cannot write through opened socket

user4592357

i open a tcp socket with startCommunication() function, which succeeds.

class SocketDescriptor : public QTcpServer
{
	Q_OBJECT
public:
	explicit SocketDescriptor(bool bServer = true, QObject *parent = 0)
        : QTcpServer(parent),
        m_bServer(bServer),
        port(0),
    {}

	bool startCommunication(QString strHostName, int nPort)
    {
        port = nPort;
        QHostInfo hostInfo = QHostInfo::fromName(strHost);

        addr = !hostInfo.addresses().isEmpty()
            ? hostInfo.addresses().front() : QHostAddress::Any;

        bool m_bStarted = listen(addr, port);
        if (!m_bStarted)
        {
            setError(QString("Server cannot be started on port %1").arg(port));
            return false;
        }
        else
        {
            addr = this->serverAddress();
            port = this->serverPort();
        }

        return m_bStarted;
    }

	bool isConnectionAvailable() const
    {
        QTcpSocket* pSocket =  getSocket();
        return (pSocket && pSocket->isOpen());
    }

public:
	bool m_bServer;
	bool m_bStarted;
	QHostAddress addr;
	int port;

protected:
	QTcpSocket *m_ClientSocket{ nullptr };
	QString m_LastError;
	QThread *m_pCommunicationThread{ nullptr };
};

this operation succeeds, then i start another application with it which expects read and write ports:

port = // get the port number

const QString sPort(Port);
auto args = QStringList() << sPort << sPort;

if (!QProcess::startDetached(sClientName, args))
{
    const QString sProgFullPath{ QString::fromLatin1(pDir) +
        QLatin1String("/bin/") + sClientName };
    if (!QProcess::startDetached(sProgFullPath, args))
        return false;
}

this succeeds, i don't get any error.
but when i send this other application a message through the socket port stored before, the isConnectionAvailable() returns false, and i cannot send the message.

if (!isConnectionAvailable())
      return;
if (m_Socket->write(MsgBytes) != -1)
      m_Socket->flush();

i want to know what i'm doing wrong.

Christian Ehrlicher

Please take a look at the QTcpServer example. Your code does ... nothing really useful.

user4592357

@Christian-Ehrlicher my code is this

open socket with above code, it returns port number
port = // get the port number

const QString sPort(Port);
auto args = QStringList() << sPort << sPort;

if (!QProcess::startDetached(sClientName, args))
{
    const QString sProgFullPath{ QString::fromLatin1(pDir) +
        QLatin1String("/bin/") + sClientName };
    if (!QProcess::startDetached(sProgFullPath, args))
        return false;
}

Christian Ehrlicher

This code can not work. Follow the QTcpServer examples.

user4592357

@Christian-Ehrlicher this is working code.
i just tried this
i created socket with above method, and run telnet localhost $port
then i sent message to telnet using this port, and it was sent...
so... is that other app faulty?

user4592357

@Christian-Ehrlicher my bad
the other app expects file descriptors, and i'm providing socket port
so, how should i work with that app?

Christian Ehrlicher

@user4592357 said in cannot write through opened socket:

so, how should i work with that app?

It's your app - how should we know?

user4592357

@Christian-Ehrlicher i mean it is accepting file descriptors, and i have socket (port).

how should i convert socket to fd?
can i do that?

Christian Ehrlicher

@user4592357 said in cannot write through opened socket:

how should i convert socket to fd?

By taking a look at the documentation: https://doc.qt.io/qt-5/qabstractsocket.html#socketDescriptor

user4592357

@Christian-Ehrlicher thanks for the link

so when i create the child app like this, when main app is closed, child app still exists, as it was started detached:

if (!QProcess::startDetached(sClientName, args))
{
    const QString sProgFullPath{ QString::fromLatin1(pDir) +
        QLatin1String("/bin/") + sClientName };
    if (!QProcess::startDetached(sProgFullPath, args))
        return false;
}

i tried to convert this to using start() instead, but looks it's not equivalent (because now error signal is async?)
what's the correct way to nest such call?

QProcess p1;
p1.start(sClientName, args);
connect(&p1, &QProcess::errorOccurred, []
{
    const QString sProgFullPath{ QString::fromLatin1(pDir) +
        QLatin1String("/bin/") + sClientName };

     QProcess p2;
     p2.start(sProgFullPath, args);
     connect(&p2, &QProcess::errorOccurred, []
     {
          // how do i return false from here?
    });
}

Christian Ehrlicher

Start both directly instead waiting for the first one.

user4592357

@Christian-Ehrlicher what if both succeed?

user4592357

@Christian-Ehrlicher what about https://doc.qt.io/qt-5/qprocess.html#execute?