Access violation while alt-tabbing from an actively updated plot widget

Hello,

I'm using Qt 4.5.1 and qwt 5.2.0 on Windows XP (Eclipse, g++). I have a graphing widget which inherits from QwtPlot. This is used to display often updated measurement data which is contained in a single curve. The graphing widget itself is refreshed with update() in the main thread using a timer, and the actual curve data is set from a measurement thread. Access to these functions is behind a single mutex. The plot also uses QwtPlotzoomer. The application seems to work fine when I don't disturb it while the plot is being updated, but I'm bumping to an access violation somewhat randomly if I alt-tab back on the application during a wrong time.

If I run the application under gdb, I also see the following warnings when I alt-tab in/out:

Code:

---

warning: QWidget::repaint: Recursive repaint detected
 
warning: QPainter::begin: Paint device returned engine == 0, type: 1

---

I have no repaint()s in the application itself, so they are performed somewhere within q(w)t. Also autoReplot is turned off.

The backtrace from Dr. Mingw is contained in the following post (character limit). gdb backtrace is not included due to some symbol-resolving problem.

Code:

---

meas.exe caused an Access Violation at location 1009a3a5 in module qwt5.dll Reading from location 09230028.
 
Registers:
eax=09230020 ebx=0022bc40 ecx=6a307128 edx=100afe38 esi=0000000a edi=0022c4dc
eip=1009a3a5 esp=0022b8d0 ebp=0022b8e8 iopl=0         nv up ei pl nz na pe nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00000202
 
Call stack:
AddrPC     AddrReturn AddrFrame  AddrStack  Params
1009A3A5   100278BD   0022B8E8   0022B8D0   06A09EF8   0001068F   0022B908   10090B89
1009A3A5  qwt5.dll:1009A3A5  QVector<double>::operator[]  qvector.h:333
	...
	template <typename T>
	inline const T &QVector<T>::operator[](int i) const
>	{ Q_ASSERT_X(i >= 0 && i < d->size, "QVector<T>::operator[]", "index out of range");
	  return d->array[i]; }
	template <typename T>
	...
 
100278BD   10090B89   0022B8F8   0022B8D0   06A09EF0   0001068F   0022BA18   10034A59
100278BD  qwt5.dll:100278BD  QwtArrayData::y  qwt_data.cpp:210
	...
	double QwtArrayData::y(size_t i) const 
	{ 
>	    return d_y[int(i)]; 
	}
 
	...
 
10090B89   10034A59   0022B908   0022B8D0   069F2850   0001068F   3F019EB8   000001D1
10090B89  qwt5.dll:10090B89E:\projects\meas\debug\qwt5.dll: No symbol found
  _ZNK12QwtPlotCurve1yEi
10034A59   1003435D   0022BA18   0022B8D0   069F2850   0022BDE0   0022BCA0   0022BC40
10034A59  qwt5.dll:10034A59  QwtPlotCurve::drawLines  qwt_plot_curve.cpp:815
	...
	            {
	                int xi = xMap.transform(x(i));
>	                int yi = yMap.transform(y(i));
 
	                polyline.setPoint(i - from, xi, yi);
	...
 
1003435D   10034230   0022BA48   0022B8D0   069F2850   0022BDE0   00000001   0022BCA0
1003435D  qwt5.dll:1003435D  QwtPlotCurve::drawCurve  qwt_plot_curve.cpp:685
	...
	            }
	            drawLines(painter, xMap, yMap, from, to);
>	            break;
	        case Sticks:
	            drawSticks(painter, xMap, yMap, from, to);
	...
 
10034230   10033A03   0022BAD8   0022B8D0   069F2850   0022BDE0   0022BCA0   0022BC40
10034230  qwt5.dll:10034230  QwtPlotCurve::draw  qwt_plot_curve.cpp:648
	...
 
	        drawCurve(painter, d_data->style, xMap, yMap, from, to);
>	        painter->restore();
 
	        if (d_data->symbol->style() != QwtSymbol::NoSymbol)
	...
 
10033A03   1002DF98   0022BAF8   0022B8D0   069F2850   0022BDE0   0022BCA0   0022BC40
10033A03  qwt5.dll:10033A03  QwtPlotCurve::draw  qwt_plot_curve.cpp:521
	...
	{
	    draw(painter, xMap, yMap, 0, -1);
>	}
 
	/*!
	...
 
1002DF98   1002DCD2   0022BB68   0022B8D0   069E7D40   0022BDE0   0022BC10   0022BC40
1002DF98  qwt5.dll:1002DF98  QwtPlot::drawItems  qwt_plot.cpp:580
	...
	                rect);
 
>	            painter->restore();
	        }
	    }
	...
 
1002DCD2   10047690   0022BD18   0022B8D0   069E7D40   0022BDE0   C0000000   00000000
1002DCD2  qwt5.dll:1002DCD2  QwtPlot::drawCanvas  qwt_plot.cpp:540
	...
	        maps[axisId] = canvasMap(axisId);
 
>	    drawItems(painter, 
	        d_data->canvas->contentsRect(), maps, QwtPlotPrintFilter());
	}
	...
 
10047690   1004728C   0022BE18   0022B8D0   069F2278   0022BF30   00000000   FFFFFFFF
10047690  qwt5.dll:10047690  QwtPlotCanvas::drawCanvas  qwt_plot_canvas.cpp:337
	...
	        ((QwtPlot *)parent())->drawCanvas(&cachePainter);
 
>	        cachePainter.end();
 
	        painter->drawPixmap(contentsRect(), *d_data->cache);
	...
 
1004728C   100470E5   0022BE88   0022B8D0   069F2278   0022BF30   00000001   0000004D
1004728C  qwt5.dll:1004728C  QwtPlotCanvas::drawContents  qwt_plot_canvas.cpp:275
	...
	        drawCanvas(painter);
 
>	        plot->setAutoReplot(doAutoReplot);
	    }
 
	...
 
100470E5   00E3888D   0022BF58   0022B8D0   069F2278   0022C5F0   0022BFC8   00DEA420
100470E5  qwt5.dll:100470E5  QwtPlotCanvas::paintEvent  qwt_plot_canvas.cpp:252
	...
	#endif
 
>	    if ( d_data->paintAttributes & PaintPacked )
	        setSystemBackground(false);
	}
	...
 
00E3888D   0127F743   0022C0C8   0022B8D0   069F2278   0022C5F0   00000000   00000001
00E3888D  QtGuid4.dll:00E3888D
adjusting sections from 0x68f40000 to 0x00dc0000, 0x97e80000
E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN7QWidget5eventEP6QEvent
0127F743   00DD85CD   0022C0E8   0022B8D0   069F2278   0022C5F0   0022C5F0   05B14CC0
0127F743  QtGuid4.dll:0127F743E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN6QFrame5eventEP6QEvent
00DD85CD   00DD8444   0022C118   0022B8D0   05B14DE8   069F2278   0022C5F0   7C9100B8
00DD85CD  QtGuid4.dll:00DD85CDE:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent
00DD8444   6A242200   0022C468   0022B8D0   0022FDE0   069F2278   0022C5F0   00000312
00DD8444  QtGuid4.dll:00DD8444E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN12QApplication6notifyEP7QObjectP6QEvent
6A242200   015BDF58   0022C508   0022B8D0   0022FDE0   069F2278   0022C5F0   0000004D
6A242200  QtCored4.dll:6A242200  QCoreApplication::notifyInternal  qcoreapplication.cpp:610
	...
	    bool returnValue;
	    try {
>	        returnValue = notify(receiver, event);
	    } catch(...) {
	        --threadData->loopLevel;
	...
 
015BDF58   00E2FD10   0022C528   0022B8D0   069F2278   0022C5F0   0022C790   0000000C
015BDF58  QtGuid4.dll:015BDF58E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN16QAccessibleEvent8setValueERK7QString
00E2FD10   00FF168A   0022C6C8   0022B8D0   069F2298   06A1C650   0022C8A0   0022C790
00E2FD10  QtGuid4.dll:00E2FD10E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN14QWidgetPrivate10drawWidgetEP12QPaintDeviceRK7QRegionRK6QPointiP8QPainterP19QWidgetBackingStore
00FF168A   00E2525E   0022C928   0022B8D0   06A0D128   0000000A   0022C958   01563590
00FF168A  QtGuid4.dll:00FF168AE:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN14QWidgetPrivate10scrollRectERK5QRectii
00E2525E   00E38F79   0022C9A8   0022B8D0   05B185D0   6A2A51D4   6A302228   0022C9E0
00E2525E  QtGuid4.dll:00E2525EE:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN14QWidgetPrivate16syncBackingStoreEv
00E38F79   012A62F0   0022CB18   0022B8D0   0022F250   06A095F0   00E040A9   00000000
00E38F79  QtGuid4.dll:00E38F79E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN7QWidget5eventEP6QEvent
012A62F0   00DD85CD   0022CC08   0022B8D0   0022F250   06A095F0   06A095F0   05B14CC0
012A62F0  QtGuid4.dll:012A62F0E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN11QMainWindow5eventEP6QEvent
00DD85CD   00DD8444   0022CC38   0022B8D0   05B14DE8   0022F250   06A095F0   6A2F7B69
00DD85CD  QtGuid4.dll:00DD85CDE:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent
00DD8444   6A242200   0022CF88   0022B8D0   0022FDE0   0022F250   06A095F0   6A2B5729
00DD8444  QtGuid4.dll:00DD8444E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN12QApplication6notifyEP7QObjectP6QEvent
6A242200   6A2B79FB   0022D028   0022B8D0   0022FDE0   0022F250   06A095F0   6A2B2CFC
6A242200  QtCored4.dll:6A242200  QCoreApplication::notifyInternal  qcoreapplication.cpp:610
	...
	    bool returnValue;
	    try {
>	        returnValue = notify(receiver, event);
	    } catch(...) {
	        --threadData->loopLevel;
	...
 
6A2B79FB   6A24339B   0022D048   0022B8D0   0022F250   06A095F0   7FFDD000   0022D0EC
6A2B79FB  QtCored4.dll:6A2B79FBE:\soft\Qt\4.5.1\bin\QtCored4.dll: No symbol found
  _ZN16QCoreApplication9sendEventEP7QObjectP6QEvent
6A24339B   6A2731BE   0022D148   0022B8D0   00000000   00000000   05B145F8   000000FF
6A24339B  QtCored4.dll:6A24339B  QCoreApplicationPrivate::sendPostedEvents  qcoreapplication.cpp:1247
	...
	#else
	        try {
>	            QCoreApplication::sendEvent(r, e);
	        } catch (...) {
	            delete e;
	...
 
6A2731BE   00E4715C   0022EFA8   0022B8D0   05B14C28   0022EFD0   05B145F8   6A17A207
6A2731BE  QtCored4.dll:6A2731BE  QEventDispatcherWin32::processEvents  qeventdispatcher_win.cpp:681
	...
	        QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
 
>	        DWORD waitRet = 0;
	        HANDLE pHandles[MAXIMUM_WAIT_OBJECTS - 1];
	        QVarLengthArray<MSG> processedTimers;
	...
 
00E4715C   6A23F38E   0022EFE8   0022B8D0   05B14C28   0022F010   00000010   00000002
00E4715C  QtGuid4.dll:00E4715CE:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _Z25qWinProcessConfigRequestsv
6A23F38E   6A23F510   0022F038   0022B8D0   0022F170   0022F0B0   00000020   6A25770E
6A23F38E  QtCored4.dll:6A23F38E  QEventLoop::processEvents  qeventloop.cpp:149
	...
	    if (flags & DeferredDeletion)
	        QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
>	    return d->threadData->eventDispatcher->processEvents(flags);
	}
 
	...
 
6A23F510   6A2428D1   0022F0E8   0022B8D0   0022F170   0022F150   00000000   00000000
6A23F510  QtCored4.dll:6A23F510  QEventLoop::exec  qeventloop.cpp:200
	...
	    try {
	        while (!d->exit)
>	            processEvents(flags | WaitForMoreEvents | EventLoopExec);
	    } catch (...) {
	        qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"
	...
 
6A2428D1   00DD6364   0022F1A8   0022B8D0   0022FDE0   00000001   0022FE08   0040F703
6A2428D1  QtCored4.dll:6A2428D1  QCoreApplication::exec  qcoreapplication.cpp:888
	...
	    self->d_func()->in_exec = true;
	    self->d_func()->aboutToQuitEmitted = false;
>	    int returnCode = eventLoop.exec();
	    threadData->quitNow = false;
	    if (self) {
	...
 
00DD6364   0040F703   0022F1B8   0022B8D0   0022F250   0048B4C4   0022F264   00040501
00DD6364  QtGuid4.dll:00DD6364E:\soft\Qt\4.5.1\bin\QtGuid4.dll: No symbol found
  _ZN12QApplication4execEv
0040F703   00432FFD   0022FE08   0022B8D0   00000001   05B14DC0   05B14D38   0000000A
0040F703  meas.exe:0040F703  qMain  main.cpp:30

---

Some possibly relevant code snippets:

Code:

---

void Graph::paintEvent(QPaintEvent *event) {
 
	if (mutex.tryLock()) {
			replot();
			QwtPlot::paintEvent(event);
 
			mutex.unlock();
	}
}

---

Code:

---

Graph::Graph(QWidget *parent): QwtPlot(parent) {
	mutex.lock();
 
    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(refresh()));
    timer->start(GRAPH_REDRAW_TIMER);
 
    setCanvasBackground(QColor(29, 100, 141));
    setFrameStyle(QFrame::NoFrame);
    setLineWidth(0);
    setCanvasLineWidth(2);
    plotLayout()->setAlignCanvasToScales(true);
 
    QwtPlotGrid *grid = new QwtPlotGrid;
    grid->setMajPen(QPen(Qt::gray, 0, Qt::DashLine));
    grid->setMinPen(QPen(Qt::gray, 0, Qt::DotLine));
    grid->enableXMin(true);
    grid->attach(this);
 
    plotLayout()->setAlignCanvasToScales(true);
 
    crv = new QwtPlotCurve();
    crv->setPen(QColor(Qt::white));
    crv->setStyle(QwtPlotCurve::Lines);
    crv->attach(this);
 
	zoomer = new QwtPlotZoomer(canvas(), true);
    zoomer->setRubberBandPen(QPen(Qt::white, 2, Qt::DotLine));
    zoomer->setTrackerPen(QPen(Qt::white));
 
	connect(zoomer,	SIGNAL(zoomed(const QwtDoubleRect &)),
			this,   SLOT(catchZoomed(const QwtDoubleRect &)));
    //canvas()->setAttribute(Qt::WA_PaintOutsidePaintEvent, true);
    zoomer->setZoomBase();
 
    setAutoReplot(false);
	mutex.unlock();
}

---

Code:

---

void Graph::refresh() {
	mutex.lock();
//	qDebug("refresh here");
	update();
	//replot();
	mutex.unlock();
}

---

Code:

---

void Graph::catchZoomed(const QwtDoubleRect &rect) {
	mutex.lock();
	if (zoomer->zoomRectIndex() == 0) {
			setAxisAutoScale(QwtPlot::yLeft);
			setAxisAutoScale(QwtPlot::xBottom);
	        zoomer->setZoomBase();
	}
	mutex.unlock();
}

---

Quote:

---

Originally Posted by dumdidum

Some possibly relevant code snippets:

---

The crash happens, when accessing the curve data. So the relevant code is how you assign the points to your curve.

Uwe

Quote:

---

Originally Posted by Uwe

The crash happens, when accessing the curve data. So the relevant code is how you assign the points to your curve.

Uwe

---

Code:

---

void Graph::setXY(double *x, double *y, int size) {
	mutex.lock();
	crv->setData(x, y, size);
	mutex.unlock();
}

---

The plot itself lives in the main thread, but this function is called from another thread.

Hurr, yeah. I get your hint now that I read the code and documents for a while: this is a threading problem. I replaced the direct function call from another thread with a signal-slot connection, which makes sure that the setData() call is handled in the appropriate thread.

Many thanks for your guidance!

Quote:

---

Originally Posted by http://doc.trolltech.com/4.2/threads.html#qobject-reentrancy

Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread.

---