An Unknown Segfault
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 ChannelWizard dialog 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 use VideoOutput, MediaCaptureSession, MediaDevices and Camera QML types for the wizard camera selection. But when the wizard closes, we have to add the channel to the Engine and now you've to destroy the Camera and MediaCaptureSession, because they still hold the camera handle. Back then, I would initialize the QCamera and QMediaCaptureSession in a per-camera capture thread, called CameraMetrics (which lived in the main thread) methods from the tbb::flow::graph nodes and lot more synchronizarion problems. Now, we ask the engine (from QML) to provide us a Channel (custom type holding per-camera half-components, including QCamera and QMediaCaptureSession) 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. Channel is used for the main thread and ChannelRaw is 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 metrics QAtomicInts.
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 (Camera probably fails silently) for the same camera. You close the app, let it save the channel configurations, launch it again, a simple click on the Add Channel button 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 ChannelWizard sometimes 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 0x5555555a08a2
Google 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.
Thank you!