qt QCoreApplication::exec() crash on ubuntu 12.04

Question

I am developing a system application with signal slot on sockets in qt, ubuntu

void Receiver::startServer()
{
....
connect(udpSocket, SIGNAL(readyRead()), this, SLOT(receiveFunc()));
}

void Receiver::receiveFunc()
{


emit receiveForwarder();
{


Handler::start()
{
receiver= new Receiver();
thread=new QThread()
receiver->moveToThread(thread);
receiver->startServer();

connect(receiver, SIGNAL(receiveForwarder()), this, SLOT(processTexts()));
}

Handler::processTexts()
{



//emit another signal that is in another thread
}

I have a processTexts function that is either a slot for a thread and also emits a signal to another thread. If I omit the second emit in Handler the problem is not solved but program crashes somewhat later. But it does not disapear. it creashes after a minute or two. program processes texts that comes from UDP socket. and sends it via another QTcpSocket to another program.

The crash signature is :

0[bt:0]/application/collector/main.cpp::exceptionHandler(int)::30
./collector(_Z16exceptionHandleri+0x56)[0x422156]
/lib/x86_64-linux-gnu/libc.so.6(+0x364c0)[0x7fbf348f54c0]
/usr/lib/x86_64-linux-gnu/libQtCore.so.4(+0x1a9b24)[0x7fbf35558b24]
/lib/x86_64-linux-gnu/libglib-2.0.so.0(g_main_context_check+0x183)[0x7fbf33db1a33]
/lib/x86_64-linux-gnu/libglib-2.0.so.0(+0x47f96)[0x7fbf33db1f96]
/lib/x86_64-linux-gnu/libglib-2.0.so.0(g_main_context_iteration+0x34)[0x7fbf33db2124]
/usr/lib/x86_64-linugnu-libQtCore.so.4(_ZN20QEventDispatcherGlib13processEventsE6QFlagsIN10QEventLoop17ProcessEven    tsFlagEE+0xd6)[0x7fbf35559426]
/usr/lib/x86_64-linux-gnu/libQtCore.so.4(_ZN10QEventLoop13processEventsE6QFlagsINS_17ProcessEventsFlagEE+0x32)       [0x7fbf35528c82]
/usr/lib/x86_64-linux-gnu/libQtCore.so.4(_ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE+0xf7)      [0x7fbf35528ed7]
  /usr/lib/x86_64-linux-gnu/libQtCore.so.4(_ZN16QCoreApplication4execEv+0x87)[0x7fbf3552df67]
  ./collector(main+0x216)[0x4234c6]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xed)[0x7fbf348e076d]

I analyzed it with valgrind and it says:

==31981== Conditional jump or move depends on uninitialised value(s)
==31981==    at 0x556AB14: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)
==31981==
==31981== Invalid read of size 2
==31981==    at 0x556AB45: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)
==31981==  Address 0xa8bf0c6 is 6 bytes inside a block of size 16 free'd
==31981==    at 0x4C2A4BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31981==    by 0x556AB44: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)

==31981==
==31981== Invalid read of size 2
==31981==    at 0x556AB4D: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)
==31981==  Address 0xa8bf0c4 is 4 bytes inside a block of size 16 free'd
==31981==    at 0x4C2A4BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31981==    by 0x556AB44: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)

==31981==
==31981== Thread 2:
==31981== Invalid write of size 2
==31981==    at 0x6C281FA: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C291D1: g_source_add_poll (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B7A9: QEventDispatcherGlib::registerSocketNotifier(QSocketNotifier*) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x5145E59: QUdpSocket::readDatagram(char*, long long, QHostAddress*, unsigned short*) (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.1)
==31981==    by 0x4205F1: LogReceiver::receiveLog() (udpSocket.cpp:52)
==31981==    by 0x42B630: LogReceiver::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_udpSocket.cpp:54)
==31981==    by 0x5555445: QObject::event(QEvent*) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553BE9B: QCoreApplication::notifyInternal(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FC69: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x556AF92: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AC99: g_main_context_dispatch (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B05F: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==  Address 0xa8bf0c6 is 6 bytes inside a block of size 16 free'd
==31981==    at 0x4C2A4BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31981==    by 0x556AB44: ??? (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x6C2AA32: g_main_context_check (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2AF95: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x6C2B123: g_main_context_iteration (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.1)
==31981==    by 0x556B3BE: QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AC81: QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553AED6: QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x553FF66: QCoreApplication::exec() (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.1)
==31981==    by 0x4234C5: main (main.cpp:170)

I analyzed the program and I removed all my bugs I do not know if it is a Qt bug or ubuntu bug or GCC or my fault.

Besides I wanted to know if anybody tried a system application with more than 20 events per second. It crashes after a while.

Thank you all in advance.

Answer 1

SocketNotifier implementation is not thread safe. That means that you are not allowed to control it from different threads.

If you have Qt compiled without QT_NO_DEBUG you will receive warning that you control it from different threads:

void QEventDispatcherGlib::unregisterSocketNotifier(QSocketNotifier *notifier)
{
    Q_ASSERT(notifier);
#ifndef QT_NO_DEBUG
    int sockfd = notifier->socket();
    if (sockfd < 0) {
        qWarning("QSocketNotifier: Internal error");
        return;
    } else if (notifier->thread() != thread()
               || thread() != QThread::currentThread()) {
        qWarning("QSocketNotifier: socket notifiers cannot be disabled from another thread");
        return;
    }   
#endif
//....
}

Please check if all member of object, that is moved to thread, are created as a child of this object. Or just switch off QT_NO_DEBUG.

BTW: I struggled with similar problem and the crash appears after few hours of usage.