QtCore4.dll_unloaded exception when usin external Wait function (LabVIEW)

[dadreamer]

This is almost certainly a LabView problem, not a Qt problem.

Maybe you're right. But I don't use Qt in my program in any ways. The vendor-provided library does, so I could conclude that something's wrong with it. I tend to believe that this is threading issues, e.g. when the library is already unloaded but some thread is still alive and tries to reach some space inside that DLL. It might be confirmed indirectly by this issue - the users were having that QtCore4.dll_unloaded crash and the developer had to change the work with threads. That fixed the crash.

I could confirm or refute my guess by rewriting the same algorithm in another IDE (MSVC, Delphi etc). It would take some time but it would be clear, if it's related to LabVIEW or not.

[d_stranz]

But I don't use Qt in my program in any ways.

I think the LabView GUI is Qt-based. However, if you say that when your developer modified the code, the crash went away, it sounds to me like the problem is the relationship between the developer's threading code and the LabView internals. Likely there was either a timing issue over resource cleanup or the threading code was causing memory corruption that was not revealed until the app went through its exit shutdown.

[dadreamer]

I think the LabView GUI is Qt-based.

I used to think the same way, but the experts on NI forums told me that Qt is used only in a few toolkits, which I don't use currently. The rest of LV graphics is written in the core without external components.

if you say that when your developer modified the code, the crash went away

Well, it was just an example of similar issue and there it was really linked to the threading issues. So my issue also might deal with it or not. But I checked out all the toolkit code and every block seems to be correct and properly configured. I have also compared the code with C examples from SICK and it matches. That's why I don't think that my code or toolkit code is the reason, but it's likely a bug in SICK's C wrapper.

Meanwhile I have found a workaround to the problem. I placed LoadLibrary( /* path to ICON library here */ ) call right before the camera(s) init. In the end of the code I'm not doing FreeLibrary. That makes LabVIEW to retain the DLL (and Qt lib's tied to it) from unloading on VI's close so it unloads only on process (exe) exit. No crash yet.

There is another workaround but it's kinda "nasty" - call ExitProcess(0) at the end, so all the threads receive their signals and finish normally. But it doesn't allow LabVIEW to do clean up work and is suitable only in exe variant (because I don't want to see LabVIEW closing when my program is stopped).

I'm not going to dig deeper because it takes a lot of free time and the things become more and more complex. On good terms the ICON C wrapper should be carefully debugged and rewritten if it's needed.

Thanks for the help.