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

CuriousPan · 17 Jun 2021, 16:36

Hello eveyone!
I know that there is a great amount of answers regarding this topic, however none of them helped me. I'm using Ubunty 20.04 via Oracle VM Virtual Box and I faced exactly the same error as all other people.
After building Qt6 source code using official Qt manual I'm trying to run different projects to test if everything works properly. Console-type projects build and run properly. There are no errors. Unfortunately, when I try to run Widget-type application it fails with well-known error.

t.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: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

My collegue, who has Qt6 working properly on his Ubuntu, saw that I don't have libqxcb.so in the .../qtbase/plugins/platforms and shared it with me. Unfortunately, it didn't help much. Only the error message has changed a bit: it says that plugin was found, but couldn't be used.
I tried different solutions from Internet, but none of them helped. One of the most popular

sudo apt install libxcb-icccm4 libxcb-image0 libxcb-keysyms1 libxcb-render-util0

had no impact.
What can be the problem? Thank you in advance.

JonB · 17 Jun 2021, 19:01

@CuriousPan
There are many posts about this on this forum where people advise what to install, you could search. I think the most likely candidate you need to apt get is libxinerama, though it might be different for Qt6. And/or maybe libxcb-util?

CuriousPan · J JonB 17 Jun 2021, 16:36

@JonB, I tried, but it turned out to be already installed, so no effect, unfortunately.

SGaist · 18 Jun 2021, 00:38

Hi,

Then start your application with the QT_DEBUG_PLUGINS environment variable set to 1 so you'll have more information about why it's failing to load.

CuriousPan · 18 Jun 2021, 06:15

@SGaist Hello.
Here's the more verbose output I got:

QFactoryLoader::QFactoryLoader() checking directory path "/home/curiouspan/qt6-build/qtbase/plugins/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqeglfs.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqeglfs.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "eglfs"
        ]
    },
    "archreq": 1,
    "className": "QEglFSIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("eglfs")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqlinuxfb.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqlinuxfb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "linuxfb"
        ]
    },
    "archreq": 1,
    "className": "QLinuxFbIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("linuxfb")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqminimal.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqminimal.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "minimal"
        ]
    },
    "archreq": 1,
    "className": "QMinimalIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("minimal")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqminimalegl.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqminimalegl.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "minimalegl"
        ]
    },
    "archreq": 1,
    "className": "QMinimalEglIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("minimalegl")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqoffscreen.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqoffscreen.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "offscreen"
        ]
    },
    "archreq": 1,
    "className": "QOffscreenIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("offscreen")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqvnc.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqvnc.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "vnc"
        ]
    },
    "archreq": 1,
    "className": "QVncIntegrationPlugin",
    "debug": true,
    "version": 393728
}


Got keys from plugin meta data QList("vnc")
QFactoryLoader::QFactoryLoader() looking at "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so"
Found metadata in lib /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so, metadata=
{
    "IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
    "MetaData": {
        "Keys": [
            "xcb"
        ]
    },
    "archreq": 0,
    "className": "QXcbIntegrationPlugin",
    "debug": false,
    "version": 393728
}


Got keys from plugin meta data QList("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/home/curiouspan/QtTestProjects/build-WidgetTestApp-Own_Desktop-Debug/platforms" ...
Cannot load library /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so: (libQt6XcbQpa.so.6: cannot open shared object file: No such file or directory)
QLibraryPrivate::loadPlugin failed on "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so" : "Cannot load library /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so: (libQt6XcbQpa.so.6: cannot open shared object file: No such file or directory)"
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: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

jsulm · 18 Jun 2021, 06:26

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

QLibraryPrivate::loadPlugin failed on "/home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so"

Looks like the XCB platform plug-in is missing (libqxcb.so)

JKSH · 18 Jun 2021, 08:44

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

Looks like the XCB platform plug-in is missing (libqxcb.so)

Or, one of its dependencies is missing.

Run ldd /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so
Look for libraries that are marked "not found"
Install those missing libraries.

hskoglund · 18 Jun 2021, 06:30

Hi, or rather, if you scroll to the right, it looks like libQt6XcbQpa.so.6 is missing:
"... Cannot load library /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so: (libQt6XcbQpa.so.6: cannot open shared object file: No such file or directory)"

JKSH · 18 Jun 2021, 08:34

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

Hi, or rather, if you scroll to the right, it looks like libQt6XcbQpa.so.6 is missing:
"... Cannot load library /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so: (libQt6XcbQpa.so.6: cannot open shared object file: No such file or directory)"

Good spot.

@CuriousPan, you say you built Qt from source. Check the configure summary and see if the XCB plugin was enabled.

CuriousPan · 18 Jun 2021, 08:42

@JKSH How can I do it?

JKSH · 18 Jun 2021, 13:45

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

@JKSH How can I do it?

When you ran the configure script at the start, it should have produced some log files. Have a look at the logs.

CuriousPan · 18 Jun 2021, 08:47

@JKSH, I entered the required command and here's the output I got

curiouspan@curiouspan-VirtualBox:~$ ldd /home/curiouspan/qt6-build/qtbase/plugins/platforms/libqxcb.so
	linux-vdso.so.1 (0x00007fffc8770000)
	libQt6XcbQpa.so.6 => not found
	libQt6Gui.so.6 => /home/curiouspan/qt6-build/qtbase/plugins/platforms/../../lib/libQt6Gui.so.6 (0x00007f0873306000)
	libQt6Core.so.6 => /home/curiouspan/qt6-build/qtbase/plugins/platforms/../../lib/libQt6Core.so.6 (0x00007f087295d000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f087277c000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f087258a000)
	libEGL.so.1 => /lib/x86_64-linux-gnu/libEGL.so.1 (0x00007f0872573000)
	libX11.so.6 => /lib/x86_64-linux-gnu/libX11.so.6 (0x00007f0872436000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f0872430000)
	libQt6DBus.so.6 => /home/curiouspan/qt6-build/qtbase/plugins/platforms/../../lib/libQt6DBus.so.6 (0x00007f0872262000)
	libGLX.so.0 => /lib/x86_64-linux-gnu/libGLX.so.0 (0x00007f087222e000)
	libOpenGL.so.0 => /lib/x86_64-linux-gnu/libOpenGL.so.0 (0x00007f0872202000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f08721dd000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f087208e000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f0872073000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f08742b7000)
	libGLdispatch.so.0 => /lib/x86_64-linux-gnu/libGLdispatch.so.0 (0x00007f0871fbb000)
	libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f0871f91000)
	libXau.so.6 => /lib/x86_64-linux-gnu/libXau.so.6 (0x00007f0871f89000)
	libXdmcp.so.6 => /lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f0871f81000)
	libbsd.so.0 => /lib/x86_64-linux-gnu/libbsd.so.0 (0x00007f0871f67000)

How can I install this libQt6XcbQpa.so.6?

JonB · 18 Jun 2021, 10:43

@CuriousPan
@JKSH asked you to look at your configure log to see what you had set to build, since you are building Qt6 yourself, not downloading it.

JKSH · 18 Jun 2021, 09:30

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

How can I install this libQt6XcbQpa.so.6?

This is part of Qt.

Since your build was missing libQt6XcbQpa.so and libqxcb.so, it probably means your system did not fulfil the requirements to build Qt's XCB plugin (again, check your configure log)

You might be able to get a copy of libQt6XcbQpa.so from your colleague, but it would be better to find out why it didn't build in the first place.

CuriousPan · J JKSH 18 Jun 2021, 08:48

@JKSH Where this file should be located? My collegue cannot find it.

CuriousPan · 18 Jun 2021, 11:19

@JonB, how can I see configure log? Is there some log file?

jsulm · 18 Jun 2021, 13:23

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

how can I see configure log? Is there some log file?

Its config.log text file in the folder where you called configure...

CuriousPan · J jsulm 18 Jun 2021, 11:19

@jsulm, undertood. Anyway, do you know where the mentioned libQt6XcbQpa.so can be located? My collegue cannot find it.

CuriousPan · 18 Jun 2021, 14:17

@jsulm, Hello.
I have found configure.summary file. Here's the content for it. Is this what you meant?

Building for: linux-g++ (x86_64, CPU features: mmx sse sse2)
Compiler: gcc 9.3.0
Build options:
  Mode ................................... debug
  Optimize debug build ................... no
  Fully optimize release builds (-O3) .... no
  Building shared libraries .............. yes
  Using C standard ....................... C11
  Using C++ standard ..................... C++17
  Using ccache ........................... no
  Using new DTAGS ........................ yes
  Generating GDB index ................... no
  Relocatable ............................ yes
  Using precompiled headers .............. yes
  Using LTCG ............................. no
  Target compiler supports:
    SSE .................................. SSE2 SSE3 SSSE3 SSE4.1 SSE4.2
    AVX .................................. AVX AVX2
    AVX512 ............................... F ER CD PF DQ BW VL IFMA VBMI
    Other x86 ............................ AES F16C RDRAND SHA
    Intrinsics without compiler architecture option  yes
  Sanitizers:
    Addresses ............................ no
    Threads .............................. no
    Memory ............................... no
    Fuzzer (instrumentation only) ........ no
    Undefined ............................ no
  Build parts ............................ libs tools
Qt modules and options:
  Qt Concurrent .......................... yes
  Qt D-Bus ............................... yes
  Qt D-Bus directly linked to libdbus .... no
  Qt Gui ................................. yes
  Qt Network ............................. yes
  Qt Sql ................................. yes
  Qt Testlib ............................. yes
  Qt Widgets ............................. yes
  Qt Xml ................................. yes
Support enabled for:
  Using pkg-config ....................... yes
  udev ................................... no
  Using system zlib ...................... no
  Zstandard support ...................... no
  Thread support ......................... yes
Qt Core:
  backtrace .............................. yes
  DoubleConversion ....................... yes
    Using system DoubleConversion ........ no
  GLib ................................... no
  ICU .................................... no
  Using system libb2 ..................... no
  Built-in copy of the MIME database ..... yes
  Tracing backend ........................ <none>
  Logging backends:
    journald ............................. no
    syslog ............................... no
    slog2 ................................ no
  PCRE2 .................................. yes
    Using system PCRE2 ................... no
  CLONE_PIDFD support in forkfd .......... yes
Qt Sql:
  SQL item models ........................ yes
Qt Network:
  getifaddrs() ........................... yes
  IPv6 ifname ............................ yes
  libproxy ............................... no
  Linux AF_NETLINK ....................... yes
  OpenSSL ................................ no
    Qt directly linked to OpenSSL ........ no
  OpenSSL 1.1 ............................ no
  DTLS ................................... no
  OCSP-stapling .......................... no
  SCTP ................................... no
  Use system proxies ..................... yes
  GSSAPI ................................. no
  Brotli Decompression Support ........... no
Qt Gui:
  Accessibility .......................... yes
  FreeType ............................... yes
    Using system FreeType ................ no
  HarfBuzz ............................... yes
    Using system HarfBuzz ................ no
  Fontconfig ............................. no
  Image formats:
    GIF .................................. yes
    ICO .................................. yes
    JPEG ................................. yes
      Using system libjpeg ............... no
    PNG .................................. yes
      Using system libpng ................ no
  Text formats:
    HtmlParser ........................... yes
    CssParser ............................ yes
    OdfWriter ............................ yes
    MarkdownReader ....................... yes
      Using system libmd4c ............... no
    MarkdownWriter ....................... yes
  EGL .................................... yes
  OpenVG ................................. no
  OpenGL:
    Desktop OpenGL ....................... yes
    OpenGL ES 2.0 ........................ no
    OpenGL ES 3.0 ........................ no
    OpenGL ES 3.1 ........................ no
    OpenGL ES 3.2 ........................ no
  Vulkan ................................. no
  Session Management ..................... yes
Features used by QPA backends:
  evdev .................................. yes
  libinput ............................... no
  INTEGRITY HID .......................... no
  mtdev .................................. no
  tslib .................................. no
  xkbcommon .............................. no
  X11 specific:
    XLib ................................. yes
    XCB Xlib ............................. no
    EGL on X11 ........................... yes
    xkbcommon-x11 ........................ no
QPA backends:
  DirectFB ............................... no
  EGLFS .................................. yes
  EGLFS details:
    EGLFS OpenWFD ........................ no
    EGLFS i.Mx6 .......................... no
    EGLFS i.Mx6 Wayland .................. no
    EGLFS RCAR ........................... no
    EGLFS EGLDevice ...................... no
    EGLFS GBM ............................ no
    EGLFS VSP2 ........................... no
    EGLFS Mali ........................... no
    EGLFS Raspberry Pi ................... no
    EGLFS X11 ............................ no
  LinuxFB ................................ yes
  VNC .................................... yes
  VK_KHR_display ......................... no
  QNX:
    lgmon ................................ no
    IMF .................................. no
  XCB:
    Using system-provided xcb-xinput ..... no
    GL integrations:
      GLX Plugin ......................... no
        XCB GLX .......................... no
      EGL-X11 Plugin ..................... yes
  Windows:
    Direct 2D ............................ no
    Direct 2D 1.1 ........................ no
    DirectWrite .......................... no
    DirectWrite 3 ........................ no
Qt Widgets:
  GTK+ ................................... no
  Styles ................................. Fusion Windows
Qt Testlib:
  Tester for item models ................. yes
Qt PrintSupport:
  CUPS ................................... no
Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. no
  OCI (Oracle) ........................... no
  ODBC ................................... no
  PostgreSQL ............................. no
  SQLite ................................. yes
    Using system provided SQLite ......... no
Core tools:
  qmake tool ............................. yes
Qt QML:
  QML network support .................... yes
  QML debugging and profiling support .... yes
  QML just-in-time compiler .............. yes
  QML sequence object .................... yes
  QML XML http request ................... yes
  QML Locale ............................. yes
Qt QML Models:
  QML list model ......................... yes
  QML delegate model ..................... yes
Qt Quick:
  AnimatedImage item ..................... yes
  Canvas item ............................ yes
  Support for Qt Quick Designer .......... yes
  Flipable item .......................... yes
  GridView item .......................... yes
  ListView item .......................... yes
  TableView item ......................... yes
  Path support ........................... yes
  PathView item .......................... yes
  Positioner items ....................... yes
  Repeater item .......................... yes
  ShaderEffect item ...................... yes
  Sprite item ............................ yes
Qt Quick Templates 2:
  Hover support .......................... yes
  Multi-touch support .................... yes
Qt Quick Controls 2:
  Styles ................................. Basic Fusion Imagine Material Universal macOS Windows

JKSH · 18 Jun 2021, 14:19

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

Features used by QPA backends:
...
  xkbcommon .............................. no
  X11 specific:
    XLib ................................. yes
    XCB Xlib ............................. no
    EGL on X11 ........................... yes
    xkbcommon-x11 ........................ no
...
QPA backends:
...
  XCB:
    Using system-provided xcb-xinput ..... no
    GL integrations:
      GLX Plugin ......................... no
        XCB GLX .......................... no
      EGL-X11 Plugin ..................... yes

You need "yes" on all of these. "no" means you were missing the dependencies.

You should:

Clean out your build tree (i.e. delete all files that were not part of the original source code)
Install all the dependencies for Ubuntu at https://wiki.qt.io/Building_Qt_5_from_Git (pay particular attention to the "Libxcb" section)
Run configure again. Make sure you get "yes" on all of the above.
Run make to build Qt.

You should now have all the XCB-related libraries.