How to know, whether our Qt 5 C++ projects make use of dynamically-loaded plug-ins.

I have used Qt Creator, with Qt version 5.7.1, to create some simplistic GUI applications as exercises. And then, I used the tool named ‘linuxdeployqt’, in order to bundle those (compiled) applications into AppImage’s, which should, in principle, run on other users’ Linux computers. (:4)  But, when using these tools, a question arose in my head, which I was also able to deduce the answer to quickly, in the form of, ‘Why does linuxdeployqt exist separately from linuxdeploy? Why does the developer need a tool which bundles library files, but which exists separately, just for C++ Qt applications? Obviously, some AppImages are not even based on that GUI library.’

And the short answer to that question is, ‘Because unlike some other application frameworks, Qt is based heavily on plug-ins, which none of my simpler exercises required explicitly.’ And, what’s so special about plug-ins? Aside from the fact that they extend the features of the application framework, plug-ins have as special property, that the application can decide to load them at run-time, and not with static awareness, at build-time. What this means is that a tool such as ‘linuxdeploy’ will scan the executable, which has been built by whichever compiler and/or IDE the developer used, will find that this executable is linked to certain shared libraries (with static awareness), but will not recognize some of the libraries which that executable needs to run, just because those are plug-ins, which the application will only decide to load at run-time.

Hence, to get the full benefit of using ‘linuxdeployqt’, the developer ‘wants to’ end up in a situation similar to the situation described Here. Granted, the person in question had run in to a bug, but his usage of the feature was correct.

This usage differs from my earlier usage, in that I never fed any ‘extra-plugins’ list to ‘linuxdeployqt’, yet, when I used the tool for creating the AppImage, my (project’s) plug-ins folder was populated with certain libraries, that were also plug-ins. And so, a legitimate question which an aspiring developer could ask would be, ‘How do I know, whether my Qt application loads plug-ins dynamically at run-time, so that I’ll know, whether I also need to specify those when bundling my AppImage?’ After all, it would seem that in certain cases, the plug-ins are in fact loaded with static awareness, which means that ‘linuxdeployqt’ can just recognize that the application is loading them, without the developer having had to make any special declaration of the fact.

One possible answer to that question might be ‘Test the AppImage, and see if it runs.’ But one problem with that answer would be, that if the executable cannot find certain plug-ins as bundled with the AppImage, the danger exists, that it may find those on the Host Machine, and that the application will end up running on some hosts, but not on other hosts, depending on what version of Qt the recipient has installed, and, depending on which plug-ins that recipient also has installed. And so, a better way must exist, for the developer to know the answer to this question.

(Updated 4/05/2021, 8h55… )

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