Executing QProcess in QThread: memory leak

sitesv

QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
    int success_count = 0;
    int pingsToDo = ip_list.count();
    QEventLoop l; // used for passive wait until process finished
    foreach(auto ip, ip_list)
    {
        QProcess ping;
        ping.setProcessChannelMode(QProcess::MergedChannels);
        connect(&ping,
                QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), &l,
                [&l, &ping, &success_count, &pingsToDo]() {
                    --pingsToDo;
                    QString output(ping.readAll()); // <<<<<<<<<<<<<<<< SIGSEGV Segmentation fault
                    if(output.contains("ttl",Qt::CaseInsensitive)){
                        success_count++;
                    }
                    if(!pingsToDo){
                        l.exit();
                    }
                }, Qt::QueuedConnection);
        ping.start("/bin/ping", QStringList() << ip << "-c" << "1");
    }
    l.exec();
    if(success_count == ip_list.count()) emit setLedStatus(0, 0, true);
    else                                 emit setLedStatus(0, 0, false);

    m_timer->start();

There is SIGSEGV Segmentation fault on ping.readAll string. :((

KroMignon

@sitesv Sorry but I was a little bit confused when I suggest you this code.
This cannot work, because QProcess is killed/destroyed at end of the for loop!
Should be:

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)
{
    auto ping = new QProcess();
    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++;
                // free memory
                ping->deleteLater();
                //  exit event loop after all pings done
                if(!pingsToDo)
                    l.exit();
            });
    ping->start("/bin/ping", QStringList() << ip << "-c" << "1");
}

// wait all pings done
l.exec();

JonB

@sitesv , @KroMignon
I will say one thing.

You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

KroMignon

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

I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

Yes, this is a good advice.

sitesv

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

You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

Maybe my first variant was good (QThread + QProcess)?

if(!myProcess) myProcess = new QProcess(this);
myProcess->start(exe_path, arguments);
myProcess->waitForFinished(500);
output = myProcess->readAll();
output_str = codec->toUnicode(output);
output_strlst = output_str.split("\r\n");
myProcess->close();
...

There is QProcess "waitForFinished" with timeout.
The app wasn't freezing with this approach.

KroMignon

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

There is QProcess "waitForFinished" with timeout.
The app wasn't freezing with this approach.

Why not, but you have to wait ping finished before starting next.
If it is what you want, then go with it.

jsulm

@sitesv I'm still wondering why you think you need a local event loop...

sitesv

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

@sitesv I'm still wondering why you think you need a local event loop...

Hi! I have done variant as you recommend. It's working!

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

I made another experiment:

Created a QThread
In QThread::run() method::

made a QTimer* object,
set him as "one shot kind",
ran QTimer, and in final
* ran QThread::exec() (for run local EventLoop)...

There is timer->start() in timeout slot, It executed, but QTimer doesn't start again.

Any ideas?

JonB

@sitesv
I don't know how/whether your issue relates this, but in your code: you set off ping.start(), but QProcess ping; is a local variable in your foreach loop and so immediately goes out of scope (not to mention, you also re-use the same local variable for each time round the loop, overwriting/destroying the previous one). Nothing should work (it might actually "crash"), I don't understand how you say it does.

On top of all of this: I think we've said it already here above, but goodness only knows why you are using a thread, with all the complications that involves? If you want to run a QProcess and the main thread to know when it's finished, it's asynchronous anyway, it would be a whole lot simpler not to have any thread. I think I said this earlier, but up to you.

And finally, while I'm on a roll: I think you are just using a /bin/ping in order to read the textual output, parse it, and see whether something is there/alive. In which case I'd be tempted to just write it myself in Qt instead of running some external command, I would have thought it's only a few lines of code. Your "parsing" of the output is beyond hokey, not even sure what you think it tells you....

KroMignon

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

Hi! I have done variant as you recommend. It's working!

I am not sure this is really working!
I think you have to (re)learn C++ object life cycle.
You create a QProcess local instance in the for loop, this object will be destroyed at loop end.

sitesv

@JonB @KroMignon
Agree with you, guys.
But it works...
I can reimplement to QProcess pointers of PingTester class...

KroMignon

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

But it works...

It don't, "it seems to work" would be the correct answer ;)

If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

If you want to do it sequentially, you have to be consistent in your choice:

foreach(auto ip, ip_list)
{
    QProcess ping;
    ping.setProcessChannelMode(QProcess::MergedChannels);
    ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
    ping.waitForFinished(5000); // wait up to 5 seconds
    QString output(ping.readAll());
    if(output.contains("ttl",Qt::CaseInsensitive)){
        success_count++;
    }
}

sitesv

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

If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

There is "-w 1" key. It helps with unaccessible IP.

JonB

@KroMignon
I would think doing it sequentially, with waitForFinished(), for his multiple IP addresses would be a poor way to do things. He has a number of IP addresses to check, these should be done in parallel....

KroMignon

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

There is "-w 1" key. It helps with unaccessible IP.

AFAIK "-w 1" will wait up to 1 second.
I am sure you will got QProcess warnings about killing a process which is still running.

sitesv

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

AFAIK "-w 1" will wait up to 1 second.

...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

KroMignon

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

...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

This is the way I would implement multiple pings in parallel:

int PingTester::doPing(const QStringList &ip_list)
{
    // to avoid issues ;)
    if(ip_list.isEmpty())
        return 0;

    int success_count = 0;
    int pingsToDo = ip_list.count();
    QEventLoop l;
    QTimer timer;
    foreach(auto ip, ip_list)
    {
        auto ping = new QProcess();
        ping->setProcessChannelMode(QProcess::MergedChannels);
        connect(timer, &QTimer::timeout, ping, &QProcess::kill);
        connect(ping, &QProcess::stateChanged,
                [&l, ping, &success_count, &pingsToDo](QProcess::ProcessState newState) {
                    if(newState != QProcess::NotRunning)
                        return;
                    --pingsToDo;
                    QString output(ping->readAll());
                    if(output.contains("ttl",Qt::CaseInsensitive))
                        success_count++;
                    // free memory
                    ping->deleteLater();
                    // exit loop when done.
                    if(!pingsToDo)
                        l.exit();
                });
        ping->start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
    }
    timer.start(1500); // kill process after 1.5 seconds if still running
    // wait all pings done
    l.exec();
    return success_count;
}

sitesv

@jsulm There is a local QEventLoop...
@KroMignon I really don't understand why you are checking state changing of QProcess...
QProcess "Finished" method is not suitable?
And why kill a process?

KroMignon

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

QProcess "Finished" method is not suitable?

I found it easier to use QProcess::stateChanged(), because there is no overload of this slot, and I am not sure if QProcess::finished(int, QProcess::ExitStatus) is triggered if process is killed.

And why kill a process?

This is a security, when calling an external application, many things can happen: execution right issues, application not exist, network issues and so on.
99.9% of time it will not be useful, but it will ensure QEventLoop will exit at the end!

sitesv

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

This is the way I would implement multiple pings in parallel:

This code doesn't work. Only one iteration.