Longstanding problem with crashing, 'pure virtual function call'

[MattPhillips]

Hi Uwe,

Is the problem that I create and attach QwtPlotItems to QwtPlots within a worker thread? Do you do this? My intuition had been that calling replot from a worker thread could fail but just making the objects could be done anywhere. Otherwise, what you're saying sounds just like what I do, except that in your case the timer is in the main thread whereas in mine it's in the worker thread, which shouldn't make any difference.

[Cah]

I believe signal emission is always thread-safe.

class CalculationThread : public QThread
 {
     Q_OBJECT
 
 public:
     CalculationThread(QObject *parent = 0);
     ~CalculationThread();
 
     void calculation(CalculationData & data);
     void run();
 
 signals:
     void dataChanged();
     void error(const QString &message);
 
 private:
     ...
     CalculationData m_data
     QMutex mutex;
     ...
 };
 
 
void CalculationThread::calculation(CalculationData & data)
 {
     QMutexLocker locker(&mutex);
     m_data = data;
     if (!isRunning())
         start();
     ...
 }
 
 
void CalculationThread::run()
 {
     mutex.lock();
 
     ...
     //do time consuming calculations
     ...
 
     if(/*data changed*/)
     	emit dataChanged(); 
     mutex.unlock();
}
 
 
class Plot: public QwtPlot
{
     ...
 
private slots:
     dataChangedCalc()
 
}
 
Plot::dataChangedCalc()
{
  ...
  // create and attach QwtPlotItems to QwtPlots 
  ...
  this->replot()
 
}
 
 
connect(&thread, SIGNAL(dataChanged()), this, SLOT(dataChangedCalc()));

class CalculationThread : public QThread
 {
     Q_OBJECT

 public:
     CalculationThread(QObject *parent = 0);
     ~CalculationThread();

     void calculation(CalculationData & data);
     void run();

 signals:
     void dataChanged();
     void error(const QString &message);

 private:
     ...
     CalculationData m_data
     QMutex mutex;
     ...
 };


void CalculationThread::calculation(CalculationData & data)
 {
     QMutexLocker locker(&mutex);
     m_data = data;
     if (!isRunning())
         start();
     ...
 }


void CalculationThread::run()
 {
     mutex.lock();

     ...
     //do time consuming calculations
     ...

     if(/*data changed*/)
     	emit dataChanged(); 
     mutex.unlock();
}


class Plot: public QwtPlot
{
     ...

private slots:
     dataChangedCalc()

}

Plot::dataChangedCalc()
{
  ...
  // create and attach QwtPlotItems to QwtPlots 
  ...
  this->replot()

}


connect(&thread, SIGNAL(dataChanged()), this, SLOT(dataChangedCalc()));

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Note: the this pointer is a pointer to a Plot

Is the problem that I create and attach QwtPlotItems to QwtPlots within a worker thread?

I assuming that you are creating and attaching many plotItems (that's your motivation for putting this activity in the worker thread)
QObject and subclasses thereof are not thread safe. QwtPlot is asubclas of QObject.

[MattPhillips]

Cah,

Yes, this is what it looks like I will have to try. My version of the '// create and attach QwtPlotItems to QwtPlots' part of 'dataChangedCalc' is in the worker thread; only the replot is in the mean thread. My hope is to gain some insight in advance as to whether this really should make the critical difference--see my previous post--but not knowing any other reasonable possibility I'll probably just try this in any case. Maybe you're right and it's the non-thread-safety of QwtPlot that's getting me; I only have one worker thread on the QwtPlot in question but maybe OS-generated calls to replot are causing the problem. I'll post my results in this thread when I have them.

[Uwe]

Attaching/detaching items from a different thread is definitely not thread safe !

To make it safe you would have at least to introduce a mutex to avoid that attaching/detaching and iterating over the items in the replot operation do conflict.

For guarding the replot it is probably best to overload QwtPlot::replot(), where you do the mutex stuff before calling the base class.

Uwe