Starting QProcess inside a thread

testus

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){
	
	connect(&proc,SIGNAL(finished(int)),this,SLOT(finishedFFMPEG())); 
}

void Recorder::start(){

	if (!recordingThread){
		recording = true;
		recordingThread.reset(new std::thread(&Recorder::recordThread, this));
	}
}

void Recorder::recordThread(){
	
	while(recording){
		//writing frame
	}
	proc.start("C:\\ffmpeg.exe", QStringList() <<"-i"<< picDir.c_str() << "-r"<< "30" << "-vcodec"<< "ffv1" << filename.c_str());
	proc.waitForStarted();      
}

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?

jeremy_k

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.

testus

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?

SGaist

Hi,

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 ?

testus

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.

jeremy_k

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";
    else
        process->moveToThread(QCoreApplication::instance()->thread());
}

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.

SGaist

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.