Blocking QTcpSocket for read/write operations

Hello guys,

I am trying to implement a blocking QTcpSocket for both reading and writing operations. The application to which I am connecting to is written in Java and for very unknown reasons the non-blocking approach doesn't work. With non-blocking approach the highest state reached is QAbstractSocket::ConnectingState and NOT Connected, although the Java client shows it got a new connection.
So this code:

Quote:

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tcpSocket = new QTcpSocket(this);
QObject::connect(tcpSocket, SIGNAL(connected()), SLOT(onConnect()), Qt::DirectConnection);
QObject::connect(tcpSocket, SIGNAL(hostFound()), SLOT(hostFound()), Qt::DirectConnection);
QObject::connect(tcpSocket, SIGNAL(readyRead()), SLOT(readFromSocket()), Qt::DirectConnection);
QObject::connect(tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
SLOT(displayError(QAbstractSocket::SocketError)), Qt::DirectConnection);

---

does not work.

By the other side, for the same unkown reasons if I'm using this code:

Quote:

---

const int Timeout = 2 * 1000;
if (!tcpSocket->waitForConnected(Timeout)) {
displayError(tcpSocket->error());
}

---

I enter the QAbstractSocket::ConnectedState ... very strage! Non-blocking approach doesn't work and the blocking on works.

The problem with the blocking approach is that I have to put the code which waits for the data into a separate thread (not to freeze the app), like this:

Quote:

---

void EQtClientSocket::run()
{
tcpSocket = new QTcpSocket;

if (!connectionRequested) {
mutex.lock();
cond.wait(&mutex);
mutex.unlock();
}

while(!quit) {

// already connected?
if(tcpSocket->state() == QAbstractSocket::ConnectedState) {
errno = EISCONN;
getWrapper()->error(NO_VALID_ID, ALREADY_CONNECTED.code(), ALREADY_CONNECTED.msg());
}

// starting to connect to server
tcpSocket->abort();
tcpSocket->connectToHost(host, port);

const int Timeout = 2 * 1000;
if (!tcpSocket->waitForConnected(Timeout)) {
displayError(tcpSocket->error());
}
else {
socketState = QAbstractSocket::ConnectedState;
onConnect();
emit connected();
qDebug() << "connection ok";
}

// set client id
while (tcpSocket->bytesAvailable() < (int)sizeof(quint16)) {
if (tcpSocket->waitForReadyRead()) {
onReceive();
qDebug() << "New data available";
}
}

mutex.lock();
cond.wait(&mutex);
mutex.unlock();
}
}

---

however I need to write something to the socket, which I can't do it from a different thread which created the socket (every QObject should be in the same thread), but I can't write something from the same thread since tcpSocket->waitForReadyRead() would block the thread in most of the time and will wait for the data until it starts again.

I'm stuck. I can't implement a read&write QTcpSocket with the blocking approach.

Please help

Maybe you have a problem with your event loop? Post a compilable example and maybe we can help.

Quote:

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Originally Posted by squidge

Maybe you have a problem with your event loop? Post a compilable example and maybe we can help.

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Thanks squidge, as per your suggestions I did some more digs and found the problem related to the event loop. This solved the problem.

Please tell me how you solved the problem of starting the event processing cycle (in fact, QCoreApplication::exec()) in Java GUI-app. I have an unsuccessful attempt to start a simple asynchronous socket client - it just does not work in JavaFX. I am trying to make a synchronous client with blocking sockets, but they also require signals and slots and therefore an event loop.