Launching a while-loop freezes program even if using another thread

Question

In my (Qt-)program I need a continuous request of a value which I get from an external source. But I did not want that this request freezes the whole program, so I created a separate thread for this function. But even if it is running in a separate thread, the GUI freezes, too. Why?

Code for the request function:

void DPC::run()
{
    int counts = 0, old_counts = 0;
    while(1)
    {
        usleep(50000);
        counts = Read_DPC();
        if(counts != old_counts)
        {
            emit currentCount(counts);
            old_counts = counts;
        }

    }
}

Read_DPC() returns an int value I want to sent to a lineEdit in my GUI.
The main class looks like

class DPC: public QThread
{
    Q_OBJECT
public:
    void run();
signals:
    void currentCount(int);
};

This code is called in the main function as:

DPC *newDPC = new DPC;
connect(newDPC, SIGNAL(currentCount(int)), SLOT(oncurrentCount(int)));
connect(newDPC, SIGNAL(finished()), newDPC, SLOT(deleteLater()));
newDPC->run();

How can I prevent this code from freezing my GUI? What am I doing wrong? Thanks!

I thought via creating a subclass which is derived from QThread? According to some threads (stackoverflow.com/questions/14545961/…) or (stackoverflow.com/questions/16501284/…) — arc_lupus
– arc_lupus, Commented Oct 15, 2014 at 17:40
Why do you call run from the main thread? Didn't you want some other thread to call run? Wasn't that the whole point of deriving from QThread? — David Schwartz
– David Schwartz, Commented Oct 15, 2014 at 17:51
How should I start the subthread if not using run? I tried using start, but that did not work, too (I simply replaced run with start, if that was correct) — arc_lupus
– arc_lupus, Commented Oct 15, 2014 at 17:52
@arc_lupus One bug at a time. Fix the call to run and then re-test, describing in as much detail as possible what goes wrong after that bug is fixed. — David Schwartz
– David Schwartz, Commented Oct 15, 2014 at 18:57

Jablonski · Accepted Answer · 2014-10-15 18:51:27Z

5

It seems that you code run in GUI thread because you use run() method to start thread, so try to call start() as documentation and many examples said.

Try:

DPC *newDPC = new DPC;
connect(newDPC, SIGNAL(currentCount(int)), SLOT(oncurrentCount(int)));
connect(newDPC, SIGNAL(finished()), newDPC, SLOT(deleteLater()));
newDPC->start();//not run

Anyways you can call thread() method or currentThread() to see in which thread some objects live.

edited Oct 15, 2014 at 18:51

answered Oct 15, 2014 at 17:44

Jablonski

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17 Comments

arc_lupus Over a year ago

Using start() leads to freeze, too.

Jablonski Over a year ago

@arc_lupus add qDebug() << thread() to your mainWindow and in the run() Did you get same threads? What exactly you get? Same threads means same adress in the output

Jablonski Over a year ago

@arc_lupus I run same code right now and try to undestand problem. Tell me please what you can see when in run you'll wrote(in while loop): qDebug() << thread() << currentThread();

Jablonski Over a year ago

@arc_lupus when I use run instead of start my GUI freeze too, call run is wrong, now I try understand why start() do not work for you.

Jablonski Over a year ago

@N1ghtLight I will, it was cool.There is no any result from voting up comment, but your contribution will not be forgotten or omitted ;)

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