Qt’s GUI Thread

If you’re a Qt developer, you surely are aware of the fact that you can only display GUI elements and access them from the main thread. This limitation as far as I know is mostly bound to the limitations of X and it isn’t to exclude that multithreading support for GUIs will be added soon.

This limitation never caused me any trouble, since the signal & slots mechanism is thread-safe and communicating between threads and GUI elements can be achieved through it. However, yesterday I needed to show a messagebox in a method and, in case the code is not executing in the main thread, show a native win32 MessageBox instead of a QMessageBox (of course, only on Windows can I do that, on other platforms when I’m not in the main thread, I won’t show anything).

Anyway, here’s a simple method to establish if we’re running the GUI thread:

bool isGuiThread()
    if (QCoreApplication::instance()->thread() == QThread::currentThread())
        return true;
    return false;

As you can see this is a pointer comparision, but can we rely on the value returned by currentThread? Yes, we can since the pointer is associated with the thread itself as we can see from the code of the method:

QThread *QThread::currentThread()
    QThreadData *data = QThreadData::current();
    Q_ASSERT(data != 0);
    return data->thread;

// thread_win.cpp

QThreadData *QThreadData::current()
    QThreadData *threadData = reinterpret_cast(TlsGetValue(qt_current_thread_data_tls_index));
    if (!threadData) {
        QThread *adopted = 0;
        if (QInternal::activateCallbacks(QInternal::AdoptCurrentThread, (void **) &adopted)) {
            threadData = QThreadData::get2(adopted);
            TlsSetValue(qt_current_thread_data_tls_index, threadData);
            adopted->d_func()->running = true;
            adopted->d_func()->finished = false;
        } else {
            threadData = new QThreadData;
            // This needs to be called prior to new AdoptedThread() to
            // avoid recursion.
            TlsSetValue(qt_current_thread_data_tls_index, threadData);
            threadData->thread = new QAdoptedThread(threadData);

        if (!QCoreApplicationPrivate::theMainThread) {
            QCoreApplicationPrivate::theMainThread = threadData->thread;
        } else {
            HANDLE realHandle = INVALID_HANDLE_VALUE;
#if !defined(Q_OS_WINCE) || (defined(_WIN32_WCE) && (_WIN32_WCE>=0x600))
                        realHandle = (HANDLE)GetCurrentThreadId();
            qt_watch_adopted_thread(realHandle, threadData->thread);
    return threadData;

qt_create_tls just calls once for every thread TlsAlloc and if the data for the current thread hasn’t been set yet, it is set with TlsSetValue. So, we can rely on a pointer comparision.

Qt: Now LGPL

Nokia today announced that its Qt cross-platform user interface (UI) and application framework for desktop and embedded platforms will be available under the open source LGPL version 2.1 license from the release of Qt 4.5.

I’ve been waiting for such a decision by Nokia and yet it really came as a surprise. Making Qt free even for closed software will hugely increase their popularity. It will also allow me to develop some crossplatform freeware utilities. I’m still working at the kernel of the CFF Explorer in my free time, but once the kernel is finished it can be used to develop some nice stuff apart from the CFF Explorer itself.

This confirms what I wrote in the article about Qt internals and reversing. This framework will be used more and more in the future.

Kudos to the best framework of all time.

Qt Internals & Reversing

Today I took a break from the larger article I’m currently writing. To relax, I wrote a smaller article about the Qt framework. I hope you enjoy.

Qt Internals & Reversing

Half of the text of this article comes from my larger paper “Dynamic C++ Proposal”. I decided that it was useful to take the part about Qt internals, put it into another article and extend it by adding a reversing part. Because of its nature, this is not the usual kind of article I write. In fact, I wrote the reversing part in less than a day. So, this is a very easy one. However, I think it is useful for people who need to reverse a Qt application and certainly wouldn’t consider reading my other paper about Dynamic C++, which doesn’t sound like a paper about Qt and, in fact, isn’t a paper about Qt: the paragraph about Qt is only one among many others. Moreover, I haven’t seen serious articles about this subject.

The first thing which needs to be considered when reversing Qt applications is what Qt brought to the C++ language. Events (inside the Qt framework) are just virtual functions, so nothing new there. This is not a C++ reversing guide. What is new in Qt are signals and slots, which rely on the dynamism of the Qt framework.

So, first thing I’m going to show how this dynamism works. The second part focus on reversing and, at that point, I will show how to obtain all the metadata one needs when disassembling a “Q_OBJECT” class.