendCloseConnectionEvent() const {
    // A hack to close XCB connection. Apparently XCB does not have any APIs for this?
    xcb_client_message_event_t event;
    memset(&event, 0, sizeof(event));

    event.response_type = XCB_CLIENT_MESSAGE;
    event.format = 32;
    event.sequence = 0;
    event.window = m_connection->clientLeader();
    event.type = m_connection->atom(QXcbAtom::_QT_CLOSE_CONNECTION);
    event.data.data32[0] = 0;

    xcb_connection_t *c = m_connection->xcb_connection();
    xcb_send_event(c, false, m_connection->clientLeader(),
                   XCB_EVENT_MASK_NO_EVENT, reinterpret_cast<const char *>(&event));
    xcb_flush(c); }

For _QT_CLOSE_CONNECTION use your own atom to signal an exit and in my case clientLeader() is some invisible window that is always present on my X11 connection. If you don't have any invisible windows that could be reused for this purpose, create one :)

With this you can terminate the thread with xcb_wait_for_event when you see this special event arriving.