Qwt trying to draw symbol even it is null

phenoboy

I am getting a strange null pointer for symbols after attaching a QwtPlotCurve to the plot. For some reason in line 409 the code goes on and tries to draw a symbol even symbol is 0x0 like you can see in the debugger.

History is that I have this compiled for 32bit and it has worked okay. Now I compiled qwt for 64bit and this problem has come up.

SGaist

Hi and welcome to devnet,

Which version of Qwt are you using ?
With which version of Qt ?
On which OS ?

phenoboy

@SGaist
Thanks!

I use qwt development branch from git on Windows 11. I use latest Qt 5.15.2

SGaist

Can you provide a minimal compilable example that shows this behaviour ?

phenoboy

@SGaist Thanks for the reply. I will try to do that.

I have multithreaded application where one thread pushes data to QwtSeriesStore. When I attach that data to plot the crash happens. I don't understand why qwt code goes so far.

JobData* gjr = JobData::instance();
 gjr->curveData()->lock();
 CustomQwtCurve* c = addCurve();
 CurveData* cd = gjr->curveData();
 c->setData(cd->getSeriesData());
 c->attach(this);

JobData runs in one thread and of course UI is on another.

After this it crashes where the original screenshot is.

Christian Ehrlicher

@phenoboy said in Qwt trying to draw symbol even it is null:

I have multithreaded application where one thread pushes data to QwtSeriesStore.

So this is the reason. You must modify the gui only from the main (gui) thread.

phenoboy

@Christian-Ehrlicher

If you look at qwt oscilloscope example the curve data is updated in another thread than GUI.
I do the same. I update only the curve data and guard it with mutexes. In replot the data is locked. But I guess this must be the reason so I'll try to come up with a small compilable example to show the error.