Comments on: Lock Free Multithreading in Qt

By: Bob

Bob — Wed, 05 Oct 2011 18:17:55 +0000

Thanks for the article, Dave! Solved my problem. I am using the Chilkat socket library for my app, but can’t use the asynchronous socket calls because they aren’t supported on Linux. Wrapping things in a QThreat wrapper was the perfect answer!

By: Dave

Dave — Fri, 22 Jul 2011 14:12:36 +0000

qtnewbie:

You could try using many small I/O operations (1-2 seconds) instead of one big one (4-5 minutes). You could also look into non-blocking I/O.

–Dave

By: qtnewbie

qtnewbie — Fri, 22 Jul 2011 06:38:50 +0000

Hi,

Thanks for the thread article,Iam using Q threads after reading this,I have 1 doubt regarding thread termination what I have to do to stop a thread if the thread is blocked(I have file I/O operation which takes around 4-5 minutes).I cannot start the event loop as the file read function is blocking the event loop in thread.Pls comment.

By: anon

anon — Wed, 27 Oct 2010 16:25:58 +0000

http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/
moveToThread(this) is deprecated

By: Dave

Dave — Sun, 20 Jun 2010 03:10:28 +0000

Okay, it turns out that subclassing QThread is not actually necessary:

http://blog.exys.org/entries/2010/QThread_affinity.html

You don’t have to write a run() method. You can just *have* a QThread in your class, and call start() on it. Then, call moveToThread(&thread) rather than moveToThread(this).

Very interesting.

By: Victor Jetten

Victor Jetten — Sun, 21 Feb 2010 16:34:44 +0000

it works now, I had commented out “movetothread(this)”. Sorry for polluting your blog! ;-)

By: Victor Jetten

Victor Jetten — Sun, 21 Feb 2010 16:24:09 +0000

sorry! I did not declare sowit under “private slots:” in the class, still getting the hang of this. However now that it runs it does not show every step in the counter, only the last one. I guess emit is not meant for passing values. I’ll check what qtimer does

By: Dave

Dave — Sun, 21 Feb 2010 16:14:29 +0000

Oh, and a couple more things:

Is your method showit() declared as a “slot”, and not just “public”, like this:

public slots:
showit(int i);

Is the show() signal declared as a signal in your thread class like this:

signals:
show(int i);

Also, do you have Q_OBJECT in your Widget class declaration? (you need Q_OBJECT for any class that has signals or slots). You need Q_OBJECT in your thread class too.

If you add Q_OBJECT, you have to re-run “qmake” by the way.

–Dave

By: Dave

Dave — Sun, 21 Feb 2010 16:11:57 +0000

Victor, do you see any warnings on the command line when you run your app (on Windows, you need to add “CONFIG += console” to your .pro file).

I ask because sometimes if you have a typo in your connect() call with your SIGNAL and SLOT text, you won’t see an error in the compiler, but on the command line, you will see a warning when connect() is called.

Also, what is “W”? This should be your thread, not your widget, right?

Lastly, have you used qDebug()? Very handy. Just #include and then you can add debug output like this:

qDebug() < < "showit called with value" << i;

Let me know.

By: Victor Jetten

Victor Jetten — Sun, 21 Feb 2010 15:58:20 +0000

thanks for helping. Showit is nothing else but:
void mod6::showit(const int i)
{
label->setNum(i);
}
where mod6 is the interface class.
In the original prog the thread is a waterbalance model that can run for several hours and gives a lot of results back to the interface at each loop (values, a graph, digital maps). During the run the interface cannot block because the user may need to do things (zoom in and so on). I was also looking at the mandelbrot example in Qt but I liked your mutex-less example.