QTcpSocket::waitForBytesWritten always returns false

Jeff Esposito

I am on Windows 10
My QT version is 5.7.0
c++

This is all part of a big app, so I have just included some parts of it. Below is the code that calls waitForBytesWritten.

//
//	writes a request to the socket and reads the server's reply
//
bool ClientSocket::WriteRead(const jlib::CReq& req, jlib ::CRsp& rsp)
{
	bool connected = (mqSocket->state() == QTcpSocket::ConnectedState);
	bool gotCount = false;
	qint64 bufoff = 0;

	if (!connected)
	{
		THROW_ERR(9999,
			<< mIpAddr.toStdString()
			<< ":"
			<< mPort
			<< " is not connected");
	}
	mqSocket->write((const char *)&req, req.mMsgLen);
	while( mqSocket->bytesToWrite() > 0 )
	{
		if ( !mqSocket->waitForBytesWritten(2000) );
		{
			THROW_ERR(9999,
				<< mIpAddr.toStdString()
				<< ":"
				<< mPort
				<< " write failed with error "
				<< mqSocket->error()
				<< ": "
				<< mqSocket->errorString().toStdString());
		}
	}
	qDebug() << "Wrote request: \n" ;
	std::string strReq = req.traceStr();
	qDebug() << QString::fromStdString(strReq);
	while (mqSocket->waitForReadyRead(kReadTimeout))
	{
		qint64 countRead = mqSocket->read(((char *)&rsp)+bufoff, sizeof(rsp));
		bufoff += countRead;
		bool hasEnufDataQueued = bufoff >= sizeof(rsp.mMsgLen)
			&& bufoff >= rsp.mMsgLen;
		if (bufoff > rsp.mMsgLen)
		{
			THROW_ERR(9999,
			<< mIpAddr.toStdString()
			<< ":"
				<< mPort
				<< " rsp too big, was "
				<< bufoff
				<< " s/b "
				<< rsp.mMsgLen);
		}
		return true;
	}
	THROW_ERR(9999,
		<< mIpAddr.toStdString()
		<< ":"
		<< mPort
		<< " read failed with error "
		<< mqSocket->error()
		<< ": "
		<< mqSocket->errorString().toStdString());
	return false;
}

Note: There is some thread stuff going on also.

This routine gets called from a "LogonWorker" thread, which is supposed to issue a logon request to the server in the background. Here is the stuff in my main, which sets up the logon thread. Maybe it has something to do with it.

QtOub::QtOub(QWidget *parent)
	: QMainWindow(parent)
{
	ui.setupUi(this);

	//
	//	Set up the logon worker thread
	//
	qRegisterMetaType<oub::CLogonReq>("oub::CLogonReq");
	qRegisterMetaType<oub::CLogonRsp>("oub::CLogonRsp");
	mpLogonThread = new QThread(this);
	mpLogonWorker = new LogonWorker;
	mpLogonWorker->moveToThread(mpLogonThread);
	connect(mpLogonWorker, SIGNAL(logonFinished(const oub::CLogonRsp&)),
		this, SLOT(onLogonFinished(const oub::CLogonRsp&)));
	connect(mpLogonWorker, SIGNAL(logonError(const QString&)),
		this, SLOT(onLogonError(const QString&)));
	connect(this, SIGNAL(startLogon(const oub::CLogonReq&)),
		mpLogonWorker, SLOT(onStartLogon(const oub::CLogonReq&)));
}

SGaist

Hi,

What's the size of your payload ? What's the speed of your network ?

VRonin

Why do you spawn a thread and use the synchronous API instead of just using the asynchronous one instead?

Jeff Esposito

The logon request is 96 bytes.

I am still developing, so the server and client are running on same windows PC, and there really is no network.

Also, the server actually sees the 96 bytes the client sent.

Jeff Esposito

@VRonin I use "threads" in general rather than "async io" to reduce complexity.

SGaist

Just to be sure I understand correctly: you are reducing complexity by adding threads using a blocking mechanism for a background task to avoid using the asynchronous nature of QTcpSocket ?

Jeff Esposito

@SGaist Yes.

VRonin

but why??

Jeff Esposito

Because the manual says its the preferred way to do it.

jsulm

@Jeff-Esposito Which manual?