Complicated QwtPlotZoomer crash, possible bug? Help request.

[philwinder]

Hi again Uwe,
I've been doing some more research on this bug, and although I couldnt find it via debugging (hard work!) I did manage to replicate the bug in a simple program.
The code attached was compiled with Qt 4.4.3 and the Qwt from the svn repository which I believe was Revision 417: version 5.2.

To get the program to crash, you have to use your mouse to zoom in and out many times. I believe the faster you zoom in and out, the faster it will crash. So I was repeatedly left clicking then right clicking at a fair pace. Make sure to do small circles too so it actually zooms in. (Note: this may cause your hand to ache!).

On a Dual core 2GHz laptop with 2G of RAM, it takes about 1.5 minutes to crash. On lower spec PC's (tested with a P4 3GHz, 512Mb RAM) it can sometimes only take 30 seconds worth of clicking.

Hopefully you can compile and run the attached code, but if you have any problems let me know.

Many thanks,
Phil Winder

Attached: qwtCrashTest.zip - source files, ui file and a pro file.

[Uwe]

I've tried your demo on Linux/X11 with Qwt 5.1/Qt 4.4.2 without problems.

But your demo code does not much, so you can expect, that the reason for it is Qt downwards and Windows specific.

I recommend to upgrade to Qt 4.4.3 build it in debug mode and rerun your demo under gdb control. And again: try to learn/understand your debugger - whenever an applicaton crashes you can see it in your debugger.

Uwe

[philwinder]

As stated in the above post, it was compiled in 4.4.3, so I cannot update any more.
But yes, it does seem to be windows specific.

[philwinder]

A quick update, unfortunately, I still haven't got round to debugging the code, but the crash is applicable to all of Qwt's examples with a zoom in. E.g. Bode, errorbars, etc.

It crashes on Xp and Vista.

I'll try and find some time to go through it this week. Hopefully. Debugging somebody else's code is a nightmare!

Cheers,
Phil

[Uwe]

I'll try and find some time to go through it this week. Hopefully. Debugging somebody else's code is a nightmare!

Phil, I didn't mean, that you should debug someones code. Believe me its completely impossible to understand ( I never tried myself ) the graphics pipeline of Qt without spending a lot of time before.

What you know is that Qt is running into an assertion in qpaintengine_raster.cpp, line 4286. Obviously CreateCompatibleDC(displayDC); returns NULL, what is the return value, when the method fails. Maybe its worth to read the MSDN docs (or google) about CreateCompatibleDC.

The assertion might have to do with the warnings before. If those are written immediately before the assertion ( do they happen in all Qwt examples too) try to find the code line, that writes them.

If you have a support license this should be enough information to contact the TrollTech support.

Uwe

[philwinder]

Hi Uwe,
I've spent a bit of time looking thought the debug window and yes, the crash is due to the Assert statement. CreateCompatibleDC is some sort of memory management function, so I can only assume that it cannot allocate the memory for the device.

I dont know why it cant allocate the memory though, because according to my memory gauge there is loads left.

Do you think that this is a Qt problem? Or a windows problem? Its obviously not something to do with Qwt (How could I ever think it was!) but I am an open source user, and hence have no service contract. Shall I attempt to report a bug?

Cheers,
Phil

[Uwe]

I'm pretty sure, that it is a Qt problem. Could you please check the warnings, that were printed before the assertion happens ?
Together with this information I will contact the Qt support,

Uwe

[philwinder]

Hi Again Uwe,
Sorry, just seen your message. I didnt manage to find any specific errors, but it was just the windows related functions returned 0, meaning that they could not allocate any memory for whatever it was.

So, usually the function CreateDIBSection on line 4289 returns 0 meaning some sort of error, then the function CreateCompatibleDC returns 0 and finally crashes the program. However, the actual order of these two have been different, ie. sometimes the CreateCompatibleDC flags an error first, sometimes the CreateDIBSection does.

I already sent a bug report to Trolltech and they referred me back to you! Message as follows:

Hi Phil,

There is no way that I can see if this is a QWT problem or a Qt problem.
QWT is an external product. Please report this to Uwe in the
forum(which I can see you have done), and he will file a bug if the
problem is Qt related.

Thanks,
Morten

Thanks,
Phil

[Uwe]

I will have a look at the problem the next time I will check Qwt on Windows ( what might take some time ).

Uwe

[jomarin]

Hi everyone,

I have a similar problem on Windows. My QwtPlot window crashes after a number of zoom in and out, that seems to depend on the machine used.

I did not experience the same problem on Solaris.

Any news on this subject ?

Thanks in advance,

Joel.

[philwinder]

Ah, I am glad it is not just me. I still have the problem, and I do think it is restricted to windows. They must handle their graphics memory management differently. Having you on board with the same problem may inspire Uwe to take a look at the problem.

[gambr]

Same problem on windows XP, VS 2003 7.1 SP1, Qt 4.4.0, Qwt 5.0.2 & 5.1.0 & 5.1.1.

Gianni