Qt - how to kill QThread

Question

I'm trying to kill/cancel a QThread. I followed the implementation of https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

I create the QThread like this:

workerThread = new QThread();
ImageProcessor* worker = new ImageProcessor();
worker->moveToThread(workerThread);
connect(workerThread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), workerThread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(workerThread, SIGNAL(finished()), workerThread, SLOT(deleteLater()));
workerThread->start();

and try to stop it on a certain UI event by calling:

workerThread->quit();

My worker looks like this:

class ImageProcessor : public QObject
{
Q_OBJECT
public:
explicit ImageProcessor(QObject *parent = 0);
~ImageProcessor();

signals:
void finished();

public slots:
void process();

private:
//...
};

Where process calls an API with heavy calculations

void ImageProcessor::process()
{
// API call...

emit finished();
}

My problem is, that after calling quit on the thread, the thread keeps running ie the calculations of the API don't stop. Because the worker only calls the API, I cannot work with a cancel check in the worker loop. Any ideas?

Thanks in advance!

Answer 1

Your heavy task is so simple that I would use QtConcurrent::run and forget about QThread.

ImageProcessor* worker = new ImageProcessor();
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
QtConcurrent::run(worker, &ImageProcessor::process);

This does exactly what you need and it will be even a bit faster since it will use thread from thread pool, instead creating a new thread.

Just remember to use Qt::AutoConnection (default value).

To stop processing gracefully you have to do a poling of some variable in your process method:

bool ImageProcessor::needEndNow() {
    QMutexLocker locker(mutex);
    return endNowRequested;
}

void ImageProcessor::requestEndNow() {
    QMutexLocker locker(mutex);
    endNowRequested = true;
}

void ImageProcessor::process()
{
   while(!needEndNow()) {
       // nextStep of processing
   }

   emit finished();
}

Answer 2

If you have something like this, without any loop:

void ImageProcessor::process()
{
    worker_api(); // that takes a lot of time
    emit finished();
}

There is little you can do with QThread. You can do, from probably better to worst:

• Maybe the API you are using has another call to stop that worker call. But since you are asking here, I guess there is no such thing.
• Launch the worker call from a different process using QProcess and kill it when no longer needed. Pros: no worries about clean-up. Contras: retrieving results may be difficult.
• QThread::terminate, but is unadvised (see the warnings in the documentation: basically, no clean-up).
• In Windows, QThread::currentThreadId gives you a number you may be able to use with other OS calls (see the warnings in the documentation).