Starting QProcess inside a thread

  • I have been struggling with this basic problem for a while now.
    I’m trying to start a QProcess from a thread. Starting the process works and the process runs correctly but my problem is that the finished() signal is never emitted.

    Here is my example:

    My class variables are

    std::atomic<bool>  recording;
    QProcess proc;
    std::unique_ptr<std::thread> recordingThread;

    The class:

    Recorder::Recorder(ParentClass *parent): QObject(parent){
    void Recorder::start(){
    	if (!recordingThread){
    		recording = true;
    		recordingThread.reset(new std::thread(&Recorder::recordThread, this));
    void Recorder::recordThread(){
    		//writing frame
    	proc.start("C:\\ffmpeg.exe", QStringList() <<"-i"<< picDir.c_str() << "-r"<< "30" << "-vcodec"<< "ffv1" << filename.c_str());
    void Recorder::stop(){
    	if (recordingThread) {
    		recording = false;
    		recordingThread->join(); recordingThread.reset();  
    void Recorder::finishedFFMPEG(){
    	qDebug() << "finished";

    start() and stop() are called from another non-GUI thread in my ParentClass.

    I tried everything from using pointers, running my recordThread() as a QThread and starting the QProcess in the stop() function but I simply never receive the finished() signal from the process. The process itself is being executed correctly. I know the problem lies in the different event loops.

    How can I achieve my goal of starting a process after the recordThread() finishes and catching the QProcess finished() signal?

  • It looks like there's no use of an event loop in the thread that starts the process. Without this or the use of a function that blocks until the process terminates, process termination won't be detected. In general, Qt's asynchronous interfaces depend on the event loop or triggering via a synchronous (relatively speaking) interface.

  • I know it has to do with the event loop but I can't find a way to make it work. I can't just block recordThread() until the process is finished because I need a new recordThread() to be able to start while the process is still running. Do you have any suggestion how I can achieve my goal?

  • Lifetime Qt Champion


    Since you are using Qt why not use a QThread ? But first thing, are you sure you need a thread at all to call QProcess ?

  • I'm not sure if you read the main post as I stated that I did try running it as a QThread. The QProcess has to run after the recordThread() finishes its task.

  • Presuming that the thread structure in use is appropriate, and the presence of a main thread running QCoreApplication::exec(), you can move the QProcess object to the main thread with QObject::moveToThread.

    void Thread::run()
        QProcess *process = new QProcess();
        connect(process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(done()));
        process->start(program, QStringList());
        if (!process->waitForStarted())
            qDebug() << "error";
    void Thread::done()
        qDebug() << "process done";

    Another option is to use a QObject previously associated with the main thread with a slot that starts the external process.

  • Lifetime Qt Champion

    Yes, I did read it but misinterpreted your use of QThread.

    You should rather split responsibilities here. Have your recording thread do only that: recording. And once it's done make it emit a signal that you will connect to a slot that will start the ffmpeg QProcess. That way you can also encapsulate ffmpeg so that you can retrieve its output to check that everything is going fine.

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