Issue emitting signal from background thread.

I have a class that I move to a background thread that constantly checks a server for updated settings. If the settings were updated then I emit a signal that forces the program to restart with the new settings. The problem I'm having is when I try to emit the signal it throws a segfault.

Where I connect this signal to the slot I want it to call.

Code:

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connect(&settings_agent, SIGNAL(updatedSettingsNotification()), LCD,SLOT(restarting()));

---

This signal that I am trying to emit:

Code:

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void SettingsFileRetrievalAgent::handleDeletedRegistration()
{
      emit updatedSettingsNotification();     //if delete registration, this can cause a crash (let it run maybe 15 seconds after startup).
}

---

This is the result from GDB Debugger:

Code:

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Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 30704.0x7c2c]
0x6a2b0e31 in QObjectPrivate::isSignalConnected (this=0x0, signal_index=3)
    at ../../include/QtCore/private/../../../../../../../ndk_buildrepos/qt-desktop/src/corelib/kernel/qobject_p.h:219
219     ../../include/QtCore/private/../../../../../../../ndk_buildrepos/qt-desktop/src/corelib/kernel/qobject_p.h: No
uch file or directory.
        in ../../include/QtCore/private/../../../../../../../ndk_buildrepos/qt-desktop/src/corelib/kernel/qobject_p.h

---

The result when I do a bt:

Code:

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#0  0x6a2b0e31 in QObjectPrivate::isSignalConnected (this=0x0, signal_index=3)
    at ../../include/QtCore/private/../../../../../../../ndk_buildrepos/qt-desktop/src/cor
#1  0x6a2164d8 in QMetaObject::activate (sender=0x28d2cc, m=0x4a80cc, local_signal_index=1
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qobject.cpp:3201
#2  0x0041dced in ?? ()
#3  0x0040fd66 in ?? ()
#4  0x0041dc73 in ?? ()
#5  0x6a205ff8 in QMetaObject::metacall (object=0x28d2cc, cl=QMetaObject::InvokeMetaMethod
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qmetaobject.cpp:237
#6  0x6a210c9d in QMetaCallEvent::placeMetaCall (this=0xe0a67b8, object=0x28d2cc)
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qobject.cpp:535
#7  0x6a212ba8 in QObject::event (this=0x28d2cc, e=0xe0a67b8)
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qobject.cpp:1217
#8  0x0111bd88 in QApplicationPrivate::notify_helper (this=0xa0d6bb8, receiver=0x28d2cc, e
    at c:\ndk_buildrepos\qt-desktop\src\gui\kernel\qapplication.cpp:4462
#9  0x011196da in QApplication::notify (this=0x28fe4c, receiver=0x28d2cc, e=0xe0a67b8)
    at c:\ndk_buildrepos\qt-desktop\src\gui\kernel\qapplication.cpp:3862
#10 0x6a201540 in QCoreApplication::notifyInternal (this=0x28fe4c, receiver=0x28d2cc, even
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qcoreapplication.cpp:731
#11 0x6a268258 in QCoreApplication::sendEvent (receiver=0x28d2cc, event=0xe0a67b8)
    at c:/ndk_buildrepos/qt-desktop/src/corelib/kernel//qcoreapplication.h:215
#12 0x6a2025eb in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, d
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qcoreapplication.cpp:1372
#13 0x6a2248e7 in qt_internal_proc (hwnd=0x440ba8, message=1025, wp=0, lp=0)
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qeventdispatcher_win.cpp:497
#14 0x75ed62fa in USER32!OffsetRect () from C:\Windows\syswow64\user32.dll
#15 0x00440ba8 in ?? ()
#16 0x75ed6d3a in USER32!IsWindow () from C:\Windows\syswow64\user32.dll
#17 0x6a22467e in qt_fast_timer_proc (timerId=0, user=4459432)
    at c:\ndk_buildrepos\qt-desktop\src\corelib\kernel\qeventdispatcher_win.cpp:429
#18 0x75ed77c4 in USER32!AnyPopup () from C:\Windows\syswow64\user32.dll
#19 0x00000000 in ?? ()

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Code:

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this=0x0

---

I'm assuming this is the problem. I don't know if it's related to how the QThreads and Signals work, I just can't seem to figure out what the problem is. Any hints or ideas as to why this is happening would be great.

show your code.
It seems 'LCD' is null.

Start a new thread and run the worker_thread()

Code:

---

QThread *settingsFileUpdaterThread = new QThread;
settings_agent.moveToThread(settingsFileUpdaterThread);
settingsFileUpdaterThread->start();
QMetaObject::invokeMethod(&settings_agent, "worker_thread", Qt::QueuedConnection);

---

Code:

---

void SettingsFileRetrievalAgent::worker_thread()
{
  while(1)
      status = updateSettingsIfPossible();
 
     if (status == RETURN_SUCCESS_CACHE_UPDATED)
     {
         emit updatedSettingsNotification(); //segfault
     }
     for( int count = 0; count < 2; ++count )
       sleep( 1 );
  }
}

---

I have other threads that are just about identical and they don't seem to have a problem when emitting a signal.

Right.
Where did you initialize 'LCD'?

Code:

---

MainWindow::MainWindow(QWidget *parent) :
  QMainWindow(parent),
  ui(new Ui::MainWindow)
{
  ui->setupUi(this);
  QMetaObject::invokeMethod(this, "startup" , Qt::QueuedConnection);
}

---

Code:

---

void MainWindow::startup()
      // Get pointer to layout manager for main window
      QLayout* layout = this->layout();
      // Add LCD screen to the main window layout at specified position
      LCD = new(IPR5000_LCD)(this);
      LCD->setGeometry(430,50,320,240);
      LCD->setParent(this);
      layout->addWidget(LCD);
}

---

does this initialization happen before you start settingsFileUpdaterThread ?
If so, what exactly "restaring()" does?
Sound like a place where all kind of mischief can happen, if "restarting()" is re-spawning threads or similar thread related stuff - if this is the case - care has to be taken that things are all correctly terminated and initialized.
If you comment out all the content of "restarting()" does it still crash (the same way)?

Code:

---

void MainWindow::startup()
      // Get pointer to layout manager for main window
      QLayout* layout = this->layout();
      // Add LCD screen to the main window layout at specified position
      LCD = new(IPR5000_LCD)(this);
      LCD->setGeometry(430,50,320,240);
      LCD->setParent(this);
      layout->addWidget(LCD);
 
      KeyManagementAgent key_management_agent;
      SettingsFileRetrievalAgent settings_agent( key_management_agent );
      connect(&settings_agent, SIGNAL(updateSettingsNotification()), LCD, SLOT(restarting()));
      // move it to its own thread and do stuff with it.
     QThread *settingsFileUpdaterThread = new QThread;
     settings_agent.moveToThread(settingsFileUpdaterThread);
     settingsFileUpdaterThread->start();
     QMetaObject::invokeMethod(&settings_agent, "worker_thread", Qt::QueuedConnection);
 
}

---

Code:

---

void IPR5000_LCD::restarting()
{
  ui->stackedWidget->setCurrentIndex(2);
}

---

Like I said previously I do this sort of thing other places without problems so I don't see any reason there would be a problem with the 'LCD', it just happens to be this thread that is giving me issues.

Commenting out the code in the restarting() method still results in a segfault. I tried printing a simple debug message and it still crashes. I don't think it's a problem with the LCD class. The LCD slot is never called because the signal is never emitted, that's how I know it cant be the LCD.

Edit: It will loop about 5-8 times before it throws a segfault.

I really appreciate the help.

1. Is 'key_management_agent' a parent for 'settings_agent' or just a parameter?
2. settings_agent is a stack variable, which dies at the end of the constructor. I would expect moveToThread() to have a problem with that - try allocating it on the heap.
This would also explain this==NULL.

Quote:

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Like I said previously I do this sort of thing other places without problems so I don't see any reason there would be a problem with the 'LCD', it just happens to be this thread that is giving me issues.

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1.LCD was a likely candidate - since I don't know your code I wanted to rule it out.
2.I fail to see the relevance of the claim that something worked somewhere else since obviously its not *exactly* the same, and secondly, obviously it doesn't work here - so it helps little that it does somewhere else.
Specially when threads are in question - since alone different hardware (same software) may produce different results, or bring bugs to the surface on one system that are not see on another.
For your own good - stop thinking "this has to work" if it doesn't - rather "I must be doing something wrong if it doesn't work".
By the way, the code as you posted it, really requires no threads.

Quote:

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Originally Posted by high_flyer

1. Is 'key_management_agent' a parent for 'settings_agent' or just a parameter?
2. settings_agent is a stack variable, which dies at the end of the constructor. I would expect moveToThread() to have a problem with that - try allocating it on the heap.
This would also explain this==NULL.

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Upon further investigation the settings_agent and the key_management_agent was being destroyed after we finished the startup function. I don't know how I managed to overlook that but I did.

Quote:

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Originally Posted by high_flyer

2.I fail to see the relevance of the claim that something worked somewhere else since obviously its not *exactly* the same, and secondly, obviously it doesn't work here - so it helps little that it does somewhere else.
Specially when threads are in question - since alone different hardware (same software) may produce different results, or bring bugs to the surface on one system that are not see on another.
For your own good - stop thinking "this has to work" if it doesn't - rather "I must be doing something wrong if it doesn't work".
By the way, the code as you posted it, really requires no threads.

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I understand, I was extremely frustrated and I overlooked the fact that the stack variable was being destroyed.


Thanks for the help.