Segmentation fault

**MarkoSan** · 14th October 2007, 14:23

I've changed the code in the way you instructed, i get same segmentation fault.

**marcel** · 14th October 2007, 14:39

Then you must have a NULL pointer somewhere. What about m_pDatabase? Are you sure it is always valid?

**MarkoSan** · 14th October 2007, 15:25

Well, I've checked with gdb, none of actors pointer have NULL or 0 value, according to gdb. What the heck is going on??

**jacek** · 14th October 2007, 15:57

Could you post the backtrace?

**MarkoSan** · 14th October 2007, 18:16

Originally Posted by jacek

Could you post the backtrace?

I am using gdb through KDevelop, how do I get backtrace?

**jacek** · 14th October 2007, 20:36

Originally Posted by MarkoSan

I am using gdb through KDevelop, how do I get backtrace?

I've never tried it, but there's a "GDB" tab where you can issue commands. In standalone GDB you do it this way:

$ gdb ./app
(gdb) run
<program crashes>
(gdb) bt
<backtrace>

**anonyme_84** · 14th October 2007, 21:57

Hi,

Originally Posted by MarkoSan

I am using gdb through KDevelop, how do I get backtrace?

With Kdevelop, you have got a tab called "backtrace" when you run your application in debug mode.

Bye.

**MarkoSan** · 15th October 2007, 09:16

Well, my friends, here is a backtrace:

Qt Code:

Switch view

(gdb) run
Starting program: /home/markofr/working_copy/qKobilica/bin/qkobilica
[Thread debugging using libthread_db enabled]
[New Thread 47848586376464 (LWP 9931)]
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.
 
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 47848586376464 (LWP 9931)]
0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x00002b8496d9e4e3 in QSqlDatabase::record () from /usr/lib/libQtSql.so.4
#2  0x00000000004072c8 in CBrowserWindow (this=0x6cff50, parent=0x663e70, pDataBase=0x66f220, sTableName=@0x7fff13f42830) at cbrowserwindow.cpp:17
#3  0x0000000000405b48 in qKobilica::browseSifrantKrajev (this=0x663e70) at qkobilica.cpp:91
#4  0x0000000000408a5d in qKobilica::qt_metacall (this=0x663e70, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fff13f42e00) at moc_qkobilica.cpp:66
#5  0x00002b8497c2f770 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#6  0x00002b8497c2f969 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#7  0x00002b849718d9ab in QAction::triggered () from /usr/lib/libQtGui.so.4
#8  0x00002b849718f4b8 in QAction::activate () from /usr/lib/libQtGui.so.4
#9  0x00002b849754fea9 in ?? () from /usr/lib/libQtGui.so.4
#10 0x00002b849755264a in QMenu::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
#11 0x00002b84971e15d9 in QWidget::event () from /usr/lib/libQtGui.so.4
#12 0x00002b849754de85 in QMenu::event () from /usr/lib/libQtGui.so.4
#13 0x00002b8497194d51 in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
#14 0x00002b8497195949 in QApplication::notify () from /usr/lib/libQtGui.so.4
#15 0x00002b8497c1e846 in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
#16 0x00002b849719fe73 in ?? () from /usr/lib/libQtGui.so.4
#17 0x00002b84971fa37c in ?? () from /usr/lib/libQtGui.so.4
#18 0x00002b84971f8239 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
#19 0x00002b8497226599 in ?? () from /usr/lib/libQtGui.so.4
#20 0x00002b8499f8ffd3 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#21 0x00002b8499f932dd in ?? () from /usr/lib/libglib-2.0.so.0
#22 0x00002b8499f9380e in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#23 0x00002b8497c45f4e in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#24 0x00002b8497225e07 in ?? () from /usr/lib/libQtGui.so.4
#25 0x00002b8497c1bb31 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#26 0x00002b8497c1bc86 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#27 0x00002b8497c1effc in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#28 0x00002b8497194928 in QApplication::exec () from /usr/lib/libQtGui.so.4
#29 0x00000000004062fd in main (argc=1, argv=0x7fff13f44638) at main.cpp:13

(gdb) run
Starting program: /home/markofr/working_copy/qKobilica/bin/qkobilica
[Thread debugging using libthread_db enabled]
[New Thread 47848586376464 (LWP 9931)]
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 47848586376464 (LWP 9931)]
0x0000000000000000 in ?? ()
(gdb) bt
#0  0x0000000000000000 in ?? ()
#1  0x00002b8496d9e4e3 in QSqlDatabase::record () from /usr/lib/libQtSql.so.4
#2  0x00000000004072c8 in CBrowserWindow (this=0x6cff50, parent=0x663e70, pDataBase=0x66f220, sTableName=@0x7fff13f42830) at cbrowserwindow.cpp:17
#3  0x0000000000405b48 in qKobilica::browseSifrantKrajev (this=0x663e70) at qkobilica.cpp:91
#4  0x0000000000408a5d in qKobilica::qt_metacall (this=0x663e70, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fff13f42e00) at moc_qkobilica.cpp:66
#5  0x00002b8497c2f770 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#6  0x00002b8497c2f969 in QMetaObject::activate () from /usr/lib/libQtCore.so.4
#7  0x00002b849718d9ab in QAction::triggered () from /usr/lib/libQtGui.so.4
#8  0x00002b849718f4b8 in QAction::activate () from /usr/lib/libQtGui.so.4
#9  0x00002b849754fea9 in ?? () from /usr/lib/libQtGui.so.4
#10 0x00002b849755264a in QMenu::mouseReleaseEvent () from /usr/lib/libQtGui.so.4
#11 0x00002b84971e15d9 in QWidget::event () from /usr/lib/libQtGui.so.4
#12 0x00002b849754de85 in QMenu::event () from /usr/lib/libQtGui.so.4
#13 0x00002b8497194d51 in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
#14 0x00002b8497195949 in QApplication::notify () from /usr/lib/libQtGui.so.4
#15 0x00002b8497c1e846 in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
#16 0x00002b849719fe73 in ?? () from /usr/lib/libQtGui.so.4
#17 0x00002b84971fa37c in ?? () from /usr/lib/libQtGui.so.4
#18 0x00002b84971f8239 in QApplication::x11ProcessEvent () from /usr/lib/libQtGui.so.4
#19 0x00002b8497226599 in ?? () from /usr/lib/libQtGui.so.4
#20 0x00002b8499f8ffd3 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#21 0x00002b8499f932dd in ?? () from /usr/lib/libglib-2.0.so.0
#22 0x00002b8499f9380e in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#23 0x00002b8497c45f4e in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#24 0x00002b8497225e07 in ?? () from /usr/lib/libQtGui.so.4
#25 0x00002b8497c1bb31 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#26 0x00002b8497c1bc86 in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#27 0x00002b8497c1effc in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#28 0x00002b8497194928 in QApplication::exec () from /usr/lib/libQtGui.so.4
#29 0x00000000004062fd in main (argc=1, argv=0x7fff13f44638) at main.cpp:13

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**marcel** · 15th October 2007, 09:23

Are you sure you instantiate the QSqlDatabase? It doesn't look like you do.
Can you post the code where you create it? It might be the fact that QSqlDatabase::addDatabase gives you a local copy and you take its address. The pointer will become invalid by the time you pass it to the constructor.
Why don't you use const refs instead?

**MarkoSan** · 15th October 2007, 09:29

Well, this is a code:

Qt Code:

Switch view

// sets up database - params QString db_user,QString db_pass,QString db_name,QString host,QString type
  m_pDatabaseConnection=new MySQLConnect(QString("username"), QString("password"), QString("premade_database"));
  Q_CHECK_PTR(m_pDatabaseConnection);

// sets up database - params QString db_user,QString db_pass,QString db_name,QString host,QString type
  m_pDatabaseConnection=new MySQLConnect(QString("username"), QString("password"), QString("premade_database"));
  Q_CHECK_PTR(m_pDatabaseConnection);

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I am quite sure I will be forced to rewrite the MySQLConection class since it obviously does not work!!!

**marcel** · 15th October 2007, 09:39

Yes, but I was referring to the internal QSqlDatabase pointer. The one from MySqlConnect.

**MarkoSan** · 15th October 2007, 09:42

Ok, just a second. Did you mean this chunk of code:

Qt Code:

Switch view

MySQLConnect::MySQLConnect(QString db_user, QString db_pass, QString db_name, QString host, QString type) {
 
  //connect to database
  QSqlDatabase db = QSqlDatabase::addDatabase(type);
  db.setHostName(host);
  db.setDatabaseName(db_name);
  db.setUserName(db_user);
  db.setPassword(db_pass);
 
  //get a pointer to a database connection
  m_pDatabase = &QSqlDatabase::database();
  Q_CHECK_PTR(m_pDatabase);
 
  //sql table model == read/write model --> navigate and modify individual sql tables
  m_pModel = new QSqlRelationalTableModel(this);
  Q_CHECK_PTR(m_pModel);
 
  //model->setTable(db_table);
  //all changes will be cached in the model until either submitAll() or revertAll() is called
  m_pModel->setEditStrategy(QSqlTableModel::OnManualSubmit);
 
  //populates the model with data from the table, using the specified filter and sort conditions
  //model->select();
}
 
//QSqlDatabase* MySQLConnect::m_pDatabase=0;
 
/*
//this function is called if a refresh button is clicked
void MySQLConnect::refresh_everything() {
        model->select();
}
 
void MySQLConnect::change_mysql() {
    //submits all pending changes to the mysql database
    model->submitAll();
}
*/

MySQLConnect::MySQLConnect(QString db_user, QString db_pass, QString db_name, QString host, QString type) {

  //connect to database
  QSqlDatabase db = QSqlDatabase::addDatabase(type);
  db.setHostName(host);
  db.setDatabaseName(db_name);
  db.setUserName(db_user);
  db.setPassword(db_pass);

  //get a pointer to a database connection
  m_pDatabase = &QSqlDatabase::database();
  Q_CHECK_PTR(m_pDatabase);

  //sql table model == read/write model --> navigate and modify individual sql tables
  m_pModel = new QSqlRelationalTableModel(this);
  Q_CHECK_PTR(m_pModel);

  //model->setTable(db_table);
  //all changes will be cached in the model until either submitAll() or revertAll() is called
  m_pModel->setEditStrategy(QSqlTableModel::OnManualSubmit);

  //populates the model with data from the table, using the specified filter and sort conditions
  //model->select();
}

//QSqlDatabase* MySQLConnect::m_pDatabase=0;

/*
//this function is called if a refresh button is clicked
void MySQLConnect::refresh_everything() {
        model->select();
}

void MySQLConnect::change_mysql() {
    //submits all pending changes to the mysql database
    model->submitAll();
}
*/

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**marcel** · 15th October 2007, 09:50

Rewrite it like this:

Qt Code:

Switch view

MySQLConnect::MySQLConnect(QString db_user, QString db_pass, QString db_name, QString host, QString type) {
  //connect to database
  m_Database = QSqlDatabase::addDatabase(type);
  m_Database.setHostName(host);
  m_Database.setDatabaseName(db_name);
  m_Database.setUserName(db_user);
  m_Database.setPassword(db_pass);
  bool openedOK =  m_Database.open();
 
  m_pModel = new QSqlRelationalTableModel(this);
  Q_CHECK_PTR(m_pModel);
  m_pModel->setEditStrategy(QSqlTableModel::OnManualSubmit);
...
}

MySQLConnect::MySQLConnect(QString db_user, QString db_pass, QString db_name, QString host, QString type) {
  //connect to database
  m_Database = QSqlDatabase::addDatabase(type);
  m_Database.setHostName(host);
  m_Database.setDatabaseName(db_name);
  m_Database.setUserName(db_user);
  m_Database.setPassword(db_pass);
  bool openedOK =  m_Database.open();
   
  m_pModel = new QSqlRelationalTableModel(this);
  Q_CHECK_PTR(m_pModel);
  m_pModel->setEditStrategy(QSqlTableModel::OnManualSubmit);
...
}

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replace m_pDatabase with m_Database (not a pointer).

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