Program crashes (SIGSEGV)

[Voldemort]

Hello,

My program crashes sometimes (SIGSEGV - Segmentation fault). This is (a part of) the code:

This shows the error message when the connection with the server was broken or stopped.

void Window_Login_Busy::show_errors(int sort_error)
{
    close();
    window_login->show();
    progressbar->setValue(0);
 
    QString text = "Aanmelden mislukt";
    if(sort_error == 1)
        text += ": Disconnected from server.";
    else if (sort_error == 2)
        text += ": Gebruikersnaam of wachtwoord is fout.";
 
    QMessageBox err;
    err.setIcon(QMessageBox::Warning);
    err.setWindowTitle("QQMsn: Error");
    err.setText(text);
    err.exec();
 
    //debug->setData("bla");
}

void Window_Login_Busy::show_errors(int sort_error)
{
    close();
    window_login->show();
    progressbar->setValue(0);

    QString text = "Aanmelden mislukt";
    if(sort_error == 1)
        text += ": Disconnected from server.";
    else if (sort_error == 2)
        text += ": Gebruikersnaam of wachtwoord is fout.";

    QMessageBox err;
    err.setIcon(QMessageBox::Warning);
    err.setWindowTitle("QQMsn: Error");
    err.setText(text);
    err.exec();

    //debug->setData("bla");
}

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and

Here someone entered the wrong password, I break the connection with the server.

while(ssl_sck_dalogin->canReadLine())
    {
        [...]
        else if(line.contains("HTTP/1.1 302 FOUND"))
        {
            debug->setData("Redirect");
 
            break;
        }
                [...]
    }

while(ssl_sck_dalogin->canReadLine())
    {
        [...]
        else if(line.contains("HTTP/1.1 302 FOUND"))
        {
            debug->setData("Redirect");

            break;
        }
                [...]
    }

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and

This is the emited slot when I close the connection, it uses the standard Qt signal:

connect(socket, SIGNAL(disconnected()), SLOT(isDisconnected()));

login_busy->show_errors(1); is the function in the first code I posted.

void cmsn::isDisconnected()
{
    debug->setData("\nDisconnected from host (Notification server).");
 
    if(login_busy->change_lbl)
        login_busy->show_errors(1);
}

void cmsn::isDisconnected()
{
    debug->setData("\nDisconnected from host (Notification server).");

    if(login_busy->change_lbl)
        login_busy->show_errors(1);
}

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When someone enters the wrong password, I get an error dialog with an Ok button, the login screen is showed and the login_busy is closed. But when you close that screen or press the Ok button, the program receives the SIGSEGV signal and crashes.

gdb says:

`/home/quinten/QQMsn_build/QQMsn' has changed; re-reading symbols.
Starting program: /home/quinten/QQMsn_build/QQMsn
Failed to read a valid object file image from memory.
[Thread debugging using libthread_db enabled]
[New Thread -1223088432 (LWP 4881)]
[New Thread -1225602160 (LWP 4882)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1223088432 (LWP 4881)]

When I don't show an MessageBox, the program does not crash and gdb says:

Starting program: /home/quinten/QQMsn_build/QQMsn
Failed to read a valid object file image from memory.
[Thread debugging using libthread_db enabled]
[New Thread -1222646064 (LWP 4927)]
[New Thread -1225159792 (LWP 4928)]
[Thread -1225159792 (LWP 4928) exited]

Program exited normally.

There are some cases where the server closes the connection (e.g. entering a wrong username). In that case, the same slot is emited, the
MessageBox shows, and when pressing OK, the MessageBox closes and the program doesn't crash.


Why does my program crash? How can I solve it? I don't understand where the error is, in my ssl class, in my window_login class, in Qt, in OpenSSL or somewhere else? When you need the whole source code, I'll post it.

[marcel]

My assumption is that calling close() in show_errors causes a lot of problems.
It will post a close event in the event loop. But then you show a modal warning message box. This will block all activity in your window, and show_errors will not exit until you close the dialog.

Try using hide() instead if you can, or show the warning message box and the login window after the main window is closed. You could do something about this in close event.

Or, in isDisconnected, call showErrors and then call login_busy->close() right after it, but remove close() from show_errors().

Perhaps it would be more helpful if you posted the entire code, so somebody can testy it.

Regards

[jacek]

Could we see the backtrace? Do you use threads in your application?

[Voldemort]

When I remove the close(); (just for testing), it still crashes, so I think the problem is not the close(); but something else.

Where can I find backtraces?

What are threads?

Source
The source is in the attachment.

Licence
GPLv2

Info
When you enter your MSN and correct password, it will stay to 100%, that's because I have to program the other things. The crash happens only when you enter a valid mailadres and a wrong password.

Requirements
Qt 4.3
CMake (don't know which version, I use CMake 2.4.6)

Compiling

mkdir QQMsn_build
cd QQMsn_build
cmake ../<source_dir>
make

mkdir QQMsn_build
cd QQMsn_build
cmake ../<source_dir>
make

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[wysota]

Where can I find backtraces?

Enter "bt" in gdb after the application crashes.

What are threads?

http://en.wikipedia.org/wiki/Thread_(computer_science)

[Voldemort]

Backtrace:

#0  0xb668ad83 in SSL_read () from /usr/lib/libssl.so
#1  0x00000004 in ?? ()
#2  0x00000004 in ?? ()
#3  0x00000000 in ?? ()

#0  0xb668ad83 in SSL_read () from /usr/lib/libssl.so
#1  0x00000004 in ?? ()
#2  0x00000004 in ?? ()
#3  0x00000000 in ?? ()

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But I don't understand a backtrace, how do you have to read it? Where can you find the crucial information in the backtrace?

I don't think I use threads.

[wysota]

First you have to compile your application with debug information (CONFIG+=debug in .pro file). Adding QMAKE_CXXFLAGS+=-ggdb will give you even more info.

[Voldemort]

But I use CMake, not QMake, how can I do the same in CMake?

[wysota]

Consult the manual on how to set CXXFLAGS for your project.

[Voldemort]

I can't find the answer in the documentation

http://www.cmake.org/HTML/Documentation.html

[wysota]

set(CMAKE_CXX_FLAGS "-ggdb")

[Voldemort]

I added the line, but there is no difference, I think I have to this:

First you have to compile your application with debug information (CONFIG+=debug in .pro file).

But how do I have to set this option?

[jacek]

#3 0x00000000 in ?? ()

It looks like you try to dereference a null pointer somewhere. Make sure you create all Qt objects after QApplication instance was created.

But I use CMake, not QMake, how can I do the same in CMake?

Run ccmake and set CMAKE_BUILD_TYPE to Debug.

[Voldemort]

1) I create QOBjects before I use QApplication, but that's because otherwise extern won't work (main.cpp). Is that the cause of the crash? How can I solve this problem?

2) How do I disable SSL from QHttp? I need a QSslSocket, so disabling SSL will cause my application won't work.

3) CMAKE_BUILD_TYPE is set to "Debug", no change in the bactrace?

[wysota]

Why not use a pointer to QObject?

[marcel]

QString version = "0.0.1 pre-alpha";
 
debugger *debug = new debugger("QQMsn_debug",version,true);
cmsn *msn = new cmsn();
Window_Login *window_login = 0;
Window_Login_Busy *login_busy = 0;

QString version = "0.0.1 pre-alpha";

debugger *debug = new debugger("QQMsn_debug",version,true);
cmsn *msn = new cmsn();
Window_Login *window_login = 0;
Window_Login_Busy *login_busy = 0;

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Why do you create those objects like that? I believe there is something drawing you back to C .

Instead of:

QString version = "0.0.1 pre-alpha";

QString version = "0.0.1 pre-alpha";

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I suggest using:

#define VERSION "0.0.1 pre-alpha"

#define VERSION "0.0.1 pre-alpha"

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Regarding the other static objects - you delete them in main .
A really "static desing" would have supposed to implement something like AllocateObject ( called before main ) and ReleaseObject( called after main ).

So, I suggest dragging the statics in main, after the QApplication is created, because I don't see any reason why you should keep them there.

As for the segfault, as jacek said you most likely have a QSslSocket in an unallocated QMsn object ( from what I have seen in your code ).