qt.qpa.plugin: Could not load the Qt platform plugin "xcb"

Puddies · 22 Jun 2021, 10:31

When starting my python application using qt, it instantly crashes with the following message.

QObject::moveToThread: Current thread (0x55f3a9a0e230) is not the object's thread (0x55f3a9c9a160).
Cannot move to target thread (0x55f3a9a0e230)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/...<username>.../.local/lib/python3.9/site-packages/cv2/qt/plugins" 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: xcb, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx.

I am on manjaro linux.
I already reinstalled libxcb and qt5-base, which did not help.

Any ideas how to fix this?

jsulm · wrote on 22 Jun 2021, 10:31

@Puddies Set QT_DEBUG_PLUGINS before starting your app and check the output, or post it here.

Puddies · 22 Jun 2021, 10:45

The output has 15543 lines. I am not allowed to post a text with that length here. Can I append a text file here?

The output starts with

QFactoryLoader::QFactoryLoader() checking directory path "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins" ...
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/qt/plugins" ...
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/ownclouddolphinactionplugin.so"
Found metadata in lib /usr/lib/qt/plugins/ownclouddolphinactionplugin.so, metadata=
{
    "IID": "org.kde.KPluginFactory",
    "archreq": 0,
    "className": "OwncloudDolphinPluginActionFactory",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ()
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/owncloudsync_vfs_suffix.so"
Found metadata in lib /usr/lib/qt/plugins/owncloudsync_vfs_suffix.so, metadata=
{
    "IID": "org.owncloud.PluginFactory",
    "MetaData": {
        "type": "vfs",
        "version": "2.8.2"
    },
    "archreq": 0,
    "className": "SuffixVfsPluginFactory",
    "debug": false,
    "version": 331520
}

and it ends with

QFactoryLoader::QFactoryLoader() checking directory path "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins" ...
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/qt/plugins" ...
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/ownclouddolphinactionplugin.so"
Found metadata in lib /usr/lib/qt/plugins/ownclouddolphinactionplugin.so, metadata=
{
    "IID": "org.kde.KPluginFactory",
    "archreq": 0,
    "className": "OwncloudDolphinPluginActionFactory",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ()
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/owncloudsync_vfs_suffix.so"
Found metadata in lib /usr/lib/qt/plugins/owncloudsync_vfs_suffix.so, metadata=
{
    "IID": "org.owncloud.PluginFactory",
    "MetaData": {
        "type": "vfs",
        "version": "2.8.2"
    },
    "archreq": 0,
    "className": "SuffixVfsPluginFactory",
    "debug": false,
    "version": 331520
}

I dont know, if that helps.

jsulm · wrote on 22 Jun 2021, 10:45

@Puddies said in qt.qpa.plugin: Could not load the Qt platform plugin "xcb":

I dont know, if that helps.

It does not. Please post whole output or at least the part where xcb platform plug-in is loaded...

Puddies · wrote on 22 Jun 2021, 10:57

I am very sorry, but I had to upload the output to my dropbox. I am not able to post a text with more than about 32000 characters here.

Here is the link with the full output:
https://www.dropbox.com/s/k8eahgu3neesxe6/output.txt?dl=0

Puddies · 22 Jun 2021, 11:19

Ok, I have found the xcb part. Maybe this helps a bit:

QFactoryLoader::QFactoryLoader() checking directory path "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so"
Found metadata in lib /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "archreq": 0,
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/usr/lib/qt/plugins/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqeglfs.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqeglfs.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "eglfs"
        ]
    },
    "archreq": 0,
    "className": "QEglFSIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("eglfs")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqlinuxfb.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqlinuxfb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "linuxfb"
        ]
    },
    "archreq": 0,
    "className": "QLinuxFbIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("linuxfb")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqminimal.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqminimal.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "minimal"
        ]
    },
    "archreq": 0,
    "className": "QMinimalIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("minimal")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqminimalegl.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqminimalegl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "minimalegl"
        ]
    },
    "archreq": 0,
    "className": "QMinimalEglIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("minimalegl")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqoffscreen.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqoffscreen.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "offscreen"
        ]
    },
    "archreq": 0,
    "className": "QOffscreenIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("offscreen")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqvnc.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqvnc.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "vnc"
        ]
    },
    "archreq": 0,
    "className": "QVncIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("vnc")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqwayland-egl.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqwayland-egl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-egl"
        ]
    },
    "archreq": 0,
    "className": "QWaylandEglPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-egl")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqwayland-generic.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqwayland-generic.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland"
        ]
    },
    "archreq": 0,
    "className": "QWaylandIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqwayland-xcomposite-egl.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqwayland-xcomposite-egl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-xcomposite-egl"
        ]
    },
    "archreq": 0,
    "className": "QWaylandXCompositeEglPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-xcomposite-egl")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqwayland-xcomposite-glx.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqwayland-xcomposite-glx.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "wayland-xcomposite-glx"
        ]
    },
    "archreq": 0,
    "className": "QWaylandXCompositeGlxPlatformIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("wayland-xcomposite-glx")
QFactoryLoader::QFactoryLoader() looking at "/usr/lib/qt/plugins/platforms/libqxcb.so"
Found metadata in lib /usr/lib/qt/plugins/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "archreq": 0,
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 331520
}


Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/usr/bin/platforms" ...
loaded library "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so"
QObject::moveToThread: Current thread (0x55871b6fa2c0) is not the object's thread (0x55871b6bee30).
Cannot move to target thread (0x55871b6fa2c0)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins" 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: xcb, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx.

JonB · 22 Jun 2021, 11:30

@Puddies
Try

ldd /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so

Any "not found"s?

Puddies · 22 Jun 2021, 11:35

@JonB

ldd: Warnung: Sie haben kein Ausführungsrecht für `/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so'
	linux-vdso.so.1 (0x00007ffe5aac9000)
	libQt5XcbQpa-70670cdb.so.5.15.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libQt5XcbQpa-70670cdb.so.5.15.0 (0x00007f53138ac000)
	libfreetype-8d3bcff4.so.6.14.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libfreetype-8d3bcff4.so.6.14.0 (0x00007f53135dc000)
	libz-d8a329de.so.1.2.7 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libz-d8a329de.so.1.2.7 (0x00007f53133c5000)
	libQt5Gui-ca084835.so.5.15.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libQt5Gui-ca084835.so.5.15.0 (0x00007f5312a2e000)
	libQt5Core-d92f2856.so.5.15.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libQt5Core-d92f2856.so.5.15.0 (0x00007f53122d4000)
	libGL.so.1 => /usr/lib/libGL.so.1 (0x00007f531224e000)
	libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f531222d000)
	libX11-xcb-1ae4fd0e.so.1.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libX11-xcb-1ae4fd0e.so.1.0.0 (0x00007f531202a000)
	libxcb-icccm-413c9f41.so.4.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-icccm-413c9f41.so.4.0.0 (0x00007f5311e24000)
	libxcb-image-e82a276d.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-image-e82a276d.so.0.0.0 (0x00007f5311c1a000)
	libxcb-shm-7a199f70.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-shm-7a199f70.so.0.0.0 (0x00007f5311a14000)
	libxcb-keysyms-21015570.so.1.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-keysyms-21015570.so.1.0.0 (0x00007f5311810000)
	libxcb-randr-a96a5a87.so.0.1.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-randr-a96a5a87.so.0.1.0 (0x00007f53115f9000)
	libxcb-render-util-43ce00f5.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-render-util-43ce00f5.so.0.0.0 (0x00007f53113f3000)
	libxcb-render-637b984a.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-render-637b984a.so.0.0.0 (0x00007f53111dd000)
	libxcb-shape-25c2b258.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-shape-25c2b258.so.0.0.0 (0x00007f5310fd7000)
	libxcb-sync-89374f40.so.1.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-sync-89374f40.so.1.0.0 (0x00007f5310dce000)
	libxcb-xfixes-9be3ba6f.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-xfixes-9be3ba6f.so.0.0.0 (0x00007f5310bc2000)
	libxcb-xinerama-ae147f87.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-xinerama-ae147f87.so.0.0.0 (0x00007f53109bd000)
	libxcb-xkb-9ba31ab3.so.1.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-xkb-9ba31ab3.so.1.0.0 (0x00007f5310794000)
	libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007f531076a000)
	libXext.so.6 => /usr/lib/libXext.so.6 (0x00007f5310755000)
	libX11.so.6 => /usr/lib/libX11.so.6 (0x00007f5310614000)
	libSM.so.6 => /usr/lib/libSM.so.6 (0x00007f531060a000)
	libICE.so.6 => /usr/lib/libICE.so.6 (0x00007f53105ed000)
	libxkbcommon-x11-c65ed502.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxkbcommon-x11-c65ed502.so.0.0.0 (0x00007f53103de000)
	libxkbcommon-71ae2972.so.0.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxkbcommon-71ae2972.so.0.0.0 (0x00007f531019d000)
	libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f5310196000)
	libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00007f530ff80000)
	libm.so.6 => /usr/lib/libm.so.6 (0x00007f530fe3c000)
	libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f530fe21000)
	libc.so.6 => /usr/lib/libc.so.6 (0x00007f530fc53000)
	libgthread-2.0.so.0 => /usr/lib/libgthread-2.0.so.0 (0x00007f530fc4e000)
	libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0x00007f530fb19000)
	libbz2-a273e504.so.1.0.6 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libbz2-a273e504.so.1.0.6 (0x00007f530f908000)
	libpng15-c2ffaf3d.so.15.13.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libpng15-c2ffaf3d.so.15.13.0 (0x00007f530f6d9000)
	/usr/lib64/ld-linux-x86-64.so.2 (0x00007f5313b5f000)
	libGLdispatch.so.0 => /usr/lib/libGLdispatch.so.0 (0x00007f530f61f000)
	libGLX.so.0 => /usr/lib/libGLX.so.0 (0x00007f530f5ec000)
	libxcb-util-4d666913.so.1.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libxcb-util-4d666913.so.1.0.0 (0x00007f530f3e5000)
	libXau-00ec42fe.so.6.0.0 => /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/../../../../opencv_python.libs/libXau-00ec42fe.so.6.0.0 (0x00007f530f1e0000)
	libXau.so.6 => /usr/lib/libXau.so.6 (0x00007f530f1d9000)
	libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007f530f1d1000)
	libuuid.so.1 => /usr/lib/libuuid.so.1 (0x00007f530f1c8000)
	libpcre.so.1 => /usr/lib/libpcre.so.1 (0x00007f530f156000)

JonB · 22 Jun 2021, 12:38

@Puddies said in qt.qpa.plugin: Could not load the Qt platform plugin "xcb":

ldd: Warnung: Sie haben kein Ausführungsrecht für /home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so'`

The dependencies look OK, but never seen this warning before. What does ls -l on it tell you?

Having said that, it did report

loaded library "/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so"

I would be looking at the

QObject::moveToThread: Current thread (0x55871b6fa2c0) is not the object's thread (0x55871b6bee30).
Cannot move to target thread (0x55871b6fa2c0)

You should not be getting this. Are you playing with threads somewhere?

Pl45m4 · wrote on 22 Jun 2021, 11:56

@Puddies said in qt.qpa.plugin: Could not load the Qt platform plugin "xcb":

ldd: Warnung: Sie haben kein Ausführungsrecht für `/home/<username>/.local/lib/python3.9/site-packages/cv2/qt/plugins/platforms/libqxcb.so'

Check your permissons on this directory. It expects execution permission. Seems strange.

@JonB It says "ldd: warning: you do not have execution permission for...."

Puddies · 22 Jun 2021, 13:01

@JonB
ls -l returns
-rw-r--r-- 1 <username> <group> 104K 10. Jun 13:34 libqxcb.so

I set execution permissions and the warning disappeared. However, this did not resolve the crash of my application.

I dont use any threads in my code. However, I dont also use QT directly, I use matplotlib for the gui (which uses qt).
But I did not explicitely use threads here.

JonB · wrote on 22 Jun 2021, 13:01

@Puddies
Normally under Linux .so files are marked executable, so would be -rwxr-xr-x. That will be the cause of the warning message. But it doesn't actually matter, since it says it loaded it.

I still think you have a threading issue as per the message, I don't know if that is what is causing your problem.

JonB · wrote on 23 Jun 2021, 09:06

Thread https://forum.qt.io/topic/127832/pyqt5-and-tensorflowlite-causes-crashes/3 shows exactly the same output and issue. If someone can answer one it answers the other....

bimhox · wrote on 6 Mar 2022, 21:00

Was someone successful in fixing it? worst case scenario, should we disintall and reinstall what application? qt ? cv2? python?

jerryhon · 4 Dec 2022, 17:40

This post is deleted!