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

gaoyiqin95 · 20 Mar 2023, 08:03

Hello everyone!
I'm new in using Qt tools.
I want to build from source Qt 5.12.12, on CentOS Linux release 7.7.1908 (Core).
I'm working on an HPC cluster so I don't have root. I installed all libraries and tools in a personal directory.
I successfully installed Qt, but while I want to launch Cube, I have an error message (already used QT_DEBUG_PLUGINS=1):

Got keys from plugin meta data ("webgl")
QFactoryLoader::QFactoryLoader() checking directory path "[MY_CUBEGUI_INSTALL_PATH]/cubegui-4.8/bin/platforms" ...
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
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, webgl.

It seems there is not libqxcb.so library in my plugins directory, here is the result of ls:

libqeglfs.so  libqlinuxfb.so  libqminimalegl.so  libqminimal.so  libqoffscreen.so  libqvnc.so  libqwebgl.so

I found similar errors in the page, but their message is:

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

which is not exactly the same as what I have.

Could anyone help me to find the problem? Thank you!

jsulm · 21 Mar 2023, 07:31

@gaoyiqin95 said in qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in "":

I successfully installed Qt

You mean you built it from source?
In that case check config.log file in the build folder, it should tell you why xcb plug-in build was disabled (probably some library is missing).

JoeCFD · wrote on 20 Mar 2023, 20:13

@gaoyiqin95
in my platforms dir of Qt-5.15.5, these libs are installed.
libqeglfs.so
libqminimalegl.so
libqoffscreen.so
libqwayland-egl.so
libqwayland-xcomposite-egl.so
libqxcb.so
libqlinuxfb.so
libqminimal.so
libqvnc.so
libqwayland-generic.so
libqwayland-xcomposite-glx.so

You may not have libqxcb.so. Check your installation flags out and make sure this one is installed.

An easy way to do this might be:
install CentOS Linux release 7.7 in a virtual machine and use online installer to install Qt under /opt/Qt(default location).
Then you can copy the whole installed Qt dir to your HPC machine.

gaoyiqin95 · 21 Mar 2023, 08:17

@jsulm
Thank you for response. But there are so much information in config.log that I can't identify what is useful for solving my problem. In contrast, here is what I have in config.summary:

Build type: linux-g++ (x86_64, CPU features: mmx sse sse2)
Compiler: gcc 9.3.0
Configuration: use_gold_linker sse2 aesni sse3 ssse3 sse4_1 sse4_2 avx avx2 avx512f avx512bw avx512cd avx512dq avx512er avx512ifma avx512pf avx512vbmi avx512vl compile_examples enable_new_dtags f16c largefile precompile_header rdrnd shani x86SimdAlways shared rpath release c++11 c++14 c++1z concurrent dbus reduce_exports reduce_relocations stl
Build options:
  Mode ................................... release
  Optimize release build for size ........ no
  Building shared libraries .............. yes
  Using C standard ....................... C11
  Using C++ standard ..................... C++1z
  Using ccache ........................... no
  Using gold linker ...................... yes
  Using new DTAGS ........................ 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 -mXXX option ...... yes
  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 ...................... yes
Qt Core:
  DoubleConversion ....................... yes
    Using system DoubleConversion ........ no
  GLib ................................... yes
  iconv .................................. yes
  ICU .................................... no
  Tracing backend ........................ <none>
  Logging backends:
    journald ............................. no
    syslog ............................... no
    slog2 ................................ no
  Using system PCRE2 ..................... no
Qt Network:
  getifaddrs() ........................... yes
  IPv6 ifname ............................ yes
  libproxy ............................... no
  Linux AF_NETLINK ....................... yes
  OpenSSL ................................ yes
    Qt directly linked to OpenSSL ........ no
  OpenSSL 1.1 ............................ no
  DTLS ................................... yes
  SCTP ................................... no
  Use system proxies ..................... yes
Qt Gui:
  Accessibility .......................... yes
  FreeType ............................... yes
    Using system FreeType ................ yes
  HarfBuzz ............................... yes
    Using system HarfBuzz ................ no
  Fontconfig ............................. yes
  Image formats:
    GIF .................................. yes
    ICO .................................. yes
    JPEG ................................. yes
      Using system libjpeg ............... yes
    PNG .................................. yes
      Using system libpng ................ 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 ............................. yes
    EGL on X11 ........................... yes
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 ...................... yes
    EGLFS GBM ............................ no
    EGLFS VSP2 ........................... no
    EGLFS Mali ........................... no
    EGLFS Raspberry Pi ................... no
    EGLFS X11 ............................ yes
  LinuxFB ................................ yes
  VNC .................................... yes
  Mir client ............................. no
Qt Sql:
  SQL item models ........................ yes
Qt Widgets:
  GTK+ ................................... no
  Styles ................................. Fusion Windows
Qt PrintSupport:
  CUPS ................................... no
Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. no
  OCI (Oracle) ........................... no
  ODBC ................................... no
  PostgreSQL ............................. no
  SQLite2 ................................ no
  SQLite ................................. yes
    Using system provided SQLite ......... no
  TDS (Sybase) ........................... no
Qt Testlib:
  Tester for item models ................. yes
Qt SerialBus:
  Socket CAN ............................. yes
  Socket CAN FD .......................... yes
Further Image Formats:
  JasPer ................................. no
  MNG .................................... no
  TIFF ................................... yes
    Using system libtiff ................. no
  WEBP ................................... yes
    Using system libwebp ................. no
Qt QML:
  QML network support .................... yes
  QML debugging and profiling support .... yes
  QML sequence object .................... yes
  QML list model ......................... yes
  QML XML http request ................... yes
  QML Locale ............................. yes
  QML delegate model ..................... yes
Qt Quick:
  Direct3D 12 ............................ no
  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 Scxml:
  ECMAScript data model for QtScxml ...... yes
Qt Gamepad:
  SDL2 ................................... no
Qt 3D:
  Assimp ................................. yes
  System Assimp .......................... no
  Output Qt3D Job traces ................. no
  Output Qt3D GL traces .................. no
  Use SSE2 instructions .................. yes
  Use AVX2 instructions .................. no
  Aspects:
    Render aspect ........................ yes
    Input aspect ......................... yes
    Logic aspect ......................... yes
    Animation aspect ..................... yes
    Extras aspect ........................ yes
Qt 3D Renderers:
  OpenGL Renderer ........................ yes
Qt 3D GeometryLoaders:
  Autodesk FBX ........................... no
Qt Wayland Client ........................ no
Qt Wayland Compositor .................... no
Qt Bluetooth:
  BlueZ .................................. no
  BlueZ Low Energy ....................... no
  Linux Crypto API ....................... no
  WinRT Bluetooth API (desktop & UWP) .... no
Qt Sensors:
  sensorfw ............................... no
Qt Quick Controls 2:
  Styles ................................. Default Fusion Imagine Material Universal
Qt Quick Templates 2:
  Hover support .......................... yes
  Multi-touch support .................... yes
Qt Positioning:
  Gypsy GPS Daemon ....................... no
  WinRT Geolocation API .................. no
Qt Location:
  Qt.labs.location experimental QML plugin . yes
  Geoservice plugins:
    OpenStreetMap ........................ yes
    HERE ................................. yes
    Esri ................................. yes
    Mapbox ............................... yes
    MapboxGL ............................. yes
    Itemsoverlay ......................... yes
QtXmlPatterns:
  XML schema support ..................... yes
Qt Multimedia:
  ALSA ................................... no
  GStreamer 1.0 .......................... no
  GStreamer 0.10 ......................... no
  Video for Linux ........................ yes
  OpenAL ................................. no
  PulseAudio ............................. no
  Resource Policy (libresourceqt5) ....... no
  Windows Audio Services ................. no
  DirectShow ............................. no
  Windows Media Foundation ............... no
Qt Tools:
  QDoc ................................... no
Qt WebEngine:
  Embedded build ......................... no
  Full debug information ................. no
  Pepper Plugins ......................... yes
  Printing and PDF ....................... yes
  Proprietary Codecs ..................... no
  Spellchecker ........................... yes
  Native Spellchecker .................... no
  WebRTC ................................. yes
  Use System Ninja ....................... no
  Geolocation ............................ yes
  WebChannel support ..................... yes
  Use v8 snapshot ........................ yes
  Kerberos Authentication ................ no
  Support qpa-xcb ........................ no
  Use ALSA ............................... no
  Use PulseAudio ......................... no
  Optional system libraries used:
    re2 .................................. no
    icu .................................. no
    libwebp, libwebpmux and libwebpdemux . no
    opus ................................. no
    ffmpeg ............................... no
    libvpx ............................... no
    snappy ............................... no
    glib ................................. yes
    zlib ................................. yes
    minizip .............................. no
    libevent ............................. no
    jsoncpp .............................. no
    protobuf ............................. no
    libxml2 and libxslt .................. no
    lcms2 ................................ no
    png .................................. no
    JPEG ................................. no
    harfbuzz ............................. no
    freetype ............................. yes
  Required system libraries:
    fontconfig ........................... yes
    dbus ................................. no
    nss .................................. no
    khr .................................. yes
    glibc ................................ yes
  Required system libraries for qpa-xcb:
    x11 .................................. yes
    libdrm ............................... yes
    xcomposite ........................... no
    xcursor .............................. no
    xi ................................... no
    xtst ................................. no

Note: Also available for Linux: linux-clang linux-icc

Note: No wayland-egl support detected. Cross-toolkit compatibility disabled.

WARNING: QDoc will not be compiled, probably because libclang could not be located. This means that you cannot build the Qt documentation.

Either ensure that llvm-config is in your PATH environment variable, or set LLVM_INSTALL_DIR to the location of your llvm installation.
On Linux systems, you may be able to install libclang by installing the libclang-dev or libclang-devel package, depending on your distribution.
On macOS, you can use Homebrew's llvm package.
On Windows, you must set LLVM_INSTALL_DIR to the installation path.

jsulm · 21 Mar 2023, 08:35

@gaoyiqin95 said in qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in "":

config.log that I can't identify what is useful for solving my problem

You can simply use the search function in your text editor to search for xcb

gaoyiqin95 · 21 Mar 2023, 08:56

@jsulm OK thanks. Here is information related to xcb I found in the log file:

looking for library xcb
Trying source 0 (type pkgConfig) of library xcb ...
+ /usr/bin/pkg-config --exists --silence-errors xcb '>=' 1.9
+ /usr/bin/pkg-config --modversion xcb '>=' 1.9
> 1.13
+ /usr/bin/pkg-config --libs-only-L xcb '>=' 1.9
>  
+ /usr/bin/pkg-config --libs-only-l xcb '>=' 1.9
> -lxcb  
+ /usr/bin/pkg-config --cflags xcb '>=' 1.9
>  
+ cd /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb && /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/qtbase/bin/qmake "CONFIG -= qt debug_and_release app_bundle lib_bundle" "CONFIG += shared use_gold_linker warn_off console single_arch" 'QMAKE_USE += xcb' 'QMAKE_LIBS_XCB = /lib64/libxcb.so' /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb
+ cd /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb && MAKEFLAGS= /usr/bin/gmake
> g++ -c -pipe -O2 -w -fPIC  -I. -I/lustre/home/acct-hpc/hpcgyq/install/qt5/qtbase/mkspecs/linux-g++ -o main.o main.cpp
> g++ -Wl,-O1 -fuse-ld=gold -o xcb main.o   /lib64/libxcb.so   
 => source accepted.
test config.qtbase_gui.libraries.xcb succeeded
looking for library xcb_xlib
Trying source 0 (type pkgConfig) of library xcb_xlib ...
+ /usr/bin/pkg-config --exists --silence-errors x11-xcb
+ /usr/bin/pkg-config --modversion x11-xcb
> 1.6.7
+ /usr/bin/pkg-config --libs-only-L x11-xcb
>  
+ /usr/bin/pkg-config --libs-only-l x11-xcb
> -lX11-xcb -lX11 -lxcb  
+ /usr/bin/pkg-config --cflags x11-xcb
>  
+ cd /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb_xlib && /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/qtbase/bin/qmake "CONFIG -= qt debug_and_release app_bundle lib_bundle" "CONFIG += shared use_gold_linker warn_off console single_arch" 'QMAKE_USE += xcb_xlib' 'QMAKE_LIBS_XCB_XLIB = /lib64/libX11-xcb.so' 'QMAKE_DEPENDS_XCB_XLIB_CC = XCB XLIB' 'QMAKE_DEPENDS_XCB_XLIB_LD = XCB XLIB' 'QMAKE_LIBS_XLIB = /lib64/libXext.so /lib64/libX11.so /lib64/libm.so' 'QMAKE_LIBS_XCB = /lib64/libxcb.so' /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb_xlib
+ cd /lustre/home/acct-hpc/hpcgyq/install/qt5/x86_build/config.tests/xcb_xlib && MAKEFLAGS= /usr/bin/gmake
> g++ -c -pipe -O2 -w -fPIC  -I. -I/lustre/home/acct-hpc/hpcgyq/install/qt5/qtbase/mkspecs/linux-g++ -o main.o main.cpp
> g++ -Wl,-O1 -fuse-ld=gold -o xcb_xlib main.o   /lib64/libX11-xcb.so /lib64/libxcb.so /lib64/libXext.so /lib64/libX11.so /lib64/libm.so   
 => source accepted.
test config.qtbase_gui.libraries.xcb_xlib succeeded

test config.qtbase_gui.tests.qpa_default_platform gave result xcb
looking for library harfbuzz
Trying source 0 (type inline) of library harfbuzz ...
None of [libharfbuzz.so libharfbuzz.a] found in [] and global paths.
  => source produced no result.

It seems that I miss library harfbuzz?

jsulm · wrote on 21 Mar 2023, 08:56

@gaoyiqin95 said in qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in "":

test config.qtbase_gui.tests.qpa_default_platform gave result xcb

It looks like xcb check is fine.
Also from what you posted before:

X11 specific:
    XLib ................................. yes
    XCB Xlib ............................. yes
    EGL on X11 ........................... yes

So, the plug-in should be built.
You will need to check the build log to see what happens during the build.
Check the build log (make) and also install step (make install).

Nilkl · 5 Jan 2024, 15:24

Maybe missing libxkbcommon.

yum install libxkbcommon libxkbcommon-devel libxkbcommon-x11 libxkbcommon-x11-devel