concurrency issues with QwtData

[sun]

Hi all,

I implemented my own QwtData which works with a ring buffer and is filled from a different thread than the plot.
Because the Plotter stops the plot with a QMutex before setting rawData

curveList[a]->setRawData(dataList[a].x(), dataList[a].y(), dataList[a].size());

curveList[a]->setRawData(dataList[a].x(), dataList[a].y(), dataList[a].size());

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everything worked fine.

void ValuePlotData::append(qint64 xValue, qint64 yValue)
{
                if(currentValuecount == arraySize)
                {
                        long newArraySize = arraySize*2;
                        double * d_x_new = new double[newArraySize];
                        double * d_y_new = new double[newArraySize];
                        for(int i=0;i<arraySize;i++)
                        {
                                d_x_new[i]=d_x[i];
                                d_y_new[i]=d_y[i];
                        }
                        delete [] d_x;
                        delete [] d_y;
                        d_x = d_x_new;
                        d_y = d_y_new;
                        arraySize = newArraySize;
                }
                d_x[currentValuecount] = xValue;
                d_y[currentValuecount] = yValue;
                currentValuecount++;
}

void ValuePlotData::append(qint64 xValue, qint64 yValue)
{
                if(currentValuecount == arraySize)
                {
                        long newArraySize = arraySize*2;
                        double * d_x_new = new double[newArraySize];
                        double * d_y_new = new double[newArraySize];
                        for(int i=0;i<arraySize;i++)
                        {
                                d_x_new[i]=d_x[i];
                                d_y_new[i]=d_y[i];
                        }
                        delete [] d_x;
                        delete [] d_y;
                        d_x = d_x_new;
                        d_y = d_y_new;
                        arraySize = newArraySize;
                }
                d_x[currentValuecount] = xValue;
                d_y[currentValuecount] = yValue;
                currentValuecount++;
}

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double ValuePlotData::x(size_t i) const
{
        return d_x[offset+(int)i];
}

double ValuePlotData::x(size_t i) const
{
        return d_x[offset+(int)i];
}

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Now I had to use a zoomer for the plot and problems started.

Whenever I zoom in or out there is a chance of the program crashing with a "call to pure virtual method".
After getting post mortem debugging ready I found out, that a replot was issued and it died as I believe on Calling .x() method

I initialize the zoomer with AutoReplot(false) but still it seems to crash because of that.

How do you take care of concurrency issues with your plot, because I don't think it is wise to fill from the gui thread for many new values.

Thanks in advance, this has allready cost me several hours.

sun

[Uwe]

Qwt doesn't take care of races at all. I remember a discussion ( almost 10 years ago ), whether QwtData should offer some sort of lock mechanisms, but in the end the decision was to leave this to the application code ( can't remember the arguments and if they are still valid today ).

A possible solution (implemented in the oscilloscope example in SVN trunk ) is to add lock/unlock methods to your data object using them in your sampling thread and the paint operations ( YourPlot::replot() or YourCurve::draw() ).

Uwe