[Development] Symbol clashes with static Qt libraries

[Kai Koehne]

Hi,

I'm wondering how we can avoid symbol clashes in static Qt libs + user code.

Reason why I'm looking into this is https://bugreports.qt.io/browse/QTBUG-67692 : We have the convention to name logging categories (which are actually functions) with 'lc', such as 'lcCanvas'. Since logging categories shouldn't be exported, this is fine for dynamic libs. However, for static builds of Qt, the symbol might clash with symbols of the same name from other libraries / user code, potentially resulting in linker errors.

(And no, we can't mark the logging category objects as static / in an anonymous namespace because they can be pre-declared by Q_DECLARE_LOGGING_CATEGORY).

Note also that the logging categories are by far not the only non-prefixed symbols that might cause clashes when statically linking. See https://paste.kde.org/poeiwjlw3 for all symbols in qtbase/lib/Qt5*.a that don't contain a q or Q , for a static Qt build of mine.

I can think of 3 approaches to tackle this:

a) Prefix all symbols with 'q', like we do for exported symbols.

This requires some bigger patches. See e.g. https://codereview.qt-project.org/#/c/235631/ for renaming all logging categories to 'qlc*' in qtbase.

b) Advise people to always configure static Qt in a namespace (-qtnamespace).

This should fix it for symbols at least in our own code. Maybe we should make it even the default for static builds in Qt 6?

c) Look into tricks like 'objcopy --localize-hidden' to hide symbols.

This would probably require some major hackery in the build system. No idea whether this is supported also on other platforms, and how hard it would be to pull it off. I'm not volunteering 😉
 
I guess I'm not the first one who looks into this, so I'm happy to hear advice/opinions 😊

Kai

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Kai Koehne, Senior Manager R&D | The Qt Company

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