Disclaimer: I am relatively new to Qt and any type of programming that revolves around Threads and Networking. I have also adopted a lot of code from Qt Examples, API, and other online examples.
All code can be found on GitHub. This code is relatively as simple as it can get minus striping out GUI. I figure supplying it this way would help as well versus just pasting the code below.
I want to use and believe I need to use Threads as I need multiple clients send a request to the server, the server run some SQL code, then spit out the results back to the client (basically deriving a MySQL Server, but specific to what I am doing). Right now though, I am just working on learning the workings of it all.
With all that being said, as the Title states.. My client can connect to the server, the server sets up the thread, and will receive data (a String) through the readReady. After the data is read in, for right now I am just trying to echo it back to the client. It will do this, but only once. Then it spits out:
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x266cca92ea0), parent's thread is serverThread(0x266cca9ed60), current thread is QThread(0x266cac772e0)
I cannot send any further data to the server unless I have the client reconnect, then after the data is sent, it will do its job but then spit out the same error and cease functioning. I have tried quite a bit of different things, but cannot seem to fix the issue. I even tried setting up a SIGNAL/SLOT for this as suggested in API:
It is important to remember that a QThread instance lives in the old thread that instantiated it, not in the new thread that calls run(). This means that all of QThread's queued slots will execute in the old thread. Thus, a developer who wishes to invoke slots in the new thread must use the worker-object approach; new slots should not be implemented directly into a subclassed QThread.
Anyway, any help would be greatly appreciated! My Code is below..
Server
ServerThread.cpp
// Project
#include "ServerDialog.h"
#include "ServerThread.h"
ServerThread::ServerThread(qintptr _socketDiscriptor, QObject *parent /*= 0*/)
: QThread(parent)
{
socketDiscriptor = _socketDiscriptor;
}
void ServerThread::run()
{
emit threadStarted(socketDiscriptor);
// Start Thread
clientSocket = new QTcpSocket;
// Set SocketDisc
if (!clientSocket->setSocketDescriptor(socketDiscriptor))
{
emit error(clientSocket->error());
return;
}
// Connect Socket and Signal
connect(clientSocket, SIGNAL(readyRead()), this, SLOT(readyRead()));
connect(clientSocket, SIGNAL(disconnected()), this, SLOT(disconnected()));
//// Loop Thread to Stay Alive for Signals and Slots
exec();
}
void ServerThread::readyRead()
{
QDataStream in(clientSocket);
in.setVersion(QDataStream::Qt_5_7);
in.startTransaction();
QString dataReceived;
in >> dataReceived;
if (!in.commitTransaction())
{
emit readyReadError(socketDiscriptor);
return;
}
emit readyReadMessage(socketDiscriptor, dataReceived);
echoData(dataReceived);
}
void ServerThread::disconnected()
{
emit threadStopped(socketDiscriptor);
clientSocket->disconnect();
clientSocket->deleteLater();
this->exit(0);
}
void ServerThread::echoData(QString &data)
{
QByteArray block;
QDataStream out(&block, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_7);
out << data;
clientSocket->write(block);
}
So in ServerThread.cpp when echoData is called, that is when the error shows up and the Socket ceases functioning.
Any and all help will be appreciated. I know there are a few other posts regarding "Cannot create children for..." in regards to Threads. But I did not find any of them helpful. The one thing that I did find interesting but did not understand was maybe using moveToThread() but a lot of mixed comments on that.
I learn best through code examples along with explanation versus just an explanation or pointer to API. Thank you!
Most of Qt network functions are asynchronous; they do not block the calling thread. There is no need to mess up with threads if you are using QTcpSockets. In fact, creating a thread for every socket is an overkill, since that thread will spend most of its time just waiting for some network operation to finish. Here is how I would implement a single-threaded echo server in Qt:
#include <QtNetwork>
#include <QtCore>
//separate class for the protocol's implementation
class EchoSocket : public QTcpSocket{
Q_OBJECT
public:
explicit EchoSocket(QObject* parent=nullptr):QTcpSocket(parent){
connect(this, &EchoSocket::readyRead, this, &EchoSocket::EchoBack);
connect(this, &EchoSocket::disconnected, this, &EchoSocket::deleteLater);
}
~EchoSocket() = default;
Q_SLOT void EchoBack(){
QByteArray receivedByteArray= readAll();
write(receivedByteArray);
disconnectFromHost();
}
};
class EchoServer : public QTcpServer{
public:
explicit EchoServer(QObject* parent= nullptr):QTcpServer(parent){}
~EchoServer() = default;
//override incomingConnection() and nextPendingConnection()
//to make them deal with EchoSockets instead of QTcpSockets
void incomingConnection(qintptr socketDescriptor){
EchoSocket* socket= new EchoSocket(this);
socket->setSocketDescriptor(socketDescriptor);
addPendingConnection(qobject_cast<QTcpSocket*>(socket));
}
EchoSocket* nextPendingConnection(){
QTcpSocket* ts= QTcpServer::nextPendingConnection();
return qobject_cast<EchoSocket*>(ts);
}
};
int main(int argc, char* argv[]){
QCoreApplication a(argc, argv);
EchoServer echoServer;
echoServer.listen(QHostAddress::Any, 9999);
QObject::connect(&echoServer, &EchoServer::newConnection, [&](){
EchoSocket* socket= echoServer.nextPendingConnection();
qDebug() << "Got new connection from: " << socket->peerAddress().toString();
});
return a.exec();
}
#include "main.moc"
Notes:
This server has the ability to handle more than one client at the same time, since there is no blocking. The thread will just respond to the event that happens with the appropriate action; So, if that event was a new connection, it will create a new EchoSocket object to handle it and prints a statement out to qDebug(), and if that event was receiving something on a previously created socket, the same thread will echo received data back and close the connection. It will never block on a single connection waiting for data to arrive nor it will block waiting for a new connection to arrive.
Since you mention using some SQL queries in response for some connections later in your project. Please avoid threading since an SQL database connection in Qt can be used only from the thread that created it, see docs here. So, You'll have to either create a new database connection for each thread (and thus for each connection) in your application (and this is beyond just overkill), or switch later to a single threaded design.
In this section, I am explaining why threading does not work for you the way you are doing it:
You should not be declaring slots in your QThread subclass, Instead, use worker QObjects and move them to QThreads as needed.
The quote you have provided in your question is the exact explanation for why you get this warning. The ServerThread instance you create will be living in the main thread (or whatever thread that created it). Now let's consider this line from your code:
connect(clientSocket, SIGNAL(readyRead()), this, SLOT(readyRead()));
The signal readyRead() will be emitted from the current ServerThread instance (since the clientSocket object that emits it lives there), However, the receiver object is the current ServerThread instance, But that lives in the main thread. Here is what the documentation says:
If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used.
Now, the main point of Qt::QueuedConnection is executing the slot in the receiver object's thread. This means that, your slots ServerThread::readyRead() and ServerThread::disconnected will get executed in the main thread. This is most likely not what you meant to do, since you'll end up accessing clientSocket from the main thread. After that, any call on clientSocket that results in child QObjects being created will result in the warning you get (you can see that QTcpSocket::write() does this here).
Mixed comments of movetothread are linked mostly to usage of it to move thread object to itself.
The quote hints that the members of QThread aren't designed to be called from worker. Strictly proper way to call signal would be by using worker object model, that was shown in Qt examples and explained a few times on QT-related blogs:
class Worker : public QObject
{
Q_OBJECT
private slots:
void onTimeout()
{
qDebug()<<"Worker::onTimeout get called from?: "<<QThread::currentThreadId();
}
};
class Thread : public QThread
{
Q_OBJECT
private:
void run()
{
qDebug()<<"From work thread: "<<currentThreadId();
QTimer timer;
Worker worker;
connect(&timer, SIGNAL(timeout()), &worker, SLOT(onTimeout()));
timer.start(1000);
exec();
}
};
worker constructed inside run() is "property" of the thread it created, so figuratively speaking, it is slaved to its context. The same effect maybe achieved if you create worker in other thread, then move it to this thread before connection was made. When you connect signal to slot of the QThread itself, you connect child thread to thread it was created by.
Use of
connect(&timer, SIGNAL(timeout()), this, SLOT(onTimeout()), Qt::DirectConnection);
or creating connection from your thread sometimes seems to achieve proper result, but not in this case, where you try use objects constructed in different threads together. Calling moveToThread(this) in constructor is a thing not recommended to do.
I am using QTcpSocket to send data between a client and a server.
The server works perfectly fine on Linux, yet on Windows it will only receive one message and not more after that. readyRead() is just never emitted again.
Does anyone know what could be the problem?
Some of the code which I deemed to be important for this question:
server sending:
void Server::sendData(Client *client, QString data)
{
if (client->socketDescriptor == socketDescriptor) {
data = data + CRLF;
// Have also tried to use socket->flush(), same problem.
socket->write(data.toUtf8());
socket->waitForBytesWritten();
}
}
server receiving:
// this slot is connected to the readyRead() signal
void Server::readData()
{
// on Windows this is called exactly once for each client
// every additional messages just don't seem to arrive at the server
QString msg = QString(socket->readAll());
emit receivedData(this, &msg);
}
I just don't get why it works on Linux, yet not on windows...
What would I need to receive data from a client?
QTcpServer Server;
if(!Server.listen("127.0.0.1", 9000)) {
return;
}
connect(Server, SIGNAL(newConnection()), this, SLOT(ReceiveData()));
Is this correct so far? What do I need in ReceiveData? Do I really need another function to receive the data? I would like to save it in a QByteArray
Thanks
As this hasn't been answered, here's a really basic example.
In your ReceiveData slot, you would need to accept the connection from the server.
In Qt's QTcpServer this is done by calling nextPendingConnection().
So the QTcpServer's newConnection slot will call your ReceiveData slot.
In your receivedata slot, you can do something like:
void ReceiveData()
{
QTcpSocket *socket = server->nextPendingConnection();
if (!socket)
return;
qDebug("Client connected");
socket->waitForReadyRead(5000);
QByteArray data = socket->readAll();
qDebug(data.constData());
socket->close();
}
Note: This is a blocking example, the waitForReadyRead will hang the thread for up to 5000 milliseconds.
To do a non blocking example, you need to connect another slot to the new socket's readyread signal.
Have you seen this example:
http://doc.qt.io/qt-5/qtnetwork-fortuneserver-server-cpp.html
PS:
Yes, you need at least one callback function to:
1) accept new connections
2) Receive and Send data on the connect(s)
I've modified the threaded fortune-server from Qt examples.
The client connects to the server and then sends a header to authenticate.
tcpSocket = new QTcpSocket();
tcpSocket->connectToHost(addr, port);
QByteArray block = "someheader";
int x = tcpSocket->write(block);
qDebug() << x;
The client seems OK here and qDebug prints the actual size of block.
On the server side I've predefined incomingConnection and I start thread to each new connection.
void Server::incomingConnection(int socketDescriptor) {
const QString &str = vec[qrand() % vec.size()];
SpellThread *thread = new SpellThread(socketDescriptor, str);
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
qDebug() << " -- incoming connection";
thread->start();
}
I'm connecting sock to check is there something to read. (sock here is QTcpServer*)
void SpellThread::run() {
qDebug() << " -- in spellthread";
connect(sock, SIGNAL(readyRead()), this, SLOT(checkBytes()));
//....
qDebug() << " -- end spellthread";
}
The first problem is that when I'm sending data from the client, readyRead is not fired. (I've added debug message in checkBytes)
Messages are:
-- incoming connection
-- in spellthread
-- end spellthread
Although the client prints the actual size of header length.
The second problem is that checkBytes currently is very bad-designed. First it checks is header OK and sets a flag, then it gets the size of message and sets another flag and finally it gets the real message. This is very clumsy. I first tried to escape signals and instead use sock->waitForReadyRead(). However it always returns false. (From the docs: "Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.").
So how to really make a client/server application in Qt with multiple clients and multiple reads/writes? I really want suggestions to improve design of my application and to solve my current two problems.
You can't use slots or socket signals with a thread without calling QThread::exec() to start an event loop within that thread/the run function.
Since your checkBytes slot belongs to QThread, it wouldn't be executed by the thread (there is a detailed article about QThreads here)
The closest example that seems to already do what you want is the Network Chat (particularly the two classes Server and Connection).
----------Edit
If you need to use threads (without any slot), the QTcpSocket object must belongs to the same thread as the one where you call waitForReadyRead. For example, with:
SpellThread::SpellThread(int socketDescriptor, const QString & str) {
tcpSocket = new QTcpSocket(); // There should be no parent to be able
// to move it to the thread
tcpSocket->moveToThread(this);
...
Or by creating the QTcpSocket object inside the run function so that it automatically belongs to that thread (it was briefly explained in the fortune example).
If you allocate the QTcpSocket dynamically, and because it won't have a parent, you should also delete it manually.
I set up a QTcpServer to listen to a Shoutcast stream. The newConnection()-signal gets fired as it should:
connect(tcpServer, SIGNAL(newConnection()), this, SLOT(handleClientComm()));
void IcecastServer::handleClientComm(){
QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
qDebug() << clientConnection->write("HTTP/1.0 200 OK\r\n\r\n" ) << endl;
clientConnection->flush();
}
How do I send HTTP 200 ?
You must extract QTcpSocket object from QTcpServer with nextPendingConnection() call when the newConnection() signal was emitted. And then you must call writeData() on the extracted QTcpSocket object.
The key here is that Listening socket (QTcpServer) is only responsible for creating Connection Sockets (or QTcpSocket) each time a new client connects. And the QTcpSocket is responsible for the actual communication with a specific client.
Maybe you can be more specific what exactly does not work for you and what have you tried? It would also be nice if you could provide us with wireshark PCAP if something does not seem to work as expected?