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Qt-interest Archive, September 2007
freeze in QPainterPath.united()

Message 1 in thread

Hi all,

  can anyone explain why following code freezes? All it does is
create some painter paths and unite them together using
QPainterPath.united()

- it freezes only when united() is called from mouseReleaseEvent()
  handler, not when it is called elsewhere, eg. in main()
- freeze happens also with QPainterPath.subtracted() and different
  input

Am i doing something wrong?

I attach the source code and the simplest input that causes the error.

Any help appreciated.


Tomas


Here is a gdb backtrace:
#0  0xb786a709 in QPathClipper::clip () from /usr/lib/libQtGui.so.4
#1  0xb7867a2f in QPathClipper::clip () from /usr/lib/libQtGui.so.4
#2  0xb782c6ce in QPainterPath::united () from /usr/lib/libQtGui.so.4
#3  0x0804a8d4 in MyWidget::mouseReleaseEvent ()
#4  0xb77743fb in QWidget::event () from /usr/lib/libQtGui.so.4
#5  0xb7722af1 in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
#6  0xb77236cb in QApplication::notify () from /usr/lib/libQtGui.so.4
#7  0xb7e7b88e in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
#8  0xb772df9b in QApplicationPrivate::QApplicationPrivate () from /usr/lib/libQtGui.so.4
#9  0xb778ea92 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
#10 0xb778c7aa in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
#11 0xb77ba66e in QX11Info::copyX11Data () from /usr/lib/libQtGui.so.4
#12 0xb7239d6c in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#13 0xb723d19f in g_main_context_check () from /usr/lib/libglib-2.0.so.0
#14 0xb723d705 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#15 0xb7ea460c in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#16 0xb77b9f24 in QX11Info::copyX11Data () from /usr/lib/libQtGui.so.4
#17 0xb7e78816 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#18 0xb7e78998 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#19 0xb7e7bfdb in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#20 0xb7722674 in QApplication::exec () from /usr/lib/libQtGui.so.4
#21 0x0804a5f4 in main ()


#include <QApplication>

#include <QApplication>
#include <QMainWindow>

#include "qpp_w.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QMainWindow mw;
    mw.setCentralWidget(new MyWidget(&mw));
    mw.show();
    app.exec();
}

#include <QApplication>

#include <QApplication>
#include <QWidget>
#include <QPainterPath>
#include <cstdio>

class MyWidget : public QWidget {
    Q_OBJECT
public:
    MyWidget(QWidget *parent) : QWidget(parent) {}
protected:
    void mouseReleaseEvent(QMouseEvent *event)
    {
        QPainterPath path;
        QPainterPath newPath;
        FILE *f = fopen("path", "r");
        char line[256];
        while (fgets(line, 255, f)) {
            int x, y;
            switch (line[0]) {
                case 'm':
                    sscanf(line, "m %d, %d\n", &x, &y);
                    newPath.moveTo(x, y);
                    break;
                case 'l':
                    sscanf(line, "l %d, %d\n", &x, &y);
                    newPath.lineTo(x, y);
                    break;
                case 'u':
                    path = path.united(newPath);
                    newPath = QPainterPath();
                    break;
                default:
                    printf("unknown command\n");
                    break;
            }
            printf(line);
        }
        fclose(f);
        printf("end of input\n");
        QApplication::exit(0);
    }
};


m  1,20

m  1,20
l 30,30
l 10,20
l 10,30
u
m  1,10
l  1,20
l 30,30
u
Message 2 in thread

Tomáš Kazmar a écrit :
> Hi all,
> 
>   can anyone explain why following code freezes? All it does is
> create some painter paths and unite them together using
> QPainterPath.united()
> 
> - it freezes only when united() is called from mouseReleaseEvent()
>   handler, not when it is called elsewhere, eg. in main()
> - freeze happens also with QPainterPath.subtracted() and different
>   input
> 
> Am i doing something wrong?
> 
> I attach the source code and the simplest input that causes the error.

I doesn't look like you're doing something wrong. In any case Qt shouldn't get 
stuck in an endless loop. Qt enters an endless loop in QPathClipper::clip() in 
file src/gui/painting/qpathclipper.cpp:

QPainterPath QPathClipper::clip(Operation op)
{
     [...]
         QList<PathVertex*> vertices;
         while (not_over)
             not_over = d->walkResultingPath(start, prev_code_owner,
                                             current, [...]);
     [...]

For some reason "not_over" remains true and Qt never gets out of this loop.

I suggest you send a bug report to Trolltech:
	http://trolltech.com/bugreport-form

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