Executing QProcess in QThread: memory leak

[SGaist]

Hi,

Can you explain why exactly do you need that ping ?

[sitesv]

@SGaist
ping needs to check the availability of pc from the internal network. I thought that this is the simplest way to do this.

[Christian Ehrlicher]

But why a thread then?

[sitesv]

@KroMignon Thank you for your variant.
I have some Qs;
Why you are using QTimer for this purpose?
doPing() is represents my code with using QProcess?
This class should create in the main thread?

[sitesv]

@Christian-Ehrlicher said in Executing QProcess in QThread: memory leak:

But why a thread then?

I need to check availability constantly during the program. I thought the QThread is a good way to do this...

[Christian Ehrlicher]

QProcess is asynchronous.

[sitesv]

@Christian-Ehrlicher said in Executing QProcess in QThread: memory leak:

QProcess is asynchronous.

"waitForFinished" is not help?

[Christian Ehrlicher]

@sitesv said in Executing QProcess in QThread: memory leak:

"waitForFinished" is not help?

Why do you want to make a asynchronous object blocking? Please read the QProcess docs and use the proper signals.

[SGaist]

The thing is: you just ping in the void, you do not even do anything with that information so why even ping ?

[sitesv]

@SGaist said in Executing QProcess in QThread: memory leak:

The thing is: you just ping in the void, you do not even do anything with that information so why even ping ?

I just skipped this code, it is unimportant... It receives an answer, analyzes it, and emits a signal to the main thread. And this is works.

[JonB]

@sitesv
As @Christian-Ehrlicher has said. You are using this thread to run a QProcess, and you call waitForFinished() on it. You would be better just running QProcess asynchronously from your main thread, and acting on signals. Then perhaps any thread memory issue will go away.

[sitesv]

@JonB
I will try it.

[SGaist]

@sitesv said in Executing QProcess in QThread: memory leak:

@SGaist said in Executing QProcess in QThread: memory leak:

The thing is: you just ping in the void, you do not even do anything with that information so why even ping ?

I just skipped this code, it is unimportant... It receives an answer, analyzes it, and emits a signal to the main thread. And this is works.

Well, do not skip that kind of details when asking questions, it can hide the issue you are asking help for and it does not allow people to understand what is really happening.

[sitesv]

@SGaist code updated

[KroMignon]

@sitesv said in Executing QProcess in QThread: memory leak:

Why you are using QTimer for this purpose?

I use QTimer here to make a passive wait. When the interval is reached, the QTimer fires the timeout() event. This will not lock the event queue. QThread::msleep() is an active wait ==> CPU usage for nothing and no signals/slots handling possible for the used thread!

doPing() is represents my code with using QProcess?

Yes, but I would not use a pointer for the QProcess.

This class should create in the main thread?

As you like. The QTimer instance has this as parent, moving the PingTester instance to another thread, will also move the QTimer instance to the same thread.

[KroMignon]

@sitesv For example a possible implementation which will not lock the event loop could be:

void PingTester::doPing()
{		
    QProcess ping;
    QEventLoop l; // used for passive wait until process finished

    ping.setProcessChannelMode(QProcess::MergedChannels);
    connect(&ping, &QProcess::finished, 
            [this, &l, &ping]() {
                QString output(ping.readAll());
                if(output.contains("ttl",Qt::CaseInsensitive))
                    emit pingSuccess();
                else
                    emit pingFailure();
                // exit event loop on process end
                l.exit();
            });
    // starting ping request
    ping.start("/bin/ping", QStringList() << "127.0.0.1" << "-c" << "1");

    // wait until ping finished
    l.exec();

    // start next ping (timer should be configured as single shot)
    m_timer->start();
}

[sitesv]

@KroMignon Thank you for the new experience.
There is a problem with 'connect'

[KroMignon]

@sitesv said in Executing QProcess in QThread: memory leak:

There is a problem with 'connect'

Can you detail more the problem?

[sitesv]

@KroMignon said in Executing QProcess in QThread: memory leak:

Can you detail more the problem?

pingtester.cpp:9:5: error: no matching member function for call to 'connect'
qobject.h:481:41: note: candidate function not viable: no overload of 'finished' matching 'const char *' for 2nd argument
qobject.h:274:13: note: candidate template ignored: couldn't infer template argument 'Func1'
qobject.h:314:13: note: candidate template ignored: couldn't infer template argument 'Func1'
qobject.h:242:43: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
qobject.h:283:13: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
qobject.h:322:13: note: candidate function template not viable: requires at least 4 arguments, but 3 were provided
qobject.h:222:36: note: candidate function not viable: requires at least 4 arguments, but 3 were provided
qobject.h:225:36: note: candidate function not viable: requires at least 4 arguments, but 3 were provided

[KroMignon]

@sitesv Oh, my bad, QProcess::finished() has an overload (https://doc.qt.io/qt-5/qprocess.html#finished)

Should be changed to:

connect(&ping, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), 
            [this, &l, &ping]() {
                QString output(ping.readAll());
                if(output.contains("ttl",Qt::CaseInsensitive))
                    emit pingSuccess();
                else
                    emit pingFailure();
                // exit event loop on process end
                l.exit();
            });