Help - QTcpSocket call errorString return wrong string in special case.

[Longnm]

I have a issue with QTcpSocket.
When I connectToHost with argument ("", 0) and looping with interval 500ms , action get errorString() make crash program.

bool stop = false;
QTcpSocket* partnerSocket = new QTcpSocket();

class TaskPool : public QRunnable
{
    void run() override
    {
        while (!stop) {
          partnerSocket -> connectToHost("", 0);
          qDebug() << this << "Socket State" << (int)partnerSocket->state();
          qDebug() << "Socket Error String : " << partnerSocket -> errorString();
          QThread::msleep(500);
        }
    }
};


int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    TaskPool* task = new TaskPool();
    QThreadPool::globalInstance()->start(task);
    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();
}

After about 20min, errorString return some wrong string : empty or unicode string \u*** and program crashed.
errorString() is member method of QIODevice, so i think QIODevice has the issue with memory.

Solution:

1. Void this case by using signal of QAbstractSocket to handle error and reconnect
2. Using waitForConnected for blocking socket when you synchonize progress.

[mrjj]

Hi and welcome to the forums
Is that code enough to reproduce it?

[aha_1980]

You need a running event loop to use QTcpSocket. With your code you block the event loop. I don't think that will work.

Also, what is the use of connecting to a host (which one?) every 500 ms. What do you think happens with the old connection in partnerSocket?

I would not be surprised if this code has memory leaks.

[Longnm]

@mrjj : oh, because i edited when type on web :D. I updated code block to confirm.
Thanks.

[Longnm]

@aha_1980 : i think, too. But i checked source code connectToHost in case return HostNotFound, I can't see something wrong to memory leak.

[SGaist]

Hi,

Since you are interested in possible failures, why not use waitForConnected ?

[Longnm]

@SGaist : I think the problem can make crash in other case , so i need investigate about it.
I changed source code using signal error() to handle error message and reconnect when state is disconnected. The program is good look.
But i can't understand about that problem.

[SGaist]

One question is: why are you retrieving the error message right after calling connectToHost since you know that this operation may take time and the function is not blocking ?

[Longnm]

@SGaist : This is bug of my friend. He unknow about not blocking socket. I investigated and fix this bug.
I think QIODevice has bug with errorString (QIODevice is valid but errorString wrong). And I can't descript about this crash issue.
Maybe I will investigate later.
Thank for your help !