Segfault when retrieving info from QSettings

[codeslicer]

Alright, in my application I have a "settings" class, with various public functions which return or set the data requested/specified. However, this line of code returns a segfault:

QString settings::getSomeInfo() {
	return internalAppSettings->value("main/info", "default").toString();
}

QString settings::getSomeInfo() {
	return internalAppSettings->value("main/info", "default").toString();
}

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Note that this file's header is included in my Main Window class, which in turn is included in main.cpp. Also, in main.cpp, I used QCoreApplication::setOrganizationName(), QCoreApplication::setApplicationName(), and QCoreApplication::setOrganizationDomain() so that I don't need to do it many times whenever I have to declare QSettings.

This is the output from GDB, the debugger:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1225369392 (LWP 17790)]
0x08063322 in settings::getSomeInfo (this=0x0) at src/settings.cpp:104
	 Use the -dograb option to enforce grabbing.
Scope for 104:
Symbol this is a variable with complex or multiple locations (DWARF2), length 4.
(gdb) 
Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1225369392 (LWP 17790)]
0x08063322 in settings::getSomeInfo (this=0x0) at src/settings.cpp:104
	 Use the -dograb option to enforce grabbing.
Scope for 104:
Symbol this is a variable with complex or multiple locations (DWARF2), length 4.
(gdb) 
Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.

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Thanks in advance ~codeslicer

[wysota]

You have a null pointer which you are trying to dereference in line 104 of src/settings.cpp. Looks like you didn't initialize your settings object.

[codeslicer]

Initialize? Is this what you mean:

settings::settings(QWidget * parent) : QWidget(parent) {
	QSettings internalAppSettings(this);
}

settings::settings(QWidget * parent) : QWidget(parent) {
	QSettings internalAppSettings(this);
}

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I did this in my constructor.

[codeslicer]

I declared it in my header file too..

private:
	//Internal QSettings declaration
	QSettings* internalAppSettings;

private:
	//Internal QSettings declaration
	QSettings* internalAppSettings;

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Could this be because the value doesn't exist? For example it has never been set before and it's returning null, but in that case wouldn't it return the string "default"? Or like the message said, are there multiple sources?

Symbol this is a variable with complex or multiple locations (DWARF2), length 4. (gdb)

I'm stumped

[wysota]

settings::settings(QWidget * parent) : QWidget(parent) {
	QSettings internalAppSettings(this);
}

settings::settings(QWidget * parent) : QWidget(parent) {
	QSettings internalAppSettings(this);
}

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This creates a local variable called internalAppSettings that gets destroyed immediately when the constructor returns. Your member variable is left uninitialized. Try this instead:

settings::settings(QWidget *parent) : QWidget(parent){
  internalAppSettings = new QSettings(this);
}

settings::settings(QWidget *parent) : QWidget(parent){
  internalAppSettings = new QSettings(this);
}

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Or better yet simply don't declare "internalAppSettings" as a pointer but instead as a regular object and then leave the constructor empty.

[codeslicer]

Thanks! I didn't know that the variable gets deleted.

[codeslicer]

Sorry for the delay, I thought it would work, but both of the solutions still return segfaults, with different errors...

Creating a QSettings object for each of the members solves the problem, but I think that's unneccesary. So... any help? I did a google search and noticed this happened in QDevelop to someone...

[codeslicer]

Just in case, attached is the relevant code:

main.cpp

int main(int argc, char ** argv)
{
...
	QCoreApplication::setOrganizationName("app");
	QCoreApplication::setApplicationName("appmakers");
	QCoreApplication::setOrganizationDomain("app.com");
...
	return app.exec();
}

int main(int argc, char ** argv)
{
...
	QCoreApplication::setOrganizationName("app");
	QCoreApplication::setApplicationName("appmakers");
	QCoreApplication::setOrganizationDomain("app.com");
...
	return app.exec();
}

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settings.cpp

#include <QtCore>
#include "settings.h"
 
settings::settings(QWidget * parent) : QWidget(parent) {
	internalAppSettings = new QSettings(this);
}
...
QString settings::getSomeInfo() {
	return internalAppSettings->value("main/info", "default").toString();
}

#include <QtCore>
#include "settings.h"

settings::settings(QWidget * parent) : QWidget(parent) {
	internalAppSettings = new QSettings(this);
}
...
QString settings::getSomeInfo() {
	return internalAppSettings->value("main/info", "default").toString();
}

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settings.h

#ifndef SETTINGS_H
#define SETTINGS_H
 
#include <QWidget>
#include <QSettings>
 
class settings : public QWidget
{
Q_OBJECT
public:
	settings(QWidget * parent = 0);
...
	QString getSomeInfo();
...
 
private:
	//Internal QSettings declaration
	QSettings *internalAppSettings;
};
#endif

#ifndef SETTINGS_H
#define SETTINGS_H

#include <QWidget>
#include <QSettings>

class settings : public QWidget
{
Q_OBJECT
public:
	settings(QWidget * parent = 0);
...
	QString getSomeInfo();
...
	
private:
	//Internal QSettings declaration
	QSettings *internalAppSettings;
};
#endif

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Personally I don't see anything wrong with this... Yet I still get a segfault:

Symbol this is a variable with complex or multiple locations (DWARF2), length 4.

Symbol this is a variable with complex or multiple locations (DWARF2), length 4.

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This is really weird Any hints?

[codeslicer]

**BUMP**

Any ideas?

[wysota]

Do you need members in those classes? I personally create and destroy the settings objects when I need them.

void C::m(p){
  QSettings s;
  QString v = s.value("xxx");
  //...
}

void C::m(p){
  QSettings s;
  QString v = s.value("xxx");
  //...
}

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[codeslicer]

Eh.. well my application needs to access QSettings pretty often... creating all those QSettings would be a waste of space and a source of confusion...

[wysota]

Currently the number of those objects is a source of confusion And you'd have to sync() all the instances before every use.