Executing QProcess in QThread: memory leak

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();
            });

sitesv

@KroMignon
Very interesting realization. I will study it.
Thank you again!

sitesv

@KroMignon
Hi!
Is it possible to make a checking ping of a list of addresses for each timer interval? I haven't used a lambda yet in my practice ((.

KroMignon

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

Is it possible to make a checking ping of a list of addresses for each timer interval? I haven't used a lambda yet in my practice ((.

Yes, it is possible.
I have give you a basic implementation example.
You just have to complete it.

If you want to learn lambda, search on internet, there are many explanations. It is not so hard to understand.
My favorite is https://blog.feabhas.com/2014/03/demystifying-c-lambdas/

sitesv

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

Yes, it is possible.

I have tried to do something like this. But is execute too slow.

QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
    int success_count = 0;
    foreach(auto ip, ip_list)
    {
        QProcess ping;
        QEventLoop l;
        ping.setProcessChannelMode(QProcess::MergedChannels);
        connect(&ping,
                QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
                [&l, &ping, &success_count]() {
                    QString output(ping.readAll());
                    if(output.contains("ttl",Qt::CaseInsensitive)) success_count++;
                    l.exit();
                });
        ping.start("/bin/ping", QStringList() << ip << "-c" << "1");
        l.exec();
    }
    if(success_count == ip_list.count())
       emit setStatus(true);
    else
       emit setStatus(false);
    m_timer->start();

jsulm

This post is deleted!

sitesv

UPD:
I found the cause of this situation. Ip's are not available. How to set a timeout for the process execution in this case?

jsulm

@sitesv Why do you actually use a local event loop? You can implement this functionality without blocking your app with a local event loop.
Also, you terminate that event loop already in the first lambda call (when the first process finishes)...

sitesv

@jsulm Local EventLoop was recommended before. Could you please advise something?

jsulm

@sitesv You know how many processes you started. So, count how many processes already finished (inside the lambda). And as soon as all processes finished you can check success_count and emit the signal. All this can be done inside lambda.

KroMignon

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

How to set a timeout for the process execution in this case?

Have you tried to look at ping parameters (ping --help)

And I would only wait once, for all pings to be done:

QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
int success_count = 0;
int pingsToDo = ip_list.count();
QEventLoop l;
foreach(auto ip, ip_list)
{
    QProcess ping;
    ping.setProcessChannelMode(QProcess::MergedChannels);
    connect(&ping,
            QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
            [&l, &ping, &success_count, &pingsToDo]() {
                --pingsToDo;
                QString output(ping.readAll());
                if(output.contains("ttl",Qt::CaseInsensitive)) success_count++;
                if(!pingsToDo)
                    l.exit();
            });
    ping.start("/bin/ping", QStringList() << ip << "-c" << "1");
}

// wait all pings done
l.exec();
if(success_count == ip_list.count())
   emit setStatus(true);
else
   emit setStatus(false);
m_timer->start();

sitesv

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

And I would only wait once, for all pings to be done:

Wow! Very nice!! Thank you!

@KroMignon
UPD: app is freezing on line l.exec() althought l.exit() is done....

sitesv

Guys, how to resolve this?

KroMignon

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

Guys, how to resolve this?

Hmm, I think it is a tricky issue. I would start with forcing QueuedConnection, to avoid threading issues:

connect(&ping,
            QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
            [&l, &ping, &success_count, &pingsToDo]() {
                ...
            }, Qt::QueuedConnection);

sitesv

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

connect(&ping,
QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
&l, &ping, &success_count, &pingsToDo {
...
}, Qt::QueuedConnection);

'connect' failed again