non GUI thread use

I make an unusual use of Qt: I've developped a Qt based library and I want to provide its services to Java too using a JNI (in addition to a standard C++ API).
Here is how it currently works:

• when the library is initalized in the context of a stand-alone application, it creates a QGraphicsScene which is rendered in a QGraphicsView
• when the library is initalized in a Java context, it creates a QGraphicsScene which is rendered in a QImage at Java client request

In both cases, I create a QApplication but in the Java context, I don't call QApplication::exec().
The QGraphicsScene is modified by the time task of a QTimer.
In the Java context, a QMutex ensures that the time task and the Java client request to get the scene bitmap data don't collide.

It basically works but it also crashes randomly and here is my current investigation state.
First of all, I've replaced the QPixmap with QImage since the final QImage rendering is made outside a GUI thread (and Qt complained about that).
Now there is a single remaining warning at client startup:
"QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread"

Here is the code of the Java context rendering scheme:

Code:

---

QImage image (w, h, QImage::Format_ARGB32_Premultiplied);
QPainter painter(&image);
painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing | QPainter::SmoothPixmapTransform);
fScene->render(&painter);
painter.end();

---

Note that it doesn't crash when I comment out the 'render' call.

I don't get anymore warning from Qt and the developer documentation only talks about QWidget that should not be used in a non GUI thread. Most of the crashes occur when modifying a QGraphicsSceneBspTreeIndex (or QGraphicsSceneBspTreeIndexPrivate) object. It looks like there are remaining unsafe operations called.

Any help would be greatly appreciated.

I guess the question and the context are maybe not familiar to this list users.
If nobody has an answer, maybe someone knows a better place to ask.
Best,

I would rather say your post isn't clear at what you are really doing. A minimal compilable example reproducing the problem might help. And if not then even just a backtrace of the crash could be helpful.

Quote:

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Originally Posted by wysota

I would rather say your post isn't clear at what you are really doing. A minimal compilable example reproducing the problem might help. And if not then even just a backtrace of the crash could be helpful.

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It's quite difficult to make a minimal compilable example since it involves a Qt based shared library, a JNI shared library and a Java virtual machine. I'll try to make one. Concerning the backtrace, here are two of them:

the first one corresponds to a timer task, i.e. it should not correspond to a JNI call

Code:

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Thread 0 Crashed:  Dispatch queue: com.apple.main-thread
0   QtGui                         	0x00000001354fbf82 QList<QGraphicsItem*>::detach_helper_grow(int, int) + 146
1   QtGui                         	0x000000013566a484 QGraphicsSceneInsertItemBspTreeVisitor::visit(QList<QGraphicsItem*>*) + 36
2   QtGui                         	0x0000000135668ab2 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 290
3   QtGui                         	0x0000000135668a13 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 131
4   QtGui                         	0x0000000135668ab2 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 290
5   QtGui                         	0x0000000135668a13 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 131
6   QtGui                         	0x0000000135668ab2 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 290
7   QtGui                         	0x0000000135668a13 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 131
8   QtGui                         	0x0000000135668ab2 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 290
9   QtGui                         	0x0000000135668a13 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 131
10  QtGui                         	0x0000000135668ab2 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 290
11  QtGui                         	0x0000000135668a13 QGraphicsSceneBspTree::climbTree(QGraphicsSceneBspTreeVisitor*, QRectF const&, int) const + 131
12  QtGui                         	0x000000013566bde5 QGraphicsSceneBspTreeIndexPrivate::_q_updateIndex() + 1013
13  QtGui                         	0x000000013566bef6 QGraphicsSceneBspTreeIndex::event(QEvent*) + 102
14  QtGui                         	0x0000000135081ddd QApplicationPrivate::notify_helper(QObject*, QEvent*) + 189
15  QtGui                         	0x0000000135088dfe QApplication::notify(QObject*, QEvent*) + 2094
16  QtCore                        	0x00000001322a2dec QCoreApplication::notifyInternal(QObject*, QEvent*) + 124
17  QtGui                         	0x0000000135081e7c qt_sendSpontaneousEvent(QObject*, QEvent*) + 44
18  QtGui                         	0x000000013503b79c QEventDispatcherMacPrivate::activateTimer(__CFRunLoopTimer*, void*) + 188
19  com.apple.CoreFoundation      	0x00007fff86484be8 __CFRunLoopRun + 6488
20  com.apple.CoreFoundation      	0x00007fff86482dbf CFRunLoopRunSpecific + 575
21  com.apple.HIToolbox           	0x00007fff8611e91a RunCurrentEventLoopInMode + 333
22  com.apple.HIToolbox           	0x00007fff8611e71f ReceiveNextEventCommon + 310
23  com.apple.HIToolbox           	0x00007fff8611e5d8 BlockUntilNextEventMatchingListInMode + 59
24  com.apple.AppKit              	0x00007fff870ebe64 _DPSNextEvent + 718
25  com.apple.AppKit              	0x00007fff870eb7a9 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 155
26  com.apple.AppKit              	0x00007fff870b148b -[NSApplication run] + 395
27  libawt.jnilib                 	0x000000011a940a04 +[AWTStarter startAWT:] + 1286
28  libawt.jnilib                 	0x000000011a904625 -[CPerformer perform] + 93
29  com.apple.Foundation          	0x00007fff86d5344f __NSThreadPerformPerform + 219
30  com.apple.CoreFoundation      	0x00007fff86485401 __CFRunLoopDoSources0 + 1361
31  com.apple.CoreFoundation      	0x00007fff864835f9 __CFRunLoopRun + 873
32  com.apple.CoreFoundation      	0x00007fff86482dbf CFRunLoopRunSpecific + 575

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the second one clearly corresponds to a JNI call

Code:

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Thread 28 Crashed:  Java: AWT-EventQueue-0
0   libSystem.B.dylib             	0x00007fffffe00f46 __memcpy + 1958
1   QtCore                        	0x0000000132a5fbbc QListData::remove(int) + 140
2   QtCore                        	0x0000000132a6011e QListData::erase(void**) + 46
3   QtGui                         	0x0000000135a95a9d QGraphicsSceneBspTreeIndexPrivate::_q_updateIndex() + 173
4   QtGui                         	0x0000000135a95f1b QGraphicsSceneBspTreeIndexPrivate::_q_updateSortCache() + 27
5   QtGui                         	0x0000000135a962ff QGraphicsSceneBspTreeIndexPrivate::estimateItems(QRectF const&, Qt::SortOrder, bool) + 271
6   QtGui                         	0x0000000135a963fb QGraphicsSceneBspTreeIndex::estimateTopLevelItems(QRectF const&, Qt::SortOrder) const + 27
7   QtGui                         	0x0000000135a9a17b QGraphicsSceneIndex::items(QRectF const&, Qt::ItemSelectionMode, Qt::SortOrder, QTransform const&) const + 315
8   QtGui                         	0x0000000135a709bf QGraphicsScene::items(QRectF const&, Qt::ItemSelectionMode) const + 111
9   QtGui                         	0x0000000135a893cf QGraphicsScene::render(QPainter*, QRectF const&, QRectF const&, Qt::AspectRatioMode) + 271
10  INScore                       	0x00000001315fba38 inscore::VSceneView::copy(unsigned int*, int, int, bool) + 280
11  INScore                       	0x000000013152e286 inscore::IGlue::getSceneView(unsigned int*, int, int, bool) + 294
12  INScore                       	0x0000000131608946 inscore::INScore::getScene(inscore::IGlue*, unsigned int*, int, int) + 22
13  libjniINScore.dylib           	0x0000000117bfa613 Java_INScore_INScore_GetBitmap + 83
14  ???                           	0x000000010301265b 0 + 4345374299
...
30  ???                           	0x00000001030013eb 0 + 4345304043
31  libclient64.dylib             	0x000000010109e249 JVM_Lseek + 185699
32  libclient64.dylib             	0x000000010109e041 JVM_Lseek + 185179
33  libclient64.dylib             	0x00000001010a8d2b JVM_StartThread + 2355
34  libclient64.dylib             	0x00000001010a8c1e JVM_StartThread + 2086
35  libclient64.dylib             	0x00000001010a8b89 JVM_StartThread + 1937
36  libclient64.dylib             	0x00000001010a8a29 JVM_StartThread + 1585
37  libclient64.dylib             	0x00000001010a88d3 JVM_StartThread + 1243
38  libclient64.dylib             	0x000000010100cad2 JNI_CreateJavaVM_Impl + 46704
39  libSystem.B.dylib             	0x00007fff859ed536 _pthread_start + 331
40  libSystem.B.dylib             	0x00007fff859ed3e9 thread_start + 13

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You have to explain clearly where the threads are. They are the most probable source of your problems.

You're right but it's quite difficult since the threads are java VM threads and there are about thirty running.
Anyway and thanks to your previous advice (to make a minimal compilable example), I think I'm on the way to find the reason (and the solution). I need to make some more investigations but when I'll have the solution, I'll post it to this forum. Right now, it seems that the problem comes from the java side.