An Unknown Segfault
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Hello,
I'm creating an application that is used to scan MTG Cards in real-time from 4 or more cameras, and output the nameplate list of each camera to a per-camera-window, which can then be captured in a streaming platform like OBS. It is in early stage and the actual detection/processing is not yet implemented, partially because of this segfault. It uses Qt, tbb queues and
tbb::flow::graph.https://github.com/ahsanullah-8bit/MTGScanner.git
At first made it work (with the integrated camera), a few problems occurred with QtMultimedia that I didn't know of (or partially known):
- You can't have any QtMultimedia in any other thread than the main thread.
- You can't use a camera device twice, at the same time.
The project uses a custom
ChannelWizarddialog component that involves 3 steps. Step 1 lets you select a camera, it has a combo box and a live preview below that. First thought would be, to just useVideoOutput,MediaCaptureSession,MediaDevicesandCameraQML types for the wizard camera selection. But when the wizard closes, we have to add the channel to theEngineand now you've to destroy theCameraandMediaCaptureSession, because they still hold the camera handle. Back then, I would initialize theQCameraandQMediaCaptureSessionin a per-camera capture thread, calledCameraMetrics(which lived in the main thread) methods from thetbb::flow::graphnodes and lot more synchronizarion problems. Now, we ask the engine (from QML) to provide us aChannel(custom type holding per-camera half-components, includingQCameraandQMediaCaptureSession) object, owned by C++ and use it to display the preview, pass it back to C++ if channel is to be created or destroy it, if canceled.Channelis used for the main thread andChannelRawis used by the EngineWorker thread, as it holds the supposedly thread safe components. Both are connected by main thread giving frames, and worker thread giving metricsQAtomicInts.The segfault occurred in the previous (commit 37913a9) and the current (commit 806aeba) architecture.
Before, if there were no channels, you add one, and launch the wizard again, it would not show anything in the preview (
Cameraprobably fails silently) for the same camera. You close the app, let it save the channel configurations, launch it again, a simple click on theAdd Channelbutton causes a segfault.Now, with the QtMultimedia moved to the main thread, I sometimes, get it at the start, or clicking a channel, or Add Channel. Disabling
ChannelWizardsometimes work. I can't debug it. It produces this call stack almost always:2 QV4::Managed::mark qv4managed_p.h 178 0x7ffff6cda402 3 QV4::MemoryManager::collectFromJSStack qv4mm.cpp 1474 0x7ffff6cda402 4 QV4::(anonymous namespace)::markJSStack unique_ptr.h 173 0x7ffff6cda51b` 5 QV4::GCStateMachine::transition qv4mm.cpp 1567 0x7ffff6cdacb7` 6 QV4::GCStateMachine::step qv4mm_p.h 89 0x7ffff6cdbd55 7 QV4::MemoryManager::runGC qv4mm.cpp 1289 0x7ffff6cdbd55 8 QV4::MemoryManager::allocate qv4mm_p.h 409 0x7ffff6cdeae4 9 QV4::MemoryManager::allocData qv4mm.cpp 1011 0x7ffff6cdeae4 10 QV4::MemoryManager::allocManaged<QV4::CallContext> qv4mm_p.h 215 0x7ffff6bbc5c6 11 QV4::ExecutionContext::newCallContext qv4context.cpp 74 0x7ffff6bbc5c6 12 QV4::Runtime::PushCallContext::call qv4runtime.cpp 1758 0x7ffff6c84ebd 13 QV4::Moth::VME::interpret qv4vme_moth.cpp 926 0x7ffff6cd0344 14 QV4::Moth::VME::exec qv4vme_moth.cpp 493 0x7ffff6cd65e3 15 QV4::doCall qv4function.cpp 53 0x7ffff6c09a8a 16 QV4::Function::call qv4function.cpp 78 0x7ffff6c09d21 17 QQmlJavaScriptExpression::evaluate qqmljavascriptexpression.cpp 238 0x7ffff6d7f293 18 QQmlBinding::evaluate qqmlbinding.cpp 195 0x7ffff6d0de8d 19 QQmlBinding::doUpdate qqmlbinding.cpp 713 0x7ffff6d0dfa9 20 QQmlBinding::update qqmlbinding.cpp 165 0x7ffff6d117b2 21 QPropertyObserverPointer::notify qproperty_p.h 916 0x7ffff4c2e1af 22 QObjectCompatProperty<QQuickItemPrivate, double, &QQuickItemPrivate::_qt_property_height_offset, &QQuickItemPrivate::setHeight, &QQuickItemPrivate::heightChanged, decltype(nullptr)>::notify qproperty_p.h 671 0x7ffff4c2e1af 23 QQuickItem::geometryChange qquickitem.cpp 3921 0x7ffff4c1aabc 24 QQuickItem::setSize qquickitem.cpp 7790 0x7ffff4c11ff0 25 QQuickControlPrivate::resizeBackground qquickcontrol.cpp 366 0x7ffff5309b1b 26 QQuickControl::geometryChange qquickcontrol.cpp 2134 0x7ffff5313ddd 27 QQuickItem::setSize qquickitem.cpp 7790 0x7ffff4c11ff0 28 QQuickGridLayoutItem::setGeometry qquickgridlayoutengine_p.h 86 0x7fffc3fbf196 29 QGridLayoutEngine::setGeometries qgridlayoutengine.cpp 1060 0x7ffff77acede 30 QQuickGridLayoutBase::rearrange qquicklinearlayout.cpp 479 0x7fffc3fbce66 31 QQuickLayout::geometryChange qquicklayout.cpp 980 0x7fffc3fb953d 32 QQuickItem::setSize qquickitem.cpp 7790 0x7ffff4c11ff0 33 QQuickAnchorsPrivate::setItemSize qquickanchors.cpp 392 0x7ffff4bda0fa 34 QQuickAnchorsPrivate::fillChanged qquickanchors.cpp 168 0x7ffff4bda0fa 35 operator() qquickitem.cpp 3909 0x7ffff4c1aa25 36 QQuickItemPrivate::notifyChangeListeners<QQuickItem::geometryChange(const QRectF&, const QRectF&)::<lambda(const QQuickItemPrivate::ChangeListener&)>> qquickitem_p.h 416 0x7ffff4c1aa25 37 QQuickItem::geometryChange qquickitem.cpp 3907 0x7ffff4c1aa25 38 QQuickItem::setHeight qquickitem.cpp 7614 0x7ffff4c114ea 39 QQuickPagePrivate::relayout qquickpage.cpp 117 0x7ffff5335ec4 40 QQuickControl::geometryChange qquickcontrol.cpp 2135 0x7ffff5313dea 41 QQuickItem::setSize qquickitem.cpp 7790 0x7ffff4c11ff0 42 QQuickAnchorsPrivate::setItemSize qquickanchors.cpp 392 0x7ffff4bda0fa 43 QQuickAnchorsPrivate::fillChanged qquickanchors.cpp 168 0x7ffff4bda0fa 44 operator() qquickitem.cpp 3909 0x7ffff4c1aa25 45 QQuickItemPrivate::notifyChangeListeners<QQuickItem::geometryChange(const QRectF&, const QRectF&)::<lambda(const QQuickItemPrivate::ChangeListener&)>> qquickitem_p.h 416 0x7ffff4c1aa25 46 QQuickItem::geometryChange qquickitem.cpp 3907 0x7ffff4c1aa25 47 QQuickItem::setSize qquickitem.cpp 7790 0x7ffff4c11ff0 48 QQuickControlPrivate::resizeContent qquickcontrol.cpp 377 0x7ffff5310594 49 QQuickControl::paddingChange qquickcontrol.cpp 2174 0x7ffff530a8f8 50 QQuickControlPrivate::setTopPadding qquickcontrol.cpp 200 0x7ffff530a8f8 51 QQmlPropertyData::writeProperty qqmlpropertydata_p.h 413 0x7ffff6d1589b 52 QQmlPropertyPrivate::write qqmlproperty.cpp 1920 0x7ffff6dd85ca 53 QQmlPropertyPrivate::writeValueProperty qqmlproperty.cpp 1328 0x7ffff6ddc1c4 54 QQmlPropertyToUnbindablePropertyBinding::update qqmlpropertytopropertybinding.cpp 111 0x7ffff6df1c67 55 QQmlNotifier::emitNotify qqmlnotifier.cpp 74 0x7ffff6dacbdd 56 doActivate<false> qobject.cpp 4147 0x7ffff63e82cb 57 QMetaObject::activate qobject.cpp 4317 0x7ffff63e0867 58 QQuickSafeArea::marginsChanged moc_qquicksafearea_p.cpp 169 0x7ffff4c63304 59 QQuickSafeArea::updateSafeArea qquicksafearea.cpp 293 0x7ffff4c63c4f 60 QQuickSafeArea::updateSafeAreasRecursively qquicksafearea.cpp 394 0x7ffff4c64ead 61 QQuickSafeArea::setAdditionalMargins qquicksafearea.cpp 194 0x7ffff4c65003 62 QQuickApplicationWindowPrivate::relayout qquickapplicationwindow.cpp 176 0x7ffff52fa62b 63 operator() qquickitem.cpp 3909 0x7ffff4c1aa25 64 QQuickItemPrivate::notifyChangeListeners<QQuickItem::geometryChange(const QRectF&, const QRectF&)::<lambda(const QQuickItemPrivate::ChangeListener&)>> qquickitem_p.h 416 0x7ffff4c1aa25 65 QQuickItem::geometryChange qquickitem.cpp 3907 0x7ffff4c1aa25 66 QQuickControl::geometryChange qquickcontrol.cpp 2133 0x7ffff5313dd0 67 QQuickItem::setWidth qquickitem.cpp 7406 0x7ffff4c10ada 68 layoutItem qquickapplicationwindow.cpp 125 0x7ffff52fa052 69 layoutItem qnumeric.h 297 0x7ffff52fa425 70 QQuickApplicationWindowPrivate::relayout qquickapplicationwindow.cpp 160 0x7ffff52fa425 71 QQuickApplicationWindowPrivate::executeBackground qquickapplicationwindow.cpp 294 0x7ffff52fae6a 72 QQuickApplicationWindow::componentComplete qquickapplicationwindow.cpp 822 0x7ffff52fda1b 73 QQmlObjectCreator::finalize qqmlobjectcreator.cpp 1617 0x7ffff6dafe42 74 QQmlComponentPrivate::complete qbipointer_p.h 133 0x7ffff6d2d540 75 QQmlComponentPrivate::completeCreate qqmlcomponent.cpp 1344 0x7ffff6d338eb 76 QQmlComponent::completeCreate qqmlcomponent.cpp 1310 0x7ffff6d34333 77 QQmlComponentPrivate::createWithProperties qqmlcomponent.cpp 998 0x7ffff6d34333 78 QQmlComponent::create qqmlcomponent.cpp 938 0x7ffff6d3463d 79 QQmlApplicationEnginePrivate::finishLoad qqmlapplicationengine.cpp 158 0x7ffff6d0cc93 80 QQmlApplicationEnginePrivate::ensureLoadingFinishes qqmlapplicationengine.cpp 185 0x7ffff6d0ce9c 81 QQmlApplicationEnginePrivate::startLoad qqmlapplicationengine.cpp 144 0x7ffff6d0d236 82 main main.cpp 29 0x5555555a08a2Google AI Search said, these are the common causes:
- Object Ownership Issues
- Dangling Pointers in Loaders
- Uninitialized C++ Objects
- Module-Specific Bugs
- Lifecycle of argc/argv
I know the call stack says, the garbage collector is having the segfault. But I looked at my code again and again, tried different LLMs, questions, posts, articles, and I'm exhausted. It might just be single line, mistake or misuse. If you got some free time on your hand, please have a look at it. Again, the project is still in development, so you might find some broken code.
It should compile on both Linux and Windows.
Update: New commits were made, recently and the problem is still there, even the QtMultimedia code is commented out in the QML Clicking on the ChannelWizard Step 1 Camera ComboBox also triggers the segfault. It's been 6 days man. I need some help.
Thank you!
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@AhsanKhan said in An Unknown Segfault:
Seriously? No, reply? It's the same with the Reddit page as well.
The original post is around 196 lines, 985 words, or 23 kilobytes of text, plus links to external resources. That's a lot to ask a forum user to read and understand. There might be a reasonable answer or interesting discussion, if the question is reduced to a more easily digestible level.
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@AhsanKhan said in An Unknown Segfault:
Seriously? No, reply? It's the same with the Reddit page as well.
We're not here for debugging your app. Please provide a minimal, compilable example to reproduce your problem.
