slot valueChanged(double) of doubleSpinBox called twice if breakpoint is set

Hi there,

I created a simple GUI Application using QT Creator 2.6 and MSVC2010 with CDB on a Win7 SP1 32-Bit system.

The QUI was created by QT Designer and has a doubleSpinBox. I setup the slot

Code:

---

valueChanged(double)

---

of the doubleSpinBox. The slot is connected via

Code:

---

connectSlotsByName

---

.

Everything works fine, but if I set a breakpoint in the slot

Code:

---

doubleSpinBox_valueChanged(double arg1)
{
}

---

there is a strange behaviour:

On a click on the arrows of the doubleSpinbox the slot

Code:

---

doubleSpinBox_valueChanged(double arg1)

---

is called TWICE and of course the value-property of the doubleSpinBox is incremented/decremented twice!
I verified this by adding a

Code:

---

qDebug << "doubleSpinBox_valueChanged hit"

---

to the slot.




Is this a known issue?

Thanks for your help in advance!

valueChanged() signal does not cause anything to be incremented or decremented. Could you explain better what you mean?

Hi wysota,

sorry if didn't make things clear. I will try to do it better:

I set a breakpoint in the slot valueChanged(double) of the doubleSpinBox. When I debug my GUI application and I click on the arrow-up-icon of the doubleSpinbox, then the breakpoint in the valueChanged(double)-Slot is hit and the old value is incremented as it should. But by clicking on F5 to continue the debugging, the breakpoint is hit again and the value of the doubleSpinBox is incremented again, i.e. twice!
As I sad, I also added a qDebug << "doubleSpinBox_valueChanged hit" in the valueChanged(double)-Slot and the message appears twice in the debugger output!

This happens only if a breakpoint is set in the valueChanged(double)-Slot. When I debug my GUI application without setting a breakpoint in the Slot, then by clickung on the arrow-up-icon the value is incremented only once and of course my qDebug-Message (as menitioned before) appears only once.

I hope that things became more clear.
Thanks for your help!

Quote:

---

Originally Posted by Marco W.

I set a breakpoint in the slot valueChanged(double) of the doubleSpinBox.

---

There is no such slot. valueChanged() is a signal. Make up your mind -- are you setting a breakpoint in the valueChanged() signal in QDoubleSpinBox or in a on_doubleSpinBox_valueChanged slot connected to that signal?

Oh, ok maybe I mixed it up.

I created a slot for the valueChanged() signal by right-clicking on the doubleSpinBox and selecting "Go to slot..." in QT Designer. The corresponding Signal and Slot are connected via QMetaObject::connectSlotsByName(MainWindow). This happens in the header file for the GUI (created by QT Designer).

So, in fact, it is the on_doubleSpinBox_valueChanged(double arg1) slot where I set my breakpoint.

Sorry for this inaccuracy and again thanks for helping me.

In that case it is not possible that the slot is executed twice if you set a breakpoint on it. If the slot is executing twice then it means you have two connections to it.

Well, but the slot is executed twice (and the value of the QdoubleSpinBox is incremented twice), if I set a breakpoint on it. I verified this by adding the qDebug() and printing a message to the debugger output.

If there is no breakpoint set, then the message is printed only once (and the value of the QdoubleSpinBox is incremented only once).


Is it possible that my Debugging Setup has some problems? Sometimes I get a message like:

*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Cored.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Widgetsd.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Guid.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\plugins\platforms\qwi ndowsd.dll

or

*** WARNING: Unable to verify checksum for MyApplicationName.exe

or (because I'm doing some openCV stuff)

*** WARNING: Unable to verify checksum for C:\opencv243\build\x86\vc10\bin\opencv_highgui243d .dll
*** ERROR: Symbol file could not be found. Defaulted to export symbols for C:\opencv243\build\x86\vc10\bin\opencv_highgui243d .dll -
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Guid.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\libGLESv2d.dll
*** ERROR: Symbol file could not be found. Defaulted to export symbols for C:\Qt\Qt5.0.1\5.0.1\msvc2010\bin\icuuc49.dll -
*** WARNING: Unable to verify checksum for MyApplicationName.exe

Thanks

Quote:

---

Originally Posted by Marco W.

Well, but the slot is executed twice (and the value of the QdoubleSpinBox is incremented twice), if I set a breakpoint on it. I verified this by adding the qDebug() and printing a message to the debugger output.

---

That's no proof. Show me the backtrace which shows that the slot is called twice.

Hm, I'd like to, but I don't have a clue how to do that. Please, could you give a link / tutorial / some advice how to do a backtrace? Thanks

---

Added after 42 minutes:

This is, what is shown on the debugger log, when I click on the arrow-up-symbol on the QdoubleSpinbox with a breakpoint set in on_valueChanged(double) slot:


Breakpoint 1 hit
eax=0031c370 ebx=00000000 ecx=0031fbc0 edx=0031c4d4 esi=00000401 edi=0031dbd4
eip=009b1418 esp=0031c344 ebp=0031c37c iopl=0 nv up ei pl nz na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000206
> 20: int temp=0;
testslider!MainWindow::on_doubleSpinBox_valueChang ed+0x28:
20 009b1418 c745f000000000 mov dword ptr [ebp-10h],0 ss:0023:0031c36c=0031c378

*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Cored.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Widgetsd.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\lib\Qt5Guid.dll
*** WARNING: Unable to verify checksum for C:\Qt\Qt5.0.1\5.0.1\msvc2010\plugins\platforms\qwi ndowsd.dll
sAn Haltepunkt 1 (1) im Thread 0 angehalten.
dNOTE: INFERIOR SPONTANEOUS STOP
sAngehalten.
dState changed from InferiorRunOk(11) to InferiorStopOk(14) [master]
<!qtcreatorcdbext.locals -t 2 -D -e inspect,local,return,watch -v -c -W 0
DEBUG: Discarding watchers
DEBUG: expandEntrySet local


/////////THEN I PRESSED F5


dNOTE: INFERIOR RUN REQUESTED
sAusführung angefordert...
dState changed from InferiorStopOk(14) to InferiorRunRequested(10) [master]
<g
dNOTE: INFERIOR RUN OK
sLäuft.
dState changed from InferiorRunRequested(10) to InferiorRunOk(11) [master]
Breakpoint 1 hit
eax=0031cf44 ebx=00000000 ecx=0031fbc0 edx=0031d0a8 esi=00000113 edi=0031dbd4
eip=009b1418 esp=0031cf18 ebp=0031cf50 iopl=0 nv up ei pl nz na po nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000202
> 20: int temp=0;
testslider!MainWindow::on_doubleSpinBox_valueChang ed+0x28:
20 009b1418 c745f000000000 mov dword ptr [ebp-10h],0 ss:0023:0031cf40=0031cf4c
sAn Haltepunkt 1 (1) im Thread 0 angehalten.
dNOTE: INFERIOR SPONTANEOUS STOP
sAngehalten.
dState changed from InferiorRunOk(11) to InferiorStopOk(14) [master]
<!qtcreatorcdbext.locals -t 3 -D -e inspect,local,return,watch -v -c -W 0
DEBUG: Discarding watchers
DEBUG: expandEntrySet local


//THEN I PRESSED F5 AGAIN


dNOTE: INFERIOR RUN REQUESTED
sAusführung angefordert...
dState changed from InferiorStopOk(14) to InferiorRunRequested(10) [master]
<g

dNOTE: INFERIOR RUN OK
sLäuft.
dState changed from InferiorRunRequested(10) to InferiorRunOk(11) [master]


So, is it that, what you requested? I hope so. Thanks again.

WIthout a backtrace (also called stacktrace) there is nothing I can do for you.

Please, could you give a link / tutorial / some advice how to do a backtrace? Thanks

Marco, wysota is talking about the window that in VS2010 is called Call Stack. He is asking where the two calls to valueChanged are being made from.

Have you, in VS2010, enabled the MS symbol import (Options/Debug/Symbols) - it will give the debugger a lot more info and make the Call stack more useful even for Debug configuration.

The first thing I would do is verify that - without a breakpoint involved - a call to OutputDebugString("stuff") within your on_doubleSpinBox_valueChanged() function results in two lines being printed in the debugger Output window. If that happens, then I would strongly suggest you take wysota's hint and triple-check that you haven't managed to connect the signal and slot together twice. That is by far the most likely explanation for seeing.

If you don't get two lines in the Output window, the call is not happening twice... something else is happening.

Yes, but I am using QT Creator, so my question was, how to get this in QT Creator =/

Is the following the content, that you asked for? It is shown right after the breakpoint in the on_dblSpinBoxThresh2_valueChanged is hit the first time.

Code:

---

0	cPMCannyWidget::on_dblSpinBoxThresh2_valueChanged	cpmcannywidget.cpp	186	0x111441a	
1	cPMCannyWidget::qt_static_metacall	moc_cpmcannywidget.cpp	120	0x1118a12	
2	cPMCannyWidget::qt_metacall	moc_cpmcannywidget.cpp	165	0x1118bdb	
3	QMetaObject::metacall	qmetaobject.cpp	308	0x66ad2230	
4	QMetaObject::activate	qobject.cpp	3456	0x66b1628f	
5	QMetaObject::activate	qobject.cpp	3316	0x66b15c11	
6	QDoubleSpinBox::valueChanged	moc_qspinbox.cpp	411	0x64e167ab	
7	QDoubleSpinBoxPrivate::emitSignals	qspinbox.cpp	1074	0x64b8f722	
8	QAbstractSpinBoxPrivate::setValue	qabstractspinbox.cpp	1678	0x64ad079d	
9	QAbstractSpinBox::stepBy	qabstractspinbox.cpp	617	0x64acd702	
10	QAbstractSpinBoxPrivate::updateState	qabstractspinbox.cpp	1571	0x64ad0232	
11	QAbstractSpinBox::mousePressEvent	qabstractspinbox.cpp	1283	0x64acf363	
12	QWidget::event	qwidget.cpp	7831	0x649ca039	
13	QAbstractSpinBox::event	qabstractspinbox.cpp	752	0x64acdb30	
14	QApplicationPrivate::notify_helper	qapplication.cpp	3394	0x649828ce	
15	QApplication::notify	qapplication.cpp	2958	0x6498093c	
16	QCoreApplication::notifyInternal	qcoreapplication.cpp	767	0x66ac74c4	
17	QCoreApplication::sendSpontaneousEvent	qcoreapplication.h	206	0x66bd7198	
18	QApplicationPrivate::sendMouseEvent	qapplication.cpp	2465	0x6497fa65	
19	QWidgetWindow::handleMouseEvent	qwidgetwindow.cpp	387	0x649fdbc0	
20	QWidgetWindow::event	qwidgetwindow.cpp	139	0x649fd134	
21	QApplicationPrivate::notify_helper	qapplication.cpp	3394	0x649828ce	
22	QApplication::notify	qapplication.cpp	2825	0x6498030a	
23	QCoreApplication::notifyInternal	qcoreapplication.cpp	767	0x66ac74c4	
24	QCoreApplication::sendSpontaneousEvent	qcoreapplication.h	206	0x66bd7198	
25	QGuiApplicationPrivate::processMouseEvent	qguiapplication.cpp	1425	0x7efe64	
26	QGuiApplicationPrivate::processWindowSystemEvent	qguiapplication.cpp	1257	0x7ef6d2	
27	QWindowSystemInterface::sendWindowSystemEventsImplementation	qwindowsysteminterface.cpp	536	0x7d8541	
28	QWindowSystemInterface::sendWindowSystemEvents	qwindowsysteminterface.cpp	516	0x7d84c9	
29	QWindowsGuiEventDispatcher::sendPostedEvents	qwindowsguieventdispatcher.cpp	86	0xf69aff4	
30	qt_internal_proc	qeventdispatcher_win.cpp	423	0x66b4fb07	
31	InternalCallWinProc	USER32		0x76bec4e7	
32	UserCallWinProcCheckWow	USER32		0x76bec5e7	
33	DispatchMessageWorker	USER32		0x76becc19	
34	DispatchMessageW	USER32		0x76becc70	
35	QEventDispatcherWin32::processEvents	qeventdispatcher_win.cpp	744	0x66b50bf3	
36	QWindowsGuiEventDispatcher::processEvents	qwindowsguieventdispatcher.cpp	78	0xf69af5e	
37	QEventLoop::processEvents	qeventloop.cpp	137	0x66ac40c1	
38	QEventLoop::exec	qeventloop.cpp	212	0x66ac421e	
39	QCoreApplication::exec	qcoreapplication.cpp	1020	0x66ac79fd	
40	QGuiApplication::exec	qguiapplication.cpp	1192	0x7ef468	
41	QApplication::exec	qapplication.cpp	2670	0x6497ffb9	
42	main	main.cpp	13	0x110242e	
43	WinMain	qtmain_win.cpp	131	0x111a8ea	
44	__tmainCRTStartup	crtexe.c	547	0x1119e10	
45	WinMainCRTStartup	crtexe.c	371	0x1119b9f	
46	BaseThreadInitThunk	kernel32		0x7701ed6c	
47	__RtlUserThreadStart	ntdll		0x7727377b	
48	_RtlUserThreadStart	ntdll		0x7727374e

---

On the second time, the breakpoint is hit:

Code:

---

0	cPMCannyWidget::on_dblSpinBoxThresh2_valueChanged	cpmcannywidget.cpp	186	0x111441a	
1	cPMCannyWidget::qt_static_metacall	moc_cpmcannywidget.cpp	120	0x1118a12	
2	cPMCannyWidget::qt_metacall	moc_cpmcannywidget.cpp	165	0x1118bdb	
3	QMetaObject::metacall	qmetaobject.cpp	308	0x66ad2230	
4	QMetaObject::activate	qobject.cpp	3456	0x66b1628f	
5	QMetaObject::activate	qobject.cpp	3316	0x66b15c11	
6	QDoubleSpinBox::valueChanged	moc_qspinbox.cpp	411	0x64e167ab	
7	QDoubleSpinBoxPrivate::emitSignals	qspinbox.cpp	1074	0x64b8f722	
8	QAbstractSpinBoxPrivate::setValue	qabstractspinbox.cpp	1678	0x64ad079d	
9	QAbstractSpinBox::stepBy	qabstractspinbox.cpp	617	0x64acd702	
10	QAbstractSpinBox::timerEvent	qabstractspinbox.cpp	1182	0x64acee20	
11	QObject::event	qobject.cpp	1052	0x66b10ab3	
12	QWidget::event	qwidget.cpp	8235	0x649cae59	
13	QAbstractSpinBox::event	qabstractspinbox.cpp	752	0x64acdb30	
14	QApplicationPrivate::notify_helper	qapplication.cpp	3394	0x649828ce	
15	QApplication::notify	qapplication.cpp	3359	0x64982450	
16	QCoreApplication::notifyInternal	qcoreapplication.cpp	767	0x66ac74c4	
17	QCoreApplication::sendEvent	qcoreapplication.h	203	0x66acc849	
18	QEventDispatcherWin32Private::sendTimerEvent	qeventdispatcher_win.cpp	572	0x66b4ffd8	
19	qt_internal_proc	qeventdispatcher_win.cpp	427	0x66b4fb5a	
20	InternalCallWinProc	USER32		0x76bec4e7	
21	UserCallWinProcCheckWow	USER32		0x76bec5e7	
22	DispatchMessageWorker	USER32		0x76becc19	
23	DispatchMessageW	USER32		0x76becc70	
24	QEventDispatcherWin32::processEvents	qeventdispatcher_win.cpp	744	0x66b50bf3	
25	QWindowsGuiEventDispatcher::processEvents	qwindowsguieventdispatcher.cpp	78	0xf69af5e	
26	QEventLoop::processEvents	qeventloop.cpp	137	0x66ac40c1	
27	QEventLoop::exec	qeventloop.cpp	212	0x66ac421e	
28	QCoreApplication::exec	qcoreapplication.cpp	1020	0x66ac79fd	
29	QGuiApplication::exec	qguiapplication.cpp	1192	0x7ef468	
30	QApplication::exec	qapplication.cpp	2670	0x6497ffb9	
31	main	main.cpp	13	0x110242e	
32	WinMain	qtmain_win.cpp	131	0x111a8ea	
33	__tmainCRTStartup	crtexe.c	547	0x1119e10	
34	WinMainCRTStartup	crtexe.c	371	0x1119b9f	
35	BaseThreadInitThunk	kernel32		0x7701ed6c	
36	__RtlUserThreadStart	ntdll		0x7727377b	
37	_RtlUserThreadStart	ntdll		0x7727374e

---

Thanks

As you can see in your second trace, there is just a single call to your slot method. The slot is not calling itself. It could be that the slot is called, then it returns and is called again but it doesn't happen as a result of the breakpoint. There is a timer event here so maybe it fires twice because stopping in the breakpoint and not continuing fast enough causes the timer to fire again. That's perfectly understandable.

Ok, thanks wysota!

The explanation concerning the timer is understandable. However, I wonder why the timer triggers only once again, and not several times: if I wait for several seconds before continuing the second break, the slot is not called again.

Is there a documentation for this, or in general, how the processing of a click on the up/down-arrow-button of a QdoubleSpinBox is done?

Quote:

---

Originally Posted by Marco W.

However, I wonder why the timer triggers only once again, and not several times

---

This is easy to explain. Timers fire as a result of event processing. When the timer fires for the first time, you block the event loop with the breakpoint. When you continue the flow returns to the event loop and the timer gets fired again. It's not that if you wait for 100 seconds and the timeout for the timer is 10 seconds, you should get 10 timeouts. You will get just one.

Ok, thanks, but I still do not understand why a timer is fired as a result of event processing and why a timer is started by clicking on the up-arrow of the QdoubleSpinBox.

After all, the result, that having a breakpoint set leads to a different behaviour than having no breakpoint set, is still strange to me.

Quote:

---

Originally Posted by Marco W.

Ok, thanks, but I still do not understand why a timer is fired as a result of event processing

---

That's how it works and you have to live with it.

Quote:

---

and why a timer is started by clicking on the up-arrow of the QdoubleSpinBox.

---

A timer is started as a result of pressing the arrow so that Qt can keep incrementing the value while you're keeping the arrow pressed.

Quote:

---

After all, the result, that having a breakpoint set leads to a different behaviour than having no breakpoint set, is still strange to me.

---

Again, you have to live with it.