QProcess cannot create pipe - Too many open files - but I don't understand why

[Mark81]

I use this function to check if an host is online:

MyClass.h

class MyClass : public QObject
{
    Q_OBJECT

public:
    explicit MyClass(QObject *parent = nullptr);

private:
    QProcess _processPing;

public slots:
    void ping();

private slots:
    void processPing_finished(int exitCode, QProcess::ExitStatus exitStatus);

};

MyClass.cpp

const int RECONNECTION_INTERVAL = 10000;

MyClass::MyClass(QObject *parent) : QObject(parent)
    connect(&_processPing, &QProcess::finished, this, &MyClass::processPing_finished);
}

void MyClass::ping()
{
    QStringList args;
    args << "-c" << "1" << "-W" << "1" << _url.host();
    _processPing.start("ping", args);
}

void MyClass::processPing_finished(int exitCode, QProcess::ExitStatus exitStatus)
{
    Q_UNUSED(exitStatus)

    _pingResult = exitCode == 0;
    if (!_pingResult) 
        QTimer::singleShot(RECONNECTION_INTERVAL, this, &MyClass::ping);
}

I have 20 instances of this class, but usually only 1-2 runs the process in a given moment. Since:

1. I store the QProcess object as a class member
2. I catch the finished() signal

I'm pretty sure I don't leave anything left open, do I?
After some time I get this warning in the console:

error : QProcessPrivate::createPipe: Cannot create pipe : Too many open files

I don't understand why it happens. ps does not show any process running:

$ ps aux | grep ping
user       21768  0.0  0.0   6476  2316 pts/1    S+   17:35   0:00 grep --color=auto ping

I have to restart the application to be able again to run the ping process. Am I using QProcess in a wrong way?

Ubuntu 22.04, Qt 6.4.0. I saw this similar question, but I don't think it's the same scenario.

[JonB]

@Mark81
You could try QProcess::close() in your finished() slot.

You could try making _processPing a QProcess *, allocating on heap, deleteLater() on finished().

I would not be using an external program like ping to check whether a host is online, but that is a different matter.

[Mark81]

@JonB said in QProcess cannot create pipe - Too many open files - but I don't understand why:

I would not be using an external program like ping to check whether a host is online, but that is a different matter.

I'm interested! What other ways would you suggest? Until now I found using ping is the most reliable one.

[JonB]

@Mark81
There have been discussions about whether pinging a server is a useful thing to do. Mostly it seems better to do communications when you have something you actually want to communicate, and handle it not there or reconnecting at that time, not pinging to find out whether it is there or to keep connection open.

There is also a possible alternative of send keep-alives on the socket instead.

If I were going to do the "ping" approach, I would look to use Qt networking rather than an external ping program. Ping is pretty simple so you should not need an external program (with QProcess, checking what ping is outputting, etc.) to do what it does. For instance, http://www.ping127001.com/pingpage/ping.text is apparently the source of ping from 1983, doubtless there are other implementations in C++ if you Google.