Custom plugin for qt is not loaded. How to debug?

Question

I wrote a custom plugin for QCanBus, that simply is a copy of the socketcan plugin but has been renamed and the identifiers have been adjusted to that new name.

I did that copying to first get the plugin recognised before I alter it.

I changed the qmake project to look that way:

TEMPLATE = lib
TARGET = qtcopysocketcanbus
CONFIG += plugin
QT = core serialbus

HEADERS += \
    copysocketcanbackend.h

SOURCES += \
    main.cpp \
    copysocketcanbackend.cpp

DISTFILES = plugin.json

and added the plugin.json like so:

{
    "Key": "copysocketcan"
}

I then renamed everything else from socketcan to copysocketcan in the main.cpp, the copysocketcan.cpp and the copysocketcan.h as well.

When I build the project, I get my *.so file which i put into $QT_PLUGIN_PATH/canbus/ on my target.

However, a quick start reveals, that qt only lists the plugins that came with the installation, not my added custom one.

I tried putting QLoggingCategory::setFilterRules(QStringLiteral("qt.canbus* = true")); as first line in my code and hoped getting more debug output, but I only get the debug output that my own application is producing. No output from the actual QCanBus.

So my questions are

How to enable the debug output for qt.canbus? Do I have to build QT with debug config for that?
Does my approach for creating a plugin reasonable?
Any ideas, why the custom plugin is not listed?

Enable `QT_DEBUG_PLUGINS` and double check the paths the other plugins are loaded from. See https://doc.qt.io/qt-5/deployment-plugins.html end of page — Th. Thielemann, Apr 20 '19 at 08:34
Thanks for that helpul comment. The debugging advice with `QT_DEBUG_PLUGINS` was the key. — darkmattercoder, Apr 23 '19 at 14:40

score 2 · Accepted Answer · answered Apr 23 '19 at 14:50

Through a helpful comment, I was able to debug the problems. The commentor suggested to use QT_DEBUG_PLUGINS to debug the plugin call. That lead to the appearance of an error message, that clearly stated, that the plugin I was trying to load because it was not a plugin and plugin metadata could not be extracted.

A bit of googling after those messages helped.

The answer to question 1 is:

Yes, you apparently have QT to build with debug information, to get the actual log output. In my case, I configured the framework with --force-debug-info

For the second question, my approach was indeed reasonable, because as answer for question 3 turned out that one has to include the moc file as soon, as a Q_OBJECT macro is used within a *.cpp file and not in a separate header, which is the case for the canbus plugins. You can read more about that here

Custom plugin for qt is not loaded. How to debug?

1 Answers1