Qt run lambda on main thread [duplicate]

Question

In my application I have a worker thread iterating through a list of files and I'm trying to update the GUI table view to indicate what the current file is. In my worker thread I have the following line when I start a new file:

emit runOnGui([this,row](){ui.tblFiles->selectRow(row);});

where runOnGui is a signal and it's handled via:

connect(this, &MainWindow::runOnGui, [this](function<void()> action){
    action();
    });

My understanding is the action() should now be running on the UI thread but sometimes, not always, this is segfaulting on me. The backtrace looks like:

#0  0x00007ffff537c28a in ?? () from /usr/lib/libQt5Core.so.5
#1  0x00007ffff537cf23 in ?? () from /usr/lib/libQt5Core.so.5
#2  0x00007ffff537d047 in ?? () from /usr/lib/libQt5Core.so.5
#3  0x00007ffff5376943 in QItemSelection::merge(QItemSelection const&, QFlags<QItemSelectionModel::SelectionFlag>) () from /usr/lib/libQt5Core.so.5
#4  0x00007ffff5379e43 in QItemSelectionModel::select(QItemSelection const&, QFlags<QItemSelectionModel::SelectionFlag>) () from /usr/lib/libQt5Core.so.5
#5  0x00007ffff6d97511 in ?? () from /usr/lib/libQt5Widgets.so.5
#6  0x0000000000465193 in MainWindow::<lambda()>::operator()(void) const (__closure=0x7fffda262710) at <...>/MainWindow.cpp:248
#7  0x00000000004682c8 in std::_Function_handler<void(), MainWindow::pushFiles()::<lambda()> >::_M_invoke(const std::_Any_data &) (__functor=...) at /usr/include/c++/6.2.1/functional:1740
#8  0x0000000000472722 in std::function<void ()>::operator()() const (this=0x7fffda262710) at /usr/include/c++/6.2.1/functional:2136
#9  0x0000000000463798 in MainWindow::MainWindow(QWidget*)::{lambda(std::function<void ()>)#1}::operator()(std::function<void ()>) const () at <...>/MainWindow.cpp:34
#10 0x0000000000469a32 in QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<std::function<void()> >, void, MainWindow::MainWindow(QWidget*)::<lambda(std::function<void()>)> >::call(MainWindow::<lambda(std::function<void()>)> &, void **) (f=..., arg=0x7fffda2628e0) at /usr/include/qt/QtCore/qobjectdefs_impl.h:501
#11 0x000000000046977b in QtPrivate::Functor<MainWindow::MainWindow(QWidget*)::<lambda(std::function<void()>)>, 1>::call<QtPrivate::List<std::function<void()> >, void>(MainWindow::<lambda(std::function<void()>)> &, void *, void **) (f=..., arg=0x7fffda2628e0) at /usr/include/qt/QtCore/qobjectdefs_impl.h:558
#12 0x0000000000468d8c in QtPrivate::QFunctorSlotObject<MainWindow::MainWindow(QWidget*)::<lambda(std::function<void()>)>, 1, QtPrivate::List<std::function<void()> >, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=1, this_=0x7cc630, r=0x7fffffffe500, a=0x7fffda2628e0, ret=0x0) at /usr/include/qt/QtCore/qobject_impl.h:198
#13 0x00007ffff53ed85e in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib/libQt5Core.so.5
#14 0x000000000047a669 in MainWindow::runOnGui(std::function<void ()>) (this=0x7fffffffe500, _t1=...) at <...>/moc_MainWindow.cpp:128
#15 0x0000000000465617 in MainWindow::pushFiles (this=0x7fffffffe500) at <...>/MainWindow.cpp:248
#16 0x0000000000476d14 in std::__invoke_impl<void, void (MainWindow::* const&)(), MainWindow*>(std::__invoke_memfun_deref, void (MainWindow::* const&)(), MainWindow*&&) ( __f=@0xa35080: (void (MainWindow::*)(MainWindow * const)) 0x4651ea <MainWindow::pushFiles()>, __t=<unknown type in <...>/simtool, CU 0x16a5cd, DIE 0x1cbd99>) at /usr/include/c++/6.2.1/functional:235
#17 0x0000000000476ca1 in std::__invoke<void (MainWindow::* const&)(), MainWindow*>(void (MainWindow::* const&)(), MainWindow*&&) ( __fn=@0xa35080: (void (MainWindow::*)(MainWindow * const)) 0x4651ea <MainWindow::pushFiles()>, __args#0=<unknown type in <...>/simtool, CU 0x16a5cd, DIE 0x1cbd99>) at /usr/include/c++/6.2.1/functional:260
#18 0x0000000000476c52 in std::_Mem_fn_base<void (MainWindow::*)(), true>::operator()<MainWindow*>(MainWindow*&&) const (this=0xa35080, __args#0=<unknown type in <...>/simtool, CU 0x16a5cd, DIE 0x1cbd99>) at /usr/include/c++/6.2.1/functional:613
#19 0x0000000000476c1d in std::_Bind_simple<std::_Mem_fn<void (MainWindow::*)()> (MainWindow*)>::_M_invoke<0ul>(std::_Index_tuple<0ul>) (this=0xa35078) at /usr/include/c++/6.2.1/functional:1400
#20 0x0000000000476b37 in std::_Bind_simple<std::_Mem_fn<void (MainWindow::*)()> (MainWindow*)>::operator()() ( this=0xa35078) at /usr/include/c++/6.2.1/functional:1389
#21 0x0000000000476ad6 in std::thread::_State_impl<std::_Bind_simple<std::_Mem_fn<void (MainWindow::*)()> (MainWindow*)> >::_M_run() (this=0xa35070) at /usr/include/c++/6.2.1/thread:196
#22 0x00007ffff4e6e31f in std::execute_native_thread_routine (__p=0xa35070) at /build/gcc/src/gcc/libstdc++-v3/src/c++11/thread.cc:83
#23 0x00007ffff743f454 in start_thread () from /usr/lib/libpthread.so.0
#24 0x00007ffff45e27df in clone () from /usr/lib/libc.so.6

Any ideas why this segfault is happening?

Misc info:

• Qt 5.7
• Arch Linux 4.7.4
• gcc 6.2.1

Answer 1

The automatism for crossing thread boundaries in signal/slot connections requires a receiver object, so that the emitting code can check if the current thread is equal or different to the thread of the receiver.

You could try the connect variant that has a reference object as the third argument or calling the slot with QMetaObject::invokeMethod() and explicitly stating Qt::QueuedConnection as the connection type.

E.g.

QMetaObject::invokeMethod(ui.tblFiles, "selectRow", Qt::QueuedConnection, Q_ARG(int, row));