QTcpSokcet connected() signal not fired

diverger

I'm working on a project with Qt on windows. I connected the QTcpSocket's connected() signal to a slot, but after call "connectToHost', my slot won't called, so I doubt the 'connected()' signal is not fired or delayed.


    proc()
    {
            ....
            connect(_socket, SIGNAL(connected()), this, SLOT(socket_connected()), Qt::DirectConnection);
    
            _socket->connectToHost(tester->ip_info().ip_addr.toString(), TCP_COMM_PORT, QTcpSocket::ReadWrite, QTcpSocket::IPv4Protocol); 
    ...
    }

Anyone can give a help? Where my code is wrong?

Best reards.

jsulm

@diverger You should connect http://doc.qt.io/qt-5/qabstractsocket.html#error-1 signal to a slot and print the error there to check whether any error occurs. And you should check whether the connection for connected actually succeeded:

qDebug() << connect(_socket, SIGNAL(connected()), this, SLOT(socket_connected()), Qt::DirectConnection);

should output true.

diverger

@jsulm Thanks for the help. It return true. Is it mandatory to call

_socket->waitForConnected(timeout);

after 'connecToHost()' ? I find if I add the clause above, my slot is called. IMHO, 'waitForxxxxx' are for blocking operations, why it will affect the async-signals?

Now, i'm a little confused about how to use the APIs Qt provided for sockets.

jsulm

@diverger Can you show more code? The whole method where you try to connect?
_socket->waitForConnected(timeout) is not needed, it should work without.
Did you try to connect the error signal as I suggested?

diverger

@jsulm I create a worker class and move it to a new thread. In the worker class's constructor, I connect the socket signals to the slots in the same class.

	connect(_socket, SIGNAL(connected()), this, SLOT(socket_connected()));
	connect(_socket, &QTcpSocket::readyRead, this, &XXXXX::read_ready);
	connect(_socket, &QTcpSocket::bytesWritten, this, &XXXXX::bytes_written);

And the main worker code is:

void XXXXX::proc_start()
{
	CommProc::proc_start();

	while (this->_is_enabled)
	{
		qDebug() << QThread::currentThreadId();

		eth_comm_command_pkt_t command_pkt = {
			MODEL_GUID,
			0, 0
		};

		eth_comm_status_pkt_t status_pkt;

		auto timeout = 5 * 1000;
		
		_socket_connected = false;
		_socket->connectToHost(tester->ip_info().ip_addr.toString(), TCP_COMM_PORT, QTcpSocket::ReadWrite, QTcpSocket::IPv4Protocol);

		if (!_socket->waitForConnected(timeout))
		{
			// error
			emit socket_error(_socket->error(), _socket->errorString());
			return;
		}

		while (!_socket_connected);
		
		// send command
		_socket_bytes_written = 0;
		command_pkt.command = 1;
		qint64 bytes_written = _socket->write((char const *)&command_pkt, sizeof(command_pkt));

		if (bytes_written < 0)
		{
			// error
			emit socket_error(_socket->error(), _socket->errorString());
			return;
		}
		
		while (!_socket->waitForBytesWritten(timeout))
		{
			emit socket_error(_socket->error(), _socket->errorString());
		}

		while (_socket_bytes_written < sizeof(command_pkt))
		{
		}

		// read status
		if (!_socket->waitForReadyRead(timeout)) {
			emit socket_error(_socket->error(), _socket->errorString());
			return;
		}

		while (_socket_bytes_ready < sizeof(status_pkt))
		{
		}

		_socket->read((char *)&status_pkt, sizeof(status_pkt));

		if (status_pkt.status == status_pkt.STA_OK)
		{
			//
			qDebug() << "Send command done";
		}
		else
		{
			qDebug() << "Command error!";
		}
	}
}

I've debuged it, the worker code indeed run in a new thread.

diverger

@jsulm I've noticed my worker main thread and the slots in the same thread, and the main thread is blocking, should it blocked the slots?

jsulm

@diverger You should remove all that while loops, because they block the event loop. Blocked event loop means: no signals can be emited/handled. I would suggest to take a look at Qt networking examples to see how Qt networking should be used. It is asynchronous event driven.

diverger

@jsulm Thanks, I'll give it a try.