[Qt-interest] Debugger for Qt threaded applications

[Andreas Pakulat]

On 12.10.09 09:45:07, Miguel Cardenas wrote:
> I need to debug a Linux Qt4 threaded application but have not a good tool.
> My current debugger is kdgb that works for simple programs. I have a
> threaded one that apparently works but randomly it causes segmentation fault
> when a button launches a thread, somtimes works, sometimes crashes and don't
> know where...
> 
> Is there a good graphic debugger that I can use to findout where is the
> problem and be able to fix it?

Valgrind's helgrind tool should be able to point you to common
threading-problems in your app. However...
 
> Or perhaps you could give me an idea on what is going on... Normally the
> first time I start the thread it works, but after 2 or 3 times it crashes.
> Note that a single thread is launched, and the button that starts the thread
> is disabled while it is running, once it finishes the button is enabled to
> use it again.
> 
> I have something like this: (pseudocode)
> 
> ----------------------------------------------------------------
> class MyThread;
> 
> class MyObject : QMainWindows {
>    Q_OBJECT
>    ...
>    QPushButton myButton;
>    MyThread *myThread; //
>    public:
>    MyObject();
>    public slots:
>    void startMyThread (void);
> }
> 
> MyObject::MyObject {
>    ...
>    myThread = new MyThread();
>    ...
>    QObject::connect(myButton,SIGNAL(clicked()),this,SLOT(startMyThread));
>    ...
> }
> 
> void MyObject::startMyThread (void) {
>     if ( myThread->isRunning() ) {
>       QMessageBox::warning (0/*parent*/, tr("Warning"),
>                             tr("ALREADY RUNNING!"));
>       return;
>    } else {
>       myThread->start();
>    }
> }

This won't work AFAIK. You have to create a new thread-object each time you
want to start it. Restarting an already-finished QThread like this won't
work.

Andreas

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