Global static QPixmapCache in Qt internals
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[Forked from https://forum.qt.io/topic/126128/qapplication-in-std-thread -- Sorry, I messed up the order of posts when I did the fork; this top post should be further down]
@kshegunov said in QApplication in std::thread:
why would this generate a crash?
Since the widget creates a timer in the background, then it's probably receiving background events/signals.
deletecould cause the event/signal handler to act on a dangling pointer -- that's how it could generate a crash. And that's whydeleteLater()can fix the crash. (Alternatively, we could change from a heap-allocated widget to a stack-allocated widget)I'm pretty sure deferred deletes are processed after the event loop has exited (see for example the
QThread::finished->QObject::deleteLater)The code below prints "Bye" but not "Widget destroyed", proving that the Widget's destructor is not run:
// widget.h #include <QWidget> class Widget : public QWidget { Q_OBJECT public: Widget(QWidget *parent = nullptr) : QWidget(parent) {} ~Widget(){ qDebug("Widget destroyed"); } }; // main.cpp #include "widget.h" #include <QApplication> #include <chrono> #include <thread> using namespace std::chrono_literals; int main(int argc, char *argv[]) { std::thread thr([&] { QApplication a(argc, argv); auto w = new Widget; w->show(); a.exec(); // delete w; // This WILL run the Widget's destructor w->deleteLater(); // This WON'T run the Widget's destructor }); std::this_thread::sleep_for(5000ms); QMetaObject::invokeMethod(qApp, &QApplication::quit); thr.join(); std::this_thread::sleep_for(5000ms); qDebug("Bye"); }Anyway, I was trying to say that @Suthiro can't simply replace
delete m_qPlotsControl;withm_qPlotsControl->deleteLater();in-place at https://stackoverflow.com/questions/67257329/weird-qpushbutton-bug . Some re-shuffling is required.Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@kshegunov said in QApplication in std::thread:
I meant the trace from
QT_FATAL_WARNINGS, so we could see what was called leading to the mentioned warning.Nothing useful from GDB:
1 qt_message_fatal(QtMsgType, QMessageLogContext const&, QString const&) [clone .isra.4] 0x68e23630 2 ?? 0x3121a30@JKSH said in QApplication in std::thread:
When main() is about to return, the program tries to unload the timer. But the unloading is done by the main() thread, not the Qt thread. Hence the complaint, "QObject::~QObject: Timers cannot be stopped from another thread".
I really doubt this as the runtime doesn't run your destructors automatically.
I don't have a better explanation for this. Do you?:
#include <QApplication> #include <QPushButton> #include <QDebug> #include <chrono> #include <thread> int main(int argc, char *argv[]) { std::thread thr([&] { QApplication a(argc, argv); QPushButton btn("Push"); btn.show(); a.exec(); qDebug("Event loop stopped"); }); // Play with the QPushButton within the next 5 seconds using namespace std::chrono_literals; std::this_thread::sleep_for(5000ms); QMetaObject::invokeMethod(qApp, &QApplication::quit); thr.join(); qDebug("Thread stopped"); std::this_thread::sleep_for(5000ms); qDebug("Waiting, waiting..."); std::this_thread::sleep_for(5000ms); qDebug("Program shutting down"); }The output sequence is:
- (Pause 5s)
- "Event loop stopped"
- "Thread stopped"
- (Pause 5s)
- "Waiting, waiting..."
- (Pause 5s)
- "Program shutting down"
- "QObject::~QObject: Timers cannot be stopped from another thread"
If you don't interact with the button, then #8 doesn't appear. No difference if qDebug is switched to std::cout with flush.
In any case if that is the case, then it's a bug in the push button, which should be posted on the tracker.
QPushButton's timers aren't alone.
For example, the global QPixmapCache object hangs around after the destruction of QApplication too.
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@JKSH said in QApplication in std::thread:
@kshegunov said in QApplication in std::thread:
I meant the trace from
QT_FATAL_WARNINGS, so we could see what was called leading to the mentioned warning.Nothing useful from GDB:
1 qt_message_fatal(QtMsgType, QMessageLogContext const&, QString const&) [clone .isra.4] 0x68e23630 2 ?? 0x3121a30Did you run this with a release Qt or with
-developer-build? :)
(The address is rather odd, though)@JKSH said in QApplication in std::thread:
If you don't interact with the button, then #8 doesn't appear. No difference if qDebug is switched to std::cout with flush.
Can't reproduce:
09:01:47: Debugging starts Event loop stopped Thread stopped Waiting, waiting... Program shutting downWhat's your kit?
I have 5.15.2 from the repo with gcc 10.2.1. -
@JKSH this actually crashes for me on btn.show()
1 __pthread_kill (x86_64) /usr/lib/system/libsystem_kernel.dylib 0x7fff2030d462 2 pthread_kill (x86_64) /usr/lib/system/libsystem_pthread.dylib 0x7fff2033b610 3 abort (x86_64) /usr/lib/system/libsystem_c.dylib 0x7fff2028e720 4 abort_message (x86_64) /usr/lib/libc++abi.dylib 0x7fff20300418 5 demangling_terminate_handler() (x86_64) /usr/lib/libc++abi.dylib 0x7fff202f1a7d 6 _objc_terminate() (x86_64h) /usr/lib/libobjc.A.dylib 0x7fff201e79b1 7 std::__terminate(void ( *)()) (x86_64) /usr/lib/libc++abi.dylib 0x7fff202ff847 8 __cxxabiv1::failed_throw(__cxxabiv1::__cxa_exception *) (x86_64) /usr/lib/libc++abi.dylib 0x7fff20302167 9 __cxa_throw (x86_64) /usr/lib/libc++abi.dylib 0x7fff2030212e 10 objc_exception_throw (x86_64h) /usr/lib/libobjc.A.dylib 0x7fff201e54f7 11 -[NSException raise] (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation 0x7fff204d5a9a 12 -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit 0x7fff22c7597d 13 -[NSWindow _initContent:styleMask:backing:defer:contentView:] (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit 0x7fff22c606d5 14 -[NSWindow initWithContentRect:styleMask:backing:defer:] (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit 0x7fff22c6016f 15 -[NSWindow initWithContentRect:styleMask:backing:defer:screen:] (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit 0x7fff22f6a2c8 16 -[QNSWindow initWithContentRect:styleMask:backing:defer:screen:platformWindow:] (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/plugins/platforms/libqcocoa.dylib 0x10b335725 17 QCocoaWindow::createNSWindow(bool) (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/plugins/platforms/libqcocoa.dylib 0x10b322cac 18 QCocoaWindow::recreateWindowIfNeeded() (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/plugins/platforms/libqcocoa.dylib 0x10b31c672 19 QCocoaWindow::initialize() (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/plugins/platforms/libqcocoa.dylib 0x10b31c0fe 20 QWindowPrivate::create(bool, unsigned long long) (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/lib/QtGui.framework/Versions/5/QtGui 0x1007aceb7 21 QWidgetPrivate::create() (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/lib/QtWidgets.framework/Versions/5/QtWidgets 0x10016c771 22 QWidget::create(unsigned long long, bool, bool) (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/lib/QtWidgets.framework/Versions/5/QtWidgets 0x10016b414 23 QWidgetPrivate::setVisible(bool) (x86_64) /Users/jonashilk/Qt/5.15.2/clang_64/lib/QtWidgets.framework/Versions/5/QtWidgets 0x10017e81d 24 main::$_0::operator()() const main.cpp 13 0x100006538 25 decltype(std::forward<main::$_0>(fp)()) std::__invoke<main::$_0>(main::$_0&&) type_traits 3545 0x10000647d 26 void std::__thread_execute<std::unique_ptr<std::__thread_struct, std::default_delete<std::__thread_struct>>, main::$_0>(std::tuple<std::unique_ptr<std::__thread_struct, std::default_delete<std::__thread_struct>>, main::$_0>&, std::__tuple_indices<>) thread 273 0x1000063e5 27 void * std::__thread_proxy<std::tuple<std::unique_ptr<std::__thread_struct, std::default_delete<std::__thread_struct>>, main::$_0>>(void *) thread 284 0x100005c16 28 _pthread_start (x86_64) /usr/lib/system/libsystem_pthread.dylib 0x7fff2033b950 29 thread_start (x86_64) /usr/lib/system/libsystem_pthread.dylib 0x7fff2033747b
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@J-Hilk said in QApplication in std::thread:
@JKSH this actually crashes for me on btn.show()
That's expected. MacOS does not support plugging into the event loop from a thread different from
main(). Should work on linux and windows, though. -
@kshegunov said in QApplication in std::thread:
Did you run this with a release Qt or with
-developer-build? :)
(The address is rather odd, though)Ah, right... I thought the Debug build would suffice. Don't have a dev build right now.
Can't reproduce:
...
What's your kit?
I have 5.15.2 from the repo with gcc 10.2.1.Same result for me on Windows 10 20H2, with all official builds of:
- Qt 5.12.10 MSVC 2017 64-bit (using MSVC 2019 compiler)
- Qt 5.14.2 MinGW 7.3.0 32-bit
- Qt 5.15.2 MSVC 2019 64-bit
Might be Windows-specific.
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@JKSH said in QApplication in std::thread:
Ah, right... I thought the Debug build would suffice. Don't have a dev build right now.
Not once in a universe lifetime.
@JKSH said in QApplication in std::thread:
Might be Windows-specific.
Possibly. Unfortunately I don't have windows on hand (or rather I'm too lazy now to boot into it). Maybe, I'll check it later.
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@kshegunov Apple does a lot of funky stuff, in that regard, let me fire up my vm
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@kshegunov said in QApplication in std::thread:
Not once in a universe lifetime.
Duly noted :-D
But before we get too sidetracked: @Suthiro's main issue isn't the message about timers. Rather, it's the crash which I believe is caused by
delete m_qPlotsControl;(see https://stackoverflow.com/questions/67257329/weird-qpushbutton-bug ).I believe it is fixable with
m_qPlotsControl->deleteLater();... but that needs to be done before QApplication::exec() returns, because QDeferredDeleteEvent won't be processed without an event loop.Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@JKSH said in QApplication in std::thread:
I believe it is fixable with m_qPlotsControl->deleteLater();... but that needs to be done before QApplication::exec() returns, because QDeferredDeleteEvent won't be processed without an event loop.
I still don't understand this. For what reason should that be the case, and why would this generate a crash? I'm pretty sure deferred deletes are processed after the event loop has exited (see for example the
QThread::finished->QObject::deleteLater), but even if that weren't the case theQPushButton's destructor is going to be run before the thread actually finishes and before the application object's destructor's run (there's no difference betweenmain()and any other function from c++'s point of view). -
@JKSH said in QApplication in std::thread:
The code below prints "Bye" but not "Widget destroyed", proving that the Widget's destructor is not run:
I meant connecting to the
aboutToQuitsignal, but anyhow it's not that important.Since the widget creates a timer in the background, then it's probably receiving background events/signals. delete could cause the event/signal handler to act on a dangling pointer -- that's how it could generate a crash.
I think this shouldn't happen, but can't investigate it currently.
Anyway, I was trying to say that @Suthiro can't simply replace delete m_qPlotsControl; with m_qPlotsControl->deleteLater(); in-place at https://stackoverflow.com/questions/67257329/weird-qpushbutton-bug . Some re-shuffling is required.
I suppose, but that timer message should go away, otherwise the code can break at any moment (this applies to your code too - the one you mentioned where you get the warning).
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I reproduced this on Windows and the message about timers comes from a destructor of a static QPixmapCache object that happens on the main() thread. That's not something the user's code does. It's in Qt's internals.
Right, so remember the rule that no QObject should outlive the application object and this means no static QObjects? Apparently Qt itself doesn't respect that rule and also creates static objects that run code in the main thread no matter where the application object is created :/
I would consider this a Qt bug and expect it to crash all over the place.
EDIT: I missed that JKSH already mentioned this, but yeah, this seems to be the culprit.
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@Chris-Kawa said in QApplication in std::thread:
I reproduced this on Windows and the message about timers comes from a destructor of a static QPixmapCache object that happens on the main() thread. That's not something the user's code does. It's in Qt's internals.
Hm, interesting. Do you have a woboq location?
Right, so remember the rule that no QObject should outlive the application object and this means no static QObjects?
Apparently Qt itself doesn't respect that rule and also creates static objects that run code in the main thread no matter where the application object is created :/Yeah. Not living up to its own expectations it seems. :(
I would consider this a Qt bug and expect it to crash all over the place.
Me too, which is what I wrote somewhere upstairs - this should be fixed at the vendor side.
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@kshegunov said in QApplication in std::thread:
Hm, interesting. Do you have a woboq location?
Instance declaration is here:
https://code.woboq.org/qt5/qtbase/src/gui/image/qpixmapcache.cpp.html#pm_cacheAnd here's Windows with MSVC crash callstack on exit:
1 QPMCache::~QPMCache qpixmapcache.cpp 269 2 ``anonymous namespace'::Q_QGS_pm_cache::innerFunction'::`2'::Holder::~Holder Qt5Guid 3 ``anonymous namespace'::Q_QGS_pm_cache::innerFunction'::`2'::`dynamic atexit destructor for 'holder'' Qt5Guid 4 initterm_e ucrtbased 5 initterm_e ucrtbased 6 initterm_e ucrtbased 7 execute_onexit_table ucrtbased 8 __scrt_dllmain_uninitialize_c utility.cpp 399 9 dllmain_crt_process_detach dll_dllmain.cpp 182 10 dllmain_crt_dispatch dll_dllmain.cpp 220 11 dllmain_dispatch dll_dllmain.cpp 293 12 _DllMainCRTStartup dll_dllmain.cpp 335 13 RtlActivateActivationContextUnsafeFast ntdll 14 LdrShutdownProcess ntdll 15 RtlExitUserProcess ntdll 16 FatalExit KERNEL32 17 wassert ucrtbased 18 wassert ucrtbased 19 exit ucrtbased 20 __scrt_common_main_seh exe_common.inl 297I used this simplified code:
int main(int argc, char *argv[]) { std::thread thr([](int argc, char *argv[]) { QApplication a(argc, argv); QPushButton btn("Push"); btn.show(); a.exec(); }, argc, argv); thr.join(); } -
Yes, this is constructed appropriately, however the deallocation should've been tied to the
QCoreApplication::aboutToQuit. I doubt anyone is going to bother fixing it though, I'm not sure if it's not going to be discarded with "this is unsupported" even ... -
Good detective work, @Chris-Kawa!
I encountered this a few years ago but had forgotten most of the details.
@kshegunov said in Global static QPixmapCache in Qt internals:
Yes, this is constructed appropriately, however the deallocation should've been tied to the
QCoreApplication::aboutToQuit. I doubt anyone is going to bother fixing it though, I'm not sure if it's not going to be discarded with "this is unsupported" even ...Since a QPixmap requires a QGuiApplication, would a simple(?) and correct fix be to allocate the QPixmapCache in the QGuiApplication constructor, and deallocate it from the QGuiApplication destructor?
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Possibly. Or it can be a pseudo-singleton (similarly to
QCoreApplication) and be initialized on demand in the instance retrieving function (dropping the global static that it currently uses). You could poke Thiago, but as written, I wouldn't hold my breath. -
There are some reports about such an issue:
https://bugreports.qt.io/browse/QTBUG-48709
https://bugreports.qt.io/browse/QTBUG-21807And it does not crash here for me on Linux and Windows (MSVC, debug-build) with 5.15.x
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@Christian-Ehrlicher said in Global static QPixmapCache in Qt internals:
And it does not crash here for me on Linux and Windows (MSVC, debug-build) with 5.15.x
We are talking about the QTimers from another thread message. JKSH, split the threads. The other stuff (the supposed crash) contines to befuddle me.
