SIGSEGV Error: Very Strange

[grayfox]

MainWindow.h

#include <QMainWindow>
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
};

#include <QMainWindow>

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
};

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MainWindow.cpp

#include "MainWindow.h"
 
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
#ifdef QT_DEBUG
    setWindowTitle("DEBUG");
#else
    setWindowTitle("RELEASE");
#endif
 
}

#include "MainWindow.h"

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
#ifdef QT_DEBUG
    setWindowTitle("DEBUG");
#else
    setWindowTitle("RELEASE");
#endif

}

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Application.h

#include <QApplication>
 
class MainWindow;
 
class Application : public QApplication
{
public:
    Application(int argc, char** argv);
    ~Application();
    MainWindow* mainWindow() const;
private:
    MainWindow* _mainWindow;
};

#include <QApplication>

class MainWindow;

class Application : public QApplication
{
public:
    Application(int argc, char** argv);
    ~Application();
    MainWindow* mainWindow() const;
private:
    MainWindow* _mainWindow;
};

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Application.cpp

#include "Application.h"
#include "MainWindow.h"
 
Application::Application(int argc, char** argv) : QApplication(argc, argv)
{
    _mainWindow = new MainWindow;
}
 
Application::~Application(){
    delete _mainWindow;
    _mainWindow = 0;
}
 
MainWindow * Application::mainWindow() const
{
    return _mainWindow;
}

#include "Application.h"
#include "MainWindow.h"

Application::Application(int argc, char** argv) : QApplication(argc, argv)
{
    _mainWindow = new MainWindow;
}

Application::~Application(){
    delete _mainWindow;
    _mainWindow = 0;
}

MainWindow * Application::mainWindow() const
{
    return _mainWindow;
}

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main.cpp

#include "Application.h"
#include "MainWindow.h"
 
int  main(int argc, char** argv)
{
    Application app(argc, argv);
    app.mainWindow()->show();
    return app.exec();
}

#include "Application.h"
#include "MainWindow.h"

int  main(int argc, char** argv)
{
    Application app(argc, argv);
    app.mainWindow()->show();
    return app.exec();
}

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The code above run perfectly fine under release build, but will cause SIGSEGV Error everytime under debug build.
My system is Ubuntu 11.04 64-bits, Qt 4.8/4.7.4

[Santosh Reddy]

When does SIGSEGV error occur?

You need to have virtual destructor

virtual ~Application()

virtual ~Application()

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[grayfox]

When does SIGSEGV error occur?

You need to have virtual destructor

virtual ~Application()

virtual ~Application()

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Exact same thing happens even after I added virtual.
The error occur in qwidget.h at line 494

inline void show() { setVisible(true); }

inline void show() { setVisible(true); }

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basically when I call "app.mainWindow()->show();" in main.

[wysota]

Does app.mainWindow() return a non-null pointer?

[grayfox]

Does app.mainWindow() return a non-null pointer?

Yes, it is not null.

[wysota]

What does the backtrace say?

[grayfox]

What does the backtrace say?

Thread 1 (Thread 0x7ffff7fca7a0 (LWP 12791)):
#0  0x00007ffff5d9b49f in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00007ffff4310382 in XSetCommand () from /usr/lib/x86_64-linux-gnu/libX11.so.6
No symbol table info available.
#2  0x00007ffff4315169 in XSetWMProperties () from /usr/lib/x86_64-linux-gnu/libX11.so.6
No symbol table info available.
#3  0x00007ffff735420a in QWidgetPrivate::create_sys(unsigned long, bool, bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#4  0x00007ffff73058e5 in QWidget::create(unsigned long, bool, bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#5  0x00007ffff730f3d1 in QWidget::setVisible(bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#6  0x0000000000402a5c in QWidget::show (this=0x609ab0) at /usr/local/Trolltech/Qt-4.8.0/include/QtGui/qwidget.h:494
No locals.
#7  0x00000000004029fd in main (argc=1, argv=0x7fffffffe6e8) at Main.cpp:7
        app = {<QApplication> = {<No data fields>}, _mainWindow = 0x609ab0}

Thread 1 (Thread 0x7ffff7fca7a0 (LWP 12791)):
#0  0x00007ffff5d9b49f in ?? () from /lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00007ffff4310382 in XSetCommand () from /usr/lib/x86_64-linux-gnu/libX11.so.6
No symbol table info available.
#2  0x00007ffff4315169 in XSetWMProperties () from /usr/lib/x86_64-linux-gnu/libX11.so.6
No symbol table info available.
#3  0x00007ffff735420a in QWidgetPrivate::create_sys(unsigned long, bool, bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#4  0x00007ffff73058e5 in QWidget::create(unsigned long, bool, bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#5  0x00007ffff730f3d1 in QWidget::setVisible(bool) () from /usr/local/Trolltech/Qt-4.8.0/lib/libQtGui.so.4
No symbol table info available.
#6  0x0000000000402a5c in QWidget::show (this=0x609ab0) at /usr/local/Trolltech/Qt-4.8.0/include/QtGui/qwidget.h:494
No locals.
#7  0x00000000004029fd in main (argc=1, argv=0x7fffffffe6e8) at Main.cpp:7
        app = {<QApplication> = {<No data fields>}, _mainWindow = 0x609ab0}

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[wysota]

Does it work if you use a stable version of Qt?

[grayfox]

Does it work if you use a stable version of Qt?

Same result with Qt 4.7.4

[wysota]

It seems odd the application crashes when setting window properties. What window manager are you using? Does the program work if you rewrite it as:

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QMainWindow mw;
  mw.setWindowTitle("DEBUG");
  mw.show();
  return app.exec();
}

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QMainWindow mw;
  mw.setWindowTitle("DEBUG");
  mw.show();
  return app.exec();
}

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