CODEWITHSUNDEEP
c++qt
Cool_Coder
Cool_Coder

Reputation: 5083

Multi-threading in Qt problematic?

I have been learning Qt for a while and a few days ago I decided to go on multi-threading by QThread. I have been following this person. Since he says that subclassing QThread is not the appropriate way but by using QObject is. I am following his method. But he has created another class which inherits from QObject. I decided to make a few changes:

class myclass
{
private:
    dowork();

private slots:
    slota();
    slotb();
    slotc();
}

myclass::dowork()
{
    QThread lett;
    QThread latt;
    QThread pltt;
    QObject lethread;
    connect(&lett, SIGNAL(started()), this, SLOT(slota()));
    lethread.moveToThread(&lett);
    QObject lathread;
    connect(&latt, SIGNAL(started()), this, SLOT(slotb()));
    lathread.moveToThread(&latt);
    QObject plthread;
    connect(&pltt, SIGNAL(started()), this, SLOT(slotc()));
    plthread.moveToThread(&pltt);

    lett.start();    
    latt.start();
    pltt.start();/*
    lett.wait();
    latt.wait();
    pltt.wait();*/
    while(lett.isRunning())
    {
        //qDebug() << "Still Running!";
    }
    lett.quit();
}

If I run the program it keeps running while loop infinitely! It doesn't come out of that at all. I used qdebug inside the implementation of the 3 slots to check whether it is entering them or not, and surprisingly none of the slots gets called. Why is that so? If I remove the comments for .wait then the program pauses.

What is wrong in this method? I want to avoid that extra class as my program is very small.

Upvotes: 2

Views: 284

Answers (1)

UmNyobe
UmNyobe

Reputation: 22910

while(lett.isRunning())
{
    //qDebug() << "Still Running!";
}
lett.quit();

Terribly wrong. The current thread is busy waiting and will be eating all the time. But this is not your problem. Your problem is certainly that the main event loop has probably not started yet, ie you didn't do QApplication.exec() yet.

Qt uses events to start threads, and when you do use movetoThread, start, specific events are queued in the current thread (ie the thread executing these methods). If the main event loop has already started, then will be processed as soon as they can. But in your case I bet that you main function is :

 int main(){
   QApplication app();
   myclass onlyclassIwanttomake;
   onlyclassIwanttomake.dowork(); 
   app.exec(); <----HERE!!!
 }

The thread will be started when the main thread executes app.exec();. When you are busy waiting, you are preventing the main thread to process the events required to start your events.

EDIT: Well, another issue is that

lett.start();    
latt.start();
pltt.start();

start 3 threads which are started, and stay idle forever. They are not doing anything, but they are running (even if you remove your loop).

This is a entry written about Qt threading. Please take the time to read it.

Upvotes: 2

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