(Another) segmentation fault

[Lebowski]

Hello.

I've started using QT recently and I quite like it so far. However I can't get past this error.

In Povezava.ui.h I have setup connection to database like this:

void Povezava::Connect_clicked()
{  
    int indeks = tipbaze->currentItem();
    if (indeks==0) {driver ="QMYSQL3";}
    if (indeks==1) {driver ="QPSQL7";}
    if (indeks==2) {driver ="QODBC3";}
 
    QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );
    baza->setHostName ( hostm );
    baza->setDatabaseName ( namem );
    baza->setUserName ( userm );
    baza->setPassword ( passm );
 
    bool ok = baza->open();
}

void Povezava::Connect_clicked()
{  
    int indeks = tipbaze->currentItem();
    if (indeks==0) {driver ="QMYSQL3";}
    if (indeks==1) {driver ="QPSQL7";}
    if (indeks==2) {driver ="QODBC3";}
    
    QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );
    baza->setHostName ( hostm );
    baza->setDatabaseName ( namem );
    baza->setUserName ( userm );
    baza->setPassword ( passm );
    
    bool ok = baza->open();
}

To copy to clipboard, switch view to plain text mode 

This part definitelly works as I've tested it in many ways.

In another form I'm checking if the connection is properly opened with this code:

void GlavnoO :: pushButton3_clicked()
{
    bool ok = FALSE;
    ok = povezava.baza->isOpen();
}

void GlavnoO :: pushButton3_clicked()
{
    bool ok = FALSE;
    ok = povezava.baza->isOpen();
}

To copy to clipboard, switch view to plain text mode 

I don't get any errors when compiling. But whet I click on this button I get segmentation fault EVERY time! I've tried it with connection opened and closed. I also did 'make clean' just in case. Nothing worked.

I have no idea how to get past this.

[zlatko]

Its ok becouse you declare baza variable in method Connect_clicked(). After returning from this method you havent access to this variable. For solving you need declare baza as class member

[Lebowski]

I already have baza declared as class member in povezava.ui.h

It's under povezava.ui.h->members->class variables->public-> QSqlDatabase *baza ;

[zlatko]

So why you redeclare it in

QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );

QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );

To copy to clipboard, switch view to plain text mode 

Right code

baza = QSqlDatabase::addDatabase ( driver )

baza = QSqlDatabase::addDatabase ( driver )

To copy to clipboard, switch view to plain text mode 

[Lebowski]

Tried that but still getting segmentation fault in the same way

[jacek]

Compile your application in debug mode and run it from a debugger. When it crashes, print the stack trace and post it here.

[Lebowski]

I've added "CONFIG += debug" into my project file but the output when my application crashes still says only "Segmentation fault".

This is debug from 'make':

[root@simon dbconn]# make --debug
GNU Make 3.80
Copyright (C) 2002 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Reading makefiles...
Updating goal targets....
File `first' does not exist.
File `all' does not exist.
Prerequisite `povezava.ui.h' is newer than target `.ui/povezava.cpp'.
Must remake target `.ui/povezava.cpp'.
/usr/lib/qt-3.3/bin/uic povezava.ui -i povezava.h -o .ui/povezava.cpp
Successfully remade target file `.ui/povezava.cpp'.
Prerequisite `.ui/povezava.cpp' is newer than target `.obj/povezava.o'.
Prerequisite `povezava.ui.h' is newer than target `.obj/povezava.o'.
Must remake target `.obj/povezava.o'.
g++ -c -pipe -Wall -W -g -DQT_SHARED -DQT_NO_DEBUG -DQT_THREAD_SUPPORT -I/usr/lib/qt-3.3/mkspecs/default -I. -I/usr/lib/qt-3.3/include -I.ui/ -I. -I.moc/ -o .obj/povezava.o .ui/povezava.cpp
/usr/lib/qt-3.3/include/qsqldatabase.h:63: warning: ‘class QSqlDriverCreatorBase’ has virtual functions but non-virtual destructor
/usr/lib/qt-3.3/include/qtooltip.h:86: warning: ‘class QToolTip’ has virtual functions but non-virtual destructor
Successfully remade target file `.obj/povezava.o'.
Prerequisite `.obj/povezava.o' is newer than target `povezava'.
Must remake target `povezava'.
g++ -o povezava .obj/main.o .obj/glavnoo.o .obj/povezava.o .obj/moc_glavnoo.o .obj/moc_povezava.o -L/usr/lib/qt-3.3/lib -L/usr/X11R6/lib -lqt-mt -lXext -lX11 -lm
Successfully remade target file `povezava'.
Must remake target `all'.
Successfully remade target file `all'.
Must remake target `first'.
Successfully remade target file `first'.
[root@simon dbconn]# ./povezava
Segmentation fault

[SkripT]

Just a way to "debug", check everytime before accesing to baza pointer if its null and show a message on screen or on the standard output, for example, in this case.

[wysota]

Try this:

$ gdb ./povezava
(gdb) run
(some output will go there and eventually the application will segfault)
(gdb) bt


and show us the result of "bt".

[Lebowski]

Here is what I get with gdb:

(gdb) run
Starting program: /home/simon/projects/dbconn/povezava
Reading symbols from shared object read from target memory...done.
Loaded system supplied DSO at 0xb9e000
[Thread debugging using libthread_db enabled]
[New Thread -1208837664 (LWP 9818)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1208837664 (LWP 9818)]
0x04d45bb5 in QSqlDatabase::isOpen (this=0xffff) at sql/qsqldatabase.cpp:772
772 return d->driver->isOpen();
(gdb) bt
#0 0x04d45bb5 in QSqlDatabase::isOpen (this=0xffff) at sql/qsqldatabase.cpp:772
#1 0x0804bcb0 in GlavnoO::test_clicked (this=0xbff491fc) at .ui/../glavnoo.ui.h:28
#2 0x0804e877 in GlavnoO::qt_invoke (this=0xbff491fc, _id=52, _o=0xbff48af8) at .moc/moc_glavnoo.cpp:87
#3 0x04ab9eb4 in QObject::activate_signal (this=0x9967440, clist=0x9969488, o=0xbff48af8) at kernel/qobject.cpp:2355
#4 0x04aba374 in QObject::activate_signal (this=0x9967440, signal=4) at kernel/qobject.cpp:2324
#5 0x04e39c7e in QButton::clicked (this=0xffff) at .moc/release-shared-mt/moc_qbutton.cpp:152
#6 0x04b59a63 in QButton::mouseReleaseEvent (this=0x9967440, e=0xbff48e54) at widgets/qbutton.cpp:836
#7 0x04af8187 in QWidget::event (this=0x9967440, e=0xbff48e54) at kernel/qwidget.cpp:4699
#8 0x04a550dd in QApplication::internalNotify (this=0xffff, receiver=0x9967440, e=0xbff48e54) at kernel/qapplication.cpp:2635
#9 0x04a5603f in QApplication::notify (this=0xbff49334, receiver=0x9967440, e=0xbff48e54) at kernel/qapplication.cpp:2421
#10 0x049ec746 in QETWidget::translateMouseEvent (this=0x9967440, event=0xbff49118) at kernel/qapplication.h:518
#11 0x049eaf01 in QApplication::x11ProcessEvent (this=0xbff49334, event=0xbff49118) at kernel/qapplication_x11.cpp:3468
#12 0x049ff008 in QEventLoop:rocessEvents (this=0x9944fe0, flags=Variable "flags" is not available.
) at kernel/qeventloop_x11.cpp:192
#13 0x04a6d82b in QEventLoop::enterLoop (this=0x9944fe0) at kernel/qeventloop.cpp:198
#14 0x04a6d736 in QEventLoop::exec (this=0x9944fe0) at kernel/qeventloop.cpp:145
#15 0x04a54aa9 in QApplication::exec (this=0xbff49334) at kernel/qapplication.cpp:2758
#16 0x0804bc31 in main (argc=1, argv=0xbff49414) at main.cpp:19
(gdb) Quit
(gdb)

[wysota]

Can we see the current code too?

[Lebowski]

Code is still the same as it was in the first post.

[wysota]

Then this code is surely invalid as you have a local variable "baza" which points to the database and a member variable "baza" which points to some garbage which probably causes a crash.

[Lebowski]

Possible. But why doesn't compiler find any errors? And how do i change the code to make it work?

[wysota]

Why should it find errors? The code is valid C++, it just doesn't do what you wnt. Zlatko already told you how to correct it.

[Lebowski]

You mean this post?

So why you redeclare it in

QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );

QSqlDatabase *baza = QSqlDatabase::addDatabase ( driver );

To copy to clipboard, switch view to plain text mode 

Right code

baza = QSqlDatabase::addDatabase ( driver )

baza = QSqlDatabase::addDatabase ( driver )

To copy to clipboard, switch view to plain text mode 

I tried that but I'm still getting segmentation fault.

[wysota]

But the code is different. Run it under debugger again. And please paste the contents of test_clicked() slot.

[Lebowski]

Hmm, I think I figured out my problem with help from all of you. I need to properly specify 'QSqlDatabase *baza; ' as a global variable. Now the n00b question: how do I do that, please? Tried to do it in my main.cpp but it doesn't work.

[wysota]

No, you don't have to do that. First of all QSqlDatabase::addDatabase() returns an object and not a pointer. And second of all you can access the database later on by calling QSqlDatabase::database(). You don't need a variable to hold that handler. Please read the reference of QSqlDatabase. It's explained there (on the very beginning of the detailed description).

[Lebowski]

Following that example I changed

1.
      void GlavnoO :: pushButton3_clicked()
      {
          bool ok = FALSE;
          ok = povezava.baza->isOpen();
      }

1.
      void GlavnoO :: pushButton3_clicked()
      {
          bool ok = FALSE;
          ok = povezava.baza->isOpen();
      }

To copy to clipboard, switch view to plain text mode 

to

void GlavnoO::pushButton3_clicked()
{
    bool ok = FALSE;
    QSqlDatabase *baza = QSqlDatabase::database();
    ok = baza->isOpen();
}

void GlavnoO::pushButton3_clicked()
{
    bool ok = FALSE;
    QSqlDatabase *baza = QSqlDatabase::database();
    ok = baza->isOpen();
}

To copy to clipboard, switch view to plain text mode 

It compiles normally but it crashes with segmentation fault again when I press that button.

And I have to use pointers cause I have this in my Qt Assistant QSqlDatabase Reference:

Static Public Members
QSqlDatabase * addDatabase ( const QString & type, const QString & connectionName = defaultConnection )
QSqlDatabase * addDatabase ( QSqlDriver * driver, const QString & connectionName = defaultConnection )
QSqlDatabase * database ( const QString & connectionName = defaultConnection, bool open = TRUE )
void removeDatabase ( const QString & connectionName )
void removeDatabase ( QSqlDatabase * db )
bool contains ( const QString & connectionName = defaultConnection )
QStringList drivers ()
void registerSqlDriver ( const QString & name, const QSqlDriverCreatorBase * creator )
bool isDriverAvailable ( const QString & name )