I've been trying to help someone in Slack[1] with a heisenbug that a Qt-based application ended up deadlocking only when run with QProcess. After some debugging, we've found out that both Qt5 and Qt6 libraries are loaded in the same process. We still don't know why this happened because all our attempts to find out why cause the problem to disappear.
Anyway, the debugging so far has shown that the use of ELF visibility has NOT sufficed to separate the two library sets sufficiently. There were still symbol clashes, causing one library to use functions from the other and thus misbehave. Though there's nothing we can do about it and we can't remove the feature right now, before Qt 7. The reason for this is, of all things, inline functions. Because those user libraries and applications don't use version scripts of their own, the Qt symbols for inline functions that end up in their dynamic symbol tables won't have a "Qt5" or "Qt6" version marker. Therefore, they match both. And there are two reasons why those inline symbols end up in the symbol table: 1) third party code being compiled without -fvisibility-inlines-hidden We can't enforce that people use this flag in their applications and libraries (though they should). This means any inline function that didn't get inlined will then be part of the dynamic symbol table of the module in question. This is the case of the user: libQt5DBus.so.5 made a call to function symbol _ZeqRK7QStringS1_ (operator==), which got resolved to the main application, which is Qt 6. 2) an apparent GCC bug even with -fvisibility-inlines-hidden For some reason, some inline functions are not hidden, even with GCC 13[2]. Since there's no apparent reason why this should happen, I think it's a GCC bug. It doesn't happen with Clang. I've discovered this when trying to understand how to mitigate the problem. There may be a third source, that of inline Q_DECL_EXPORT variables. I don't think we have any such in our code today and Qt 5 couldn't use them, so this is not a problem right now. [1] https://cpplang.slack.com/archives/C29936TQC/p1683900238213659 [2] in debug mode uic.cpp.o, all "QStirng::" functions are HIDDEN, except for QString::clear(). -- Thiago Macieira - thiago.macieira (AT) intel.com Cloud Software Architect - Intel DCAI Cloud Engineering
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