Qt World Summit: Submit your Presentation

QXcbConnection: XCB error (QWarning)

  • I recently ported a Qt4 application to Qt5.4.1 (also tested Qt5.5.0 beta). I've experienced a handful of odd behavior since then. One of the problems involves the xcb platform plugin on Linux (mainly RHEL 6.6) platforms. Namely, on some platforms I receive a host of qwarnings related to xcb errors:

    QXcbConnection: XCB error: 2 (BadValue), sequence: 888, resource id: 2562, major code: 130 (Unknown), minor code: 3
    QXcbConnection: XCB error: 147 (Unknown), sequence: 161, resource id: 0, major code 143 (Unkown), minor code: 20

    Interestingly, I do not see any negative effects despite the warnings.

    Some of these errors are issued on launch. Others occur simply when moving the main window around. These issues seem to multiply when running over X11 forwarding or when running in high-dpi mode (QT_DEVICE_PIXEL_RATIO=2). Conversely, I've run on some Linux machines that do not display any warnings at all. Furthermore, each machine I've tested on is running from the same build of my application.

    Should I be concerned with these warnings, and if so, does anyone have any ideas how I might debug this? Has anyone run into a solution for these warnings?

    I'd be glad to answer any more information if it will help to diagnose the problem. Thanks for your help!

  • Compiled and rebuilt with Qt5.5.0 (beta).

    Interestingly, the warnings I received when running over VNC or when in high-dpi mode (QT_DEVICE_PIXEL_RATIO=2) no longer appear with Qt5.5.0. So, this is a definite improvement.

    However, I still receive the 147 (Unknown) error on launch when running over X11 forwarding.