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.

Dynamic C++

As anticipated, I just finished my Dynamic C++ proposal. I’d like to thank my friend Quake2 for his support during the last month.

I wrote this document because I needed to express myself about this subject. Despite the fact that C++ is one of the most used programming languages, especially for serious projects, it gets much criticism for being messy, bloated, complicate etc. I believe these critics miss the point. Yes, C++ is a very powerful programming language and that’s why it is difficult. And this is also why sometimes C++ source codes are poorly written. I don’t believe in improvements of C++ resulting in a new programming language. All attempts in that direction have failed. I think that C++ is here to stay for many reasons. Not only because of the amount of code already available in C++, but also because at the moment there isn’t a better programming language for large projects. The only thing I want is for C++ to evolve, but not by losing compatibility with older code or by removing some features. No, I’d like C++ to evolve in a healthy and compatible way. This paper contains the suggestions to achieve this and I will demonstrate technically how it can be implemented at low level.

Everybody should be warned that the material contained in this paper is purely theoretical. The first idea behind this paper came out while working on a particular project. At the time I discovered myself in need of particular dynamic features. So, for many months I had some ideas in the background of my mind and decided eventually to write them down. So, in this paper I’m going to talk about the current status of dynamism for C++, why dynamism is important and what could be done. At the time of writing (November 2008) the new C++0x (or C++09) standard has not yet been introduced. However, I will talk about it throughout this paper when the related topic is affected by it.