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Editorial Team
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Asked: 2026-05-23T08:19:55+00:00

I use a third-party DLL (FTD2xx) to communicate with an external device. Using Qt4,

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I use a third-party DLL (FTD2xx) to communicate with an external device. Using Qt4, in debug mode everything works fine, but the release crashes silently after successfully completing a called function. It seems to crash at return, but if I write something to the console (with qDebug) at the end of the function, sometimes it does not crash there, but a few, or few dozen lines later.

I suspect a not properly cleaned stack, what the debug build can survive, but the release chokes on it. Did someone encounter a similar problem? The DLL itself cannot be changed, as the source is not available.

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1 Answer

  1. Editorial Team

    It seems the reduction of the optimization level was the only way around. The DLL itself might have problems, as a program which does nothing but calls a single function from that DLL crashes the same way if optimization is turned on.

    Fortunately, the size and speed lost by the change in optimization level is negligible.

    Edit: for anyone with similar problems on Qt 5.0 or higher: If you change the optimization level (for example, to QMAKE_CXXFLAGS_RELEASE = -O0), it’s usually not enough to just rebuild the application. A full “clean all” is required.

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