[qt6] Kvantum is not installed on your system (Kvantum style)

[paulcarroty]

Got the error on kvantummanager:

Kvantum is not installed on your system.
Please first install the Kvantum style plugin!

Installed with prefix=/opt, the kvantumpreview works and qt6ct is able to pick the Kvantum theme. Environment - GNOME 42 on Wayland.

Qt 6.2.1
strace.log

[tsujan]

It says what it sees: libkvantum.so isn't recognized as a Qt style plugin by itself when installed in /opt; only a Qt platform plugin like qt6ct may recognize it.

This is how Qt works — more precisely, QStyleFactory.

[paulcarroty]

Ohh, thanks.

[tsujan]

There's something I don't understand — or, perhaps, have misunderstood. Even if you use prefix=/opt, libkvantum.so should be installed in Qt's plugin install directory, and so, recognized. Where is ibkvantum.so in your case?

[paulcarroty]

libkvantum.so is in /opt and the Qt's plugin path is fixed with QT_PLUGIN_PATH environment variable.

[tsujan]

Thanks! That explains why the plugin isn't seen by Qt.

I think if you run kvantummanager after exporting QT_PLUGIN_PATH to the same place in a terminal emulator, it'll start (exporting it globally might have serious side effects).

[paulcarroty]

Nope, same error, with globally and locally QT_PLUGIN_PATH.

[tsujan]

Strange! Qt doc explains it here: https://doc.qt.io/qt-6/deployment-plugins.html.

If that EV sets the plugin path, then QStyleFactory::keys() should see it, while your test shows that QStyleFactory doesn't consult QT_PLUGIN_PATH. That may be an inconsistency (or bug) in Qt.

Kvantum Manager needs QStyleFactory::keys() to check if Kvantum is installed as a Qt style plugin. because it should be styled only by Kvantum.

[paulcarroty]

Yeah:

For example, on Windows, if the application is in C:\Program Files\MyApp and it has a style plugin, Qt looks in C:\Program Files\MyApp\styles.

I will test the next ways:

• libkvantum.so symlink in /usr /to /opt
• edit plugin paths in qt.conf
• qt5 build

[paulcarroty]

Update: the libkvantum.so symlink and qt.conf both failed, but the q5 build works out the box, with local or global QT_PLUGIN_PATH.

[tsujan]

q5 build works out the box

So, there may be a regression in Qt6. The documentation is the same.

[paulcarroty]

Reported to https://bugreports.qt.io/browse/QTBUG-112493

[tsujan]

Do you have the Qt5 plugin alongside the Qt6 one? Kvantum Manager is compiled against Qt5 by default.

In the above discussion, I supposed that you somehow enforced a Qt6 compilation for Kvantum Manager (that's possible with qmake). If not, and if you don't have the Qt5 plugin, there is no bug in Qt6 in this regard.

[paulcarroty]

Do you have the Qt5 plugin?

Yeah, just checked it to be sure.

you somehow enforced a Qt6 compilation for Kvantum Manager (that's possible with qmake)

Yes.

[paulcarroty]

It happened 'cause this command failed silently:

DESTDIR=%{buildroot} cmake --install build6

Fixed by

install -Dm644 build6/style/libkvantum.so %{buildroot}/usr/lib64/qt6/plugins/styles/libkvantum.so