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Semaphore and multithread



  • Hi
    I have a simple app with one function reading interrupt and with ui.label writing the required value

    I have to divide the function(that checks interrupt) and the ui-label.

    Expert suggest me to put semaphore in.

    How can I do ? 
    
    
    *MainWindow::MainWindow(QWidget* parent) :
        QMainWindow(parent),
        ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
       
        qDebug() << "Program started!"
    
        QTimer* timer = new QTimer(this);
        connect(timer, SIGNAL(timeout()), this, SLOT(poll()));
        //timer->start(1000);
        timer->start(500);
    }
    
    
    
    unsigned char MainWindow::poll()
    {
    
    //------------------------------------------------------------------------------
    unsigned char MainWindow::poll()
    {
        unsigned short distance;
            qDebug() << " Polling started";
    
             check_for_interrupt(ADDR, &distance); 
            
                qDebug() << "valore distanza:" << distance;
     
                ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    
        
            }
    
    --------------------*
    

  • Lifetime Qt Champion

    @Montanaro said in Semaphore and multithread:

    I have to divide the function(that checks interrupt) and the ui-label.

    What do you mean by "devide"?
    How are "Semaphore and multithread" involved here? I can't see anything in the code you posted.



  • Well, now I have only one thread.

    But ui.label and check_for_interrupt use the same channel.
    So maybe the ui.label and check_for_interrupt collide-

    So I need to put ui.label in one thread and check_for_interrupt in other thread and lock/unlock the channel/device they use.


  • Lifetime Qt Champion

    @Montanaro Not sure what you mean by "channel".
    Is check_for_interrupt a blocking call? If not you do not need a second thread.

    If you do need a second thread, then put check_for_interrupt there and emit a signal when you get new data. Connect that signal to a slot in your MainWindow where you then update the label.



  • @jsulm

    yes. I think the use the same channel (frame buffer).

    You suggest this was?

    unsigned char MainWindow::poll()
    {
    unsigned short m_distance;
    unsigned short distance;
    qDebug() << " Polling started";

         check_for_interrupt(ADDR, &distance); 
        
            qDebug() << "distance value:" << distance;  
    

    if(distance!=m_distance)
    { m_distance=distance;
    emit ui->label->setText("the value of distance is mm : " + QString::number(distance));
    }

        }
    


  • @Montanaro
    Assuming check_for_interrupt() is non-blocking (returns with an answer immediately) this is OK, if there's no other way to check fro the interrupt. If it blocks then this is not OK.

    Your code would be "nicer" if you emitted a signal when the interrupt arrives instead of putting your code for what to do into MainWindow::poll(), but that's up to you.

    If this is the actual code of that method, then the (misnamed here) m_distance local variable is not initialized. I would expect the compiler to warn you about this.


  • Lifetime Qt Champion

    @Montanaro said in Semaphore and multithread:

    You suggest this was?

    No.
    Your code is not even valid:

    emit ui->label->setText("the value of distance is mm : " + QString::number(distance));
    

    This is not how signals/slots work.
    Please read https://doc.qt.io/qt-5/signalsandslots.html



  • @jsulm
    mmmm I can write:

    Mainwindows writing {
    ui->label->setText("the value of distance is mm : " + QString::number(distance))
    }

    and then :

    unsigned char MainWindow::poll()
    {
    unsigned short m_distance;
    unsigned short distance;
    qDebug() << " Polling started";
    check_for_interrupt(ADDR, &distance);

        qDebug() << "distance value:" << distance;  
    

    if(distance!=m_distance)
    { m_distance=distance;
    emit writing
    }
    }

    ?

    Anyway, I m not using semaphore (unlock/lock) but I must use semaphore.


  • Lifetime Qt Champion

    @Montanaro said in Semaphore and multithread:

    but I must use semaphore.

    Why?



  • @Christian-Ehrlicher
    It' s my task.

    ui.label and check_for_interrupt use the same resource(frame buffer).
    Now when I use only ui.label, the code works; when I use only check_for_interrupt, the code works
    but when I use together, the code works bad: the results of check_for_interrupt (this function read a time of flight sensor) is blocked on only one value and it's phisically not possible.
    So I have to show if they collide and then I have to solve the problem.
    I think to use semaphore to assign the resource.


  • Lifetime Qt Champion

    @Montanaro You still did not answer this question: is check_for_interrupt blocking or not?
    As you do not use threads in the code you posted I don't see the need for semaphores at all.
    First you call check_for_interrupt, then ui->label->setText - they are NOT executed at the same time.



  • @jsulm

    Well … I dont know if check_for_interrupt blocks the resource. I suppose that and I want to detect that.

    I think I can use that:
    https://doc.qt.io/archives/qt-4.8/qmutexlocker.html


  • Lifetime Qt Champion

    @Montanaro said in Semaphore and multithread:

    check_for_interrupt blocks the resource

    I'm not talking about blocking a resource. I'm simply asking whether check_for_interrupt blocks for longer time or returns immediately.
    And again: why do you think you need semaphore if you do not use any threads?


  • Moderators

    @Montanaro

    here's the thing, multithreading /parallelization is not trivial. You'll need a good understanding of what you're doing or you end up, at best, wasting lots of time or at worst you shoot yourself in the foot.

    Expert suggest me to put semaphore in.

    I highly doubt that expert status. What is the reasoning behind that statement, please explain

    think I can use that:
    https://doc.qt.io/archives/qt-4.8/qmutexlocker.html

    The point of a Semaphore is not to use a mutex

    does check_for_interrupt actually take time at all ? And if it does how long. And what does it do exactly. There are other ways to do this, much more neater/higher level ones that mutex or semaphores especially in Qt



  • @jsulm
    well Experts tell me that code (ui or check_for_interrupt) can occupy the resource while the other line of code ( ui or check_for_interrupt) starts. I 'm beginner so I dont know if it's true but it's mandatory to check that for me.



  • @Montanaro
    You have now been asked about 3 times whether your check_for_interrupt() is blocking or returns immediately? Are you going to answer to help those who are trying to help you, or are you going to just ignore the question?

    On a separate matter, it is up to you whether you want to follow the "experts'" advice you have received elsewhere, or whether you want to listen to the "experts" here as well? I do not count myself among the "experts", but there are others posting here who are experts (especially in matters Qt, which your other advisors may not be), so you might want to weigh their comments against those you have received from elsewhere.



  • @J-Hilk

    I dont know.
    He (my senior) suggests me to use that :
    https://doc.qt.io/archives/qt-4.8/qmutexlocker.html
    so I must try.

    I have two task:

    1. checking_for_interrupt. this function reads the time of flight sensor and its results are for example 120, 150, 180 (millimeter) when I dont use ui. And those results are phisically correct
    2. only ui shows me the number and text on my display, for example: "the distance is …. mm"

    but when I use together (ui and check_for_interrupt), the results of time of flight are: 3000, 3000, 3000, 3000, 3000 and it 's not possible


  • Lifetime Qt Champion

    @Montanaro Do you mean if you comment out

    ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    

    in

    unsigned char MainWindow::poll()
    {
        unsigned short distance;
            qDebug() << " Polling started";
    
             check_for_interrupt(ADDR, &distance); 
            
                qDebug() << "valore distanza:" << distance;
     
                ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    
        
            }
    

    qDebug() << "valore distanza:" << distance;
    prints correct values?



  • @JonB

    check_for_interrupt returns immediately but always the same result ( I visually dont note difference )

    Well… I understand your suggest but if I dont try to put mutex in, I dont accomplish my task and my senior dont countenance me :(

    @jsulm : no. check_for_interrupt (with ui) prints uncorrect values. it's the problem

    so thanks to all :)


  • Lifetime Qt Champion

    @Montanaro said in Semaphore and multithread:

    no. check_for_interrupt (with ui) prints uncorrect values. it's the problem

    That's why I segested to to comment out this line

    ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    

    and see what happens.
    Or do you mean something different if you write "with ui", because I really don't understand the problem.
    "when I dont use ui" - what does this mean exactly? Do you mean an app without UI works?



  • @jsulm

    I wrote the code without ui and it works. Then I wrote ui->label->setText("il valore della distanza in mm è: " + QString::number(distance)); into my code and code works with time of 1 second (its resultes are correct) and code doesnt work with time of 500 ms (its results are bad)

    I dont understant why.

    Senior suggest me that the resource is occuped by the other process

    @jsulm I think you are right.
    If with ui.label or without ui.label, I Always have ui in my code. But I'm not the boss and I have task :D


  • Moderators

    @Montanaro
    ok, just to make sure, so that I'm not going crazy:

    unsigned char MainWindow::poll()
    {
        unsigned short distance;
        qDebug() << " Polling started";
    
        check_for_interrupt(ADDR, &distance); 
            
        qDebug() << "valore distanza:" << distance;
     
        //ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    }
    

    Results in the correct read of
    120 distance
    150 distance
    180 distance

    but

    unsigned char MainWindow::poll()
    {
        unsigned short distance;
        qDebug() << " Polling started";
    
        check_for_interrupt(ADDR, &distance); 
            
        qDebug() << "valore distanza:" << distance;
     
        ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    }
    

    results in
    3000 distance
    3000 distance
    3000 distance

    ?



  • @Montanaro
    First, please answer @J-Hilk's latest question.

    There must be more going on in your code somewhere than you show... And for my part at least, I don't understand what the "resource" you refer to is....

    I suggest you briefly test one more case. Try having:

        qDebug() << "valore distanza:" << QString::number(distance);  // <- this is changed, it uses `QString::number`
     
        // next line is still commented out
        //ui->label->setText("il valore della distanza in mm è: " + QString::number(distance));
    

    Does this produce right or wrong output? If by any chance this also produces wrong output, it suggests that any call to a Qt method is causing a problem, not the label->setText() itself.



  • @J.Hilk

    Yes. your summary is right

    @JonB thanks. I try your suggest (In check_interrupt, I use ioctl to read i2c from dev/usb/myDevice)



  • @Montanaro
    Depending on your call to ioctl, it could be a blocking or non-blocking call, which is what we have asked you about with success....


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