Question

After googling around I found out that segmentation faults are given when the program is pointed to use memory that it doesn’t have access to.

I’ve recently started experiencing these errors after I tried making a custom button class that would enclose, in its clicked() signal, an integer.

Here’s the custom button class:

.h:

#include <QtGui>

#ifndef CUSTOMBUTTON_H
#define CUSTOMBUTTON_H

class CustomButton : public QPushButton        //This simple class allows us to map arguments in the Widget's cicked() signal.
{                                              //In the project, it's used in the delete-edit buttons so the program knows which part of the Movie/Theater/Screening list we're referring to
 Q_OBJECT

public:
 CustomButton(QString name,int num, QWidget *parent = 0);

signals:
 void clicked(int pos);

private:
 QSignalMapper *signalMapper;
};

#endif // CUSTOMBUTTON_H

.cpp:

#include "custombutton.h"

CustomButton::CustomButton(QString name,int num = 0, QWidget *parent) //Our constructor
    : QPushButton(name,parent)
{   //Our button's now created and shown, through the superconstructor. Let's take care of its clicked() signal.

    signalMapper = new QSignalMapper(this);

    connect(this, SIGNAL(clicked()), signalMapper, SLOT(map()));
    signalMapper->setMapping(this, num);


    connect(signalMapper, SIGNAL(mapped(int)),this, SIGNAL(clicked(int)));

}

And then in my main code I do:

CustomButton *edit_button = new CustomButton("Edit",i,0);
edit_button->setFixedWidth(30);
connect(edit_button,SIGNAL(clicked(int)),this,SLOT(edit_movie(int))); //When the user decides to edit a movie, we set the class'es working_with var to the position of the film in the MovieList and we set the screen to the appropriate one.


CustomButton *del_button = new CustomButton("Delete",i,0);            //We pass i as an argument to the button, so it can relate to the movie it's set next to.
del_button->setFixedWidth(45);
connect(del_button,SIGNAL(clicked(int)),this,SLOT(del_movie(int)));

Where i is the number I want it to have in the signal.

Thing is, thorugh the debugger it doesn’t give me a segfault if I press the Delete button. It all happens in the edit one.

As I’m still pretty confused and in a situation where I don’t even know WHAT to ask about, if you need anything else from me please say so and I will provide.

Here’s the backtrace, which I know not how to read:

0 QHash<QObject*, QString>::findNode qhash.h 884 0×69e98594
1 QHash<QObject*, QString>::contains qhash.h 874 0×69e98568
2 QSignalMapper::map qsignalmapper.cpp 267 0×69debe0c
3 QSignalMapper::map qsignalmapper.cpp 257 0×69debda2
4 QSignalMapper::qt_static_metacall moc_qsignalmapper.cpp 64 0×69debfce
5 QMetaObject::activate qobject.cpp 3547 0×69de9baf
6 QAbstractButton::clicked moc_qabstractbutton.cpp 220 0×10cb4b8
7 QAbstractButtonPrivate::emitClicked qabstractbutton.cpp 548 0xe2e517
8 QAbstractButtonPrivate::click qabstractbutton.cpp 541 0xe2e495
9 QAbstractButton::mouseReleaseEvent qabstractbutton.cpp 1123 0xe2f941
10 QWidget::event qwidget.cpp 8362 0xae63de
11 QAbstractButton::event qabstractbutton.cpp 1082 0xe2f7cc
12 QPushButton::event qpushbutton.cpp 683 0xecfeba
13 QApplicationPrivate::notify_helper qapplication.cpp 4554 0xa9c020
14 QApplication::notify qapplication.cpp 4097 0xa9a26a
15 QCoreApplication::notifyInternal qcoreapplication.cpp 876 0×69dd3b76
16 QCoreApplication::sendSpontaneousEvent qcoreapplication.h 234 0×113137e
17 QApplicationPrivate::sendMouseEvent qapplication.cpp 3163 0xa98ad6
18 QETWidget::translateMouseEvent qapplication_win.cpp 3363 0xb03171
19 QtWndProc qapplication_win.cpp 1696 0xafdf66
20 USER32!IsDialogMessageW C:\Windows\syswow64\user32.dll 0 0×76726238
21 USER32!RegisterSystemThread C:\Windows\syswow64\user32.dll 0 0×767278b0
22 ?? 0 0×30000
23 USER32!AllowForegroundActivation C:\Windows\syswow64\user32.dll 0 0×767268ea
24 qt_is_translatable_mouse_event qapplication_win.cpp 1463 0xafd465
25 USER32!GetMessageExtraInfo C:\Windows\syswow64\user32.dll 0 0×76727d31
26 ?? 0 

Any heads up as to what might be causing the problem? As I said, the segfault only occurs if I run the program through the Debugger and when I press the “Edit” button, of the CustomButton class. When I normally build&run, the program works 9/10 times. Sometimes, clicking the edit button results in a crash. This erratic behaviour is what caused me to look for help here.

As when I click the button I’m taken to a new screen, I’m suspecting that the problem might lie in the decstructor? Does a blank destructor correctly deconstruct elements of objects of the CustomButton class, seeing as the actual object is passed to the base class constructor? Maybe I’ve got a leak there?

Answer 1

Unless knowing where you create the objects, it’s hard to tell why there is a segfault in QSignalMapper. The pointer to your signal mapper might be invalid at some point in time, so there might be an unwanted deletion.

In some cases, a complete rebuild of your application might be helpful. Also re-running qmake. Just should be on your checklist when you can’t explain why there is a segfault …;)

There is an alternative to QSignalMapper: keep your signal clicked(int) and implement a private slot:

private slots:
    this_clicked() {
        emit clicked(num);
    }

Then put in the constructor:

connect(this, SIGNAL(clicked()), SLOT(this_clicked()));