(Linux) qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.

[bb00000001]

I'm trying to run XP-Pen pentablet drivers on my system-- they launch just fine under my normal user, but because they are drivers I need to run them as root, which gives me the following error:

sleepy@magi ~/Programs/Pentablet/Linux_Pentablet_V1.2.13.1 $ sudo QT_DEBUG_PLUGINS=1 ./Pentablet_Driver
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib64/qt5/plugins/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/KWinQpaPlugin.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/KWinQpaPlugin.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-org.kde.kwin.qpa"
        ]
    },
    "archreq": 2,
    "className": "KWinIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-org.kde.kwin.qpa")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqminimal.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqminimal.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "minimal"
        ]
    },
    "archreq": 2,
    "className": "QMinimalIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("minimal")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqoffscreen.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqoffscreen.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "offscreen"
        ]
    },
    "archreq": 2,
    "className": "QOffscreenIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("offscreen")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqwayland-egl.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqwayland-egl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-egl"
        ]
    },
    "archreq": 2,
    "className": "QWaylandEglPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-egl")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqwayland-generic.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqwayland-generic.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland"
        ]
    },
    "archreq": 2,
    "className": "QWaylandIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqwayland-xcomposite-egl.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqwayland-xcomposite-egl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-xcomposite-egl"
        ]
    },
    "archreq": 2,
    "className": "QWaylandXCompositeEglPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-xcomposite-egl")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqwayland-xcomposite-glx.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqwayland-xcomposite-glx.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-xcomposite-glx"
        ]
    },
    "archreq": 2,
    "className": "QWaylandXCompositeGlxPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-xcomposite-glx")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib64/qt5/plugins/platforms/libqxcb.so"
Found metadata in lib /usr/lib64/qt5/plugins/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "archreq": 2,
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/home/sleepy/Programs/Pentablet/Linux_Pentablet_V1.2.13.1/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/home/sleepy/Programs/Pentablet/Linux_Pentablet_V1.2.13.1/platforms/libqxcb.so"
Found metadata in lib /home/sleepy/Programs/Pentablet/Linux_Pentablet_V1.2.13.1/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 330241
}


Got keys from plugin meta data ("xcb")
loaded library "/usr/lib64/qt5/plugins/platforms/libqxcb.so"
No protocol specified
qt.qpa.xcb: could not connect to display :0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: wayland-org.kde.kwin.qpa, minimal, offscreen, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Aborted

I'm using Gentoo and I have xcb installed:

sleepy@magi ~/Programs/Pentablet/Linux_Pentablet_V1.2.13.1 $ eix xcb
* dev-perl/X11-XCB
     Available versions:  0.170.0-r1^t {test}
     Homepage:            https://metacpan.org/release/X11-XCB
     Description:         Perl bindings for libxcb

[I] x11-base/xcb-proto
     Available versions:  1.14-r1 {ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32" PYTHON_TARGETS="python3_6 python3_7 python3_8"}
     Installed versions:  1.14-r1(08:32:13 PM 05/08/2020)(ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32" PYTHON_TARGETS="python3_7 python3_8 -python3_6")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/proto/xcbproto
     Description:         X C-language Bindings protocol headers

[I] x11-libs/libxcb
     Available versions:  1.14(0/1.12)^t {doc selinux static-libs test +xkb ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  1.14(0/1.12)^t(08:32:42 PM 05/08/2020)(xkb -doc -selinux -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb
     Description:         X C-language Bindings library

[I] x11-libs/xcb-util
     Available versions:  0.4.0-r1^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.4.0-r1^t(08:07:25 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-util
     Description:         X C-language Bindings sample implementations

[I] x11-libs/xcb-util-cursor
     Available versions:  0.1.3-r2^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.1.3-r2^t(08:07:58 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-cursor
     Description:         X C-language Bindings sample implementations

[I] x11-libs/xcb-util-image
     Available versions:  0.4.0-r1^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.4.0-r1^t(08:07:41 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-image
     Description:         X C-language Bindings sample implementations

[I] x11-libs/xcb-util-keysyms
     Available versions:  0.4.0-r1^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.4.0-r1^t(08:03:51 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-keysyms
     Description:         X C-language Bindings sample implementations

[I] x11-libs/xcb-util-renderutil
     Available versions:  0.3.9-r2^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.3.9-r2^t(08:04:24 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-render-util
     Description:         X C-language Bindings sample implementations

[I] x11-libs/xcb-util-wm
     Available versions:  0.4.1-r2^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Installed versions:  0.4.1-r2^t(08:04:09 AM 05/22/2020)(-doc -static-libs -test ABI_MIPS="-n32 -n64 -o32" ABI_RISCV="-lp64 -lp64d" ABI_S390="-32 -64" ABI_X86="32 64 -x32")
     Homepage:            https://xcb.freedesktop.org/ https://gitlab.freedesktop.org/xorg/lib/libxcb-wm
     Description:         X C-language Bindings sample implementations

* x11-libs/xcb-util-xrm
     Available versions:  1.3^t {doc static-libs test ABI_MIPS="n32 n64 o32" ABI_RISCV="lp64 lp64d" ABI_S390="32 64" ABI_X86="32 64 x32"}
     Homepage:            https://xcb.freedesktop.org/
     Description:         X C-language Bindings sample implementations

[I] x11-misc/xcb
     Available versions:  2.4-r1 {motif}
     Installed versions:  2.4-r1(05:04:39 PM 08/29/2020)(-motif)
     Homepage:            http://oldhome.schmorp.de/marc/xcb.html
     Description:         Marc Lehmann's improved X Cut Buffers

Found 11 matches

Running the application under polkit does not fix the issue. How can I fix this?

[hskoglund]

Hi, I think this is not a really a Qt question, because you get the error "No protocol specified, qt.qpa.xcb: could not connect to display :0"

Try copying your .Xauthority file into /root, say like this:
sudo cp ~/.Xauthority /root

Otherwise, there are lots of hits googling that error, here is one that might help:
https://unix.stackexchange.com/questions/118811/why-cant-i-run-gui-apps-from-root-no-protocol-specified

[bb00000001]

@hskoglund said in (Linux) qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.:

Hi, I think this is not a really a Qt question, because you get the error "No protocol specified, qt.qpa.xcb: could not connect to display :0"

Try copying your .Xauthority file into /root, say like this:
sudo cp ~/.Xauthority /root

Otherwise, there are lots of hits googling that error, here is one that might help:
https://unix.stackexchange.com/questions/118811/why-cant-i-run-gui-apps-from-root-no-protocol-specified

Ah yup, that fixed it. Thank you!