Undefined symbol when trying to load a Wayland custom shell extension client plugin

[Brenton]

I'm having an issue with writing a custom shell integration client plugin for Wayland. I'm trying to write a plugin for wlr-layer-shell like layer-shell-qt but with minimal dependencies. I can build the plugin but when I try to use it I get an "undefined symbol" error. The custom shell example builds and runs fine, but when I try to use other Wayland protocols (like xdg-shell) I get the error.

As a test I've copied xdg-shell from QtWayland 6.8.3, renamed it to "new-shell", and modified the CMakeLists.txt to be similar to the custom shell example. Then I build the project with cmake -B build && cmake --build build and run test.sh.

The output I get (omitting the non-relevant output) is this:

qt.core.library: "/home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so" cannot load: Cannot load library /home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so: /home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so: undefined symbol: xdg_activation_v1_interface
qt.core.plugin.loader: QLibraryPrivate::loadPlugin failed on "/home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so" : "Cannot load library /home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so: /home/brenton/Development/new-shell/plugins/wayland-shell-integration/libnew-shell.so: undefined symbol: xdg_activation_v1_interface"
qt.qpa.wayland: Loading shell integration failed.
qt.qpa.wayland: Attempted to load the following shells QList("new-shell")
qt.qpa.plugin: Could not load the Qt platform plugin "wayland" 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, wayland-egl, vnc, xcb, minimalegl, vkkhrdisplay, offscreen, eglfs, minimal, linuxfb.

./test.sh: line 11: 47588 Aborted                 (core dumped) qml --verbose test.qml

So the error is undefined symbol: xdg_activation_v1_interface, but xdg-activation-v1.xml is included in qt_generate_wayland_protocol_client_sources. It's not just an issue with xdg_activation_v1_interface, I have seen the error for various functions that are in the files generated by qt_generate_wayland_protocol_client_sources.

Does anyone know what the issue could be? Obviously xdg-shell in QtWayland works, and layer-shell-qt works too. I have the issue with layer-shell-qt although I have to make modifications to the CMakeLists.txt to build it. I've also tried using Qt 6.9.1 and the newest version of cmake.

Thanks

[SGaist]

Hi and welcome to devnet,

What do you get if you run ldd on that plug-in ?

[Brenton]

Hey @SGaist, this is what I get:

$ ldd libnew-shell.so 
	linux-vdso.so.1 (0x00007b09d8fa6000)
	libwayland-client.so.0 => /lib/x86_64-linux-gnu/libwayland-client.so.0 (0x00007b09d8f18000)
	libQt6WaylandClient.so.6 => /opt/Qt/6.8.3/gcc_64/lib/libQt6WaylandClient.so.6 (0x00007b09d8e0c000)
	libQt6Gui.so.6 => /opt/Qt/6.8.3/gcc_64/lib/libQt6Gui.so.6 (0x00007b09d8200000)
	libGLX.so.0 => /lib/x86_64-linux-gnu/libGLX.so.0 (0x00007b09d8dd9000)
	libOpenGL.so.0 => /lib/x86_64-linux-gnu/libOpenGL.so.0 (0x00007b09d8dac000)
	libQt6Core.so.6 => /opt/Qt/6.8.3/gcc_64/lib/libQt6Core.so.6 (0x00007b09d7a00000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007b09d7600000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007b09d8d7e000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007b09d7200000)
	libffi.so.8 => /lib/x86_64-linux-gnu/libffi.so.8 (0x00007b09d8d72000)
	libwayland-cursor.so.0 => /lib/x86_64-linux-gnu/libwayland-cursor.so.0 (0x00007b09d8d66000)
	libGL.so.1 => /lib/x86_64-linux-gnu/libGL.so.1 (0x00007b09d8cdf000)
	libxkbcommon.so.0 => /lib/x86_64-linux-gnu/libxkbcommon.so.0 (0x00007b09d81b7000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007b09d7917000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007b09d8cda000)
	libEGL.so.1 => /lib/x86_64-linux-gnu/libEGL.so.1 (0x00007b09d8cc6000)
	libfontconfig.so.1 => /lib/x86_64-linux-gnu/libfontconfig.so.1 (0x00007b09d8166000)
	libX11.so.6 => /lib/x86_64-linux-gnu/libX11.so.6 (0x00007b09d74c3000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007b09d8cc1000)
	libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007b09d70b7000)
	libQt6DBus.so.6 => /opt/Qt/6.8.3/gcc_64/lib/libQt6DBus.so.6 (0x00007b09d6fe5000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007b09d814a000)
	libfreetype.so.6 => /lib/x86_64-linux-gnu/libfreetype.so.6 (0x00007b09d6f19000)
	libgthread-2.0.so.0 => /lib/x86_64-linux-gnu/libgthread-2.0.so.0 (0x00007b09d8cba000)
	libGLdispatch.so.0 => /lib/x86_64-linux-gnu/libGLdispatch.so.0 (0x00007b09d6e61000)
	libicui18n.so.73 => /opt/Qt/6.8.3/gcc_64/lib/libicui18n.so.73 (0x00007b09d6a00000)
	libicuuc.so.73 => /opt/Qt/6.8.3/gcc_64/lib/libicuuc.so.73 (0x00007b09d6600000)
	libicudata.so.73 => /opt/Qt/6.8.3/gcc_64/lib/libicudata.so.73 (0x00007b09d4600000)
	libzstd.so.1 => /lib/x86_64-linux-gnu/libzstd.so.1 (0x00007b09d6da7000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007b09d8cb3000)
	/lib64/ld-linux-x86-64.so.2 (0x00007b09d8fa8000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007b09d811e000)
	libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007b09d80f5000)
	libpcre2-8.so.0 => /lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007b09d7429000)
	libdbus-1.so.3 => /lib/x86_64-linux-gnu/libdbus-1.so.3 (0x00007b09d78c8000)
	libbz2.so.1.0 => /lib/x86_64-linux-gnu/libbz2.so.1.0 (0x00007b09d78b4000)
	libpng16.so.16 => /lib/x86_64-linux-gnu/libpng16.so.16 (0x00007b09d6d6f000)
	libbrotlidec.so.1 => /lib/x86_64-linux-gnu/libbrotlidec.so.1 (0x00007b09d78a6000)
	libXau.so.6 => /lib/x86_64-linux-gnu/libXau.so.6 (0x00007b09d8ca9000)
	libXdmcp.so.6 => /lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007b09d789e000)
	libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007b09d6920000)
	libbrotlicommon.so.1 => /lib/x86_64-linux-gnu/libbrotlicommon.so.1 (0x00007b09d68fd000)
	libbsd.so.0 => /lib/x86_64-linux-gnu/libbsd.so.0 (0x00007b09d7888000)
	libcap.so.2 => /lib/x86_64-linux-gnu/libcap.so.2 (0x00007b09d741c000)
	libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007b09d64b8000)
	liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007b09d68db000)
	liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007b09d68a9000)
	libmd.so.0 => /lib/x86_64-linux-gnu/libmd.so.0 (0x00007b09d6d60000)
	libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007b09d6884000)

[Axel Spoerl]

Is the generated xdg-activation-v1-protocol.c linked in your CMakeLists.txt?

[Brenton]

@Axel-Spoerl not directly, but I believe that should be handled by the qt_generate_wayland_protocol_client_sources function.

I've tried adding the generated files like this but I still have the same run time error:

target_sources(new-shell PUBLIC
        build/qwayland-xdg-activation-v1.cpp
        build/qwayland-xdg-activation-v1.h
        build/qwayland-xdg-decoration-unstable-v1.cpp
        build/qwayland-xdg-decoration-unstable-v1.h
        build/qwayland-xdg-dialog-v1.cpp
        build/qwayland-xdg-dialog-v1.h
        build/qwayland-xdg-foreign-unstable-v2.cpp
        build/qwayland-xdg-foreign-unstable-v2.h
        build/qwayland-xdg-shell.cpp
        build/qwayland-xdg-shell.h
        build/wayland-xdg-activation-v1-client-protocol.h
        build/wayland-xdg-activation-v1-protocol.c
        build/wayland-xdg-decoration-unstable-v1-client-protocol.h
        build/wayland-xdg-decoration-unstable-v1-protocol.c
        build/wayland-xdg-dialog-v1-client-protocol.h
        build/wayland-xdg-dialog-v1-protocol.c
        build/wayland-xdg-foreign-unstable-v2-client-protocol.h
        build/wayland-xdg-foreign-unstable-v2-protocol.c
        build/wayland-xdg-shell-client-protocol.h
        build/wayland-xdg-shell-protocol.c
)

[Axel Spoerl]

If a library exists and an application’s related symbols can’t be resolved, it can be any combination of

• version mismatch
• missing user permissions
• not linked
  Hard to guess from here. You can only try that locally

[Brenton]

I've figured out the problem, in the project definition in CMakeLists.txt I had CXX but not C, which meant that the .c files weren't processed.

[Christian Ehrlicher]

@Brenton said in Undefined symbol when trying to load a Wayland custom shell extension client plugin:

n the project definition in CMakeLists.txt I had CXX but not C

Simply leave it empty so CMake will auto-determine the needed compilers.

[Brenton]

@Christian-Ehrlicher said in Undefined symbol when trying to load a Wayland custom shell extension client plugin:

Simply leave it empty so CMake will auto-determine the needed compilers.

Yeah, if it's omitted it will default to C and CXX. The custom shell example has LANGUAGES CXX but that still works for the example.