Server thread inside a qt class (need mutex?) - c++

Question

I made this server class that starts a thread when new connection comes in. It works ok with some cases, but it's not very stable. I am trying to solve where it breaks. My debugger tells me something about qmutex. If anyone can spot the problem. ty
It connects with parent with signal&slots and gets data back also.
Here is the header:
#ifndef FORTUNESERVER_H
#define FORTUNESERVER_H
#include <QStringList>
#include <QTcpServer>
#include <QThread>
#include <QTcpSocket>
#include <string>
using namespace std;
class FortuneServer : public QTcpServer
{
Q_OBJECT
public:
FortuneServer(QObject *parent = 0);
public slots:
void procesServerString(string serverString);
void getStringToThread(string serverString);
protected:
void incomingConnection(int socketDescriptor);
private:
QStringList fortunes;
signals:
void procesServerStringToParent(string serverString);
void getStringToThreadSignal(string serverString);
};
class FortuneThread : public QObject
{
Q_OBJECT
public:
FortuneThread(int socketDescriptor, QObject *parent);
public slots:
void getString();
void sendString(string sendoutString);
signals:
void error(QTcpSocket::SocketError socketError);
void fromThreadString(string serverString);
void finished();
private:
int socketDescriptor;
QString text;
QTcpSocket tcpSocket;
};
#endif
and cc:
#include <stdlib.h>
#include <QtNetwork>
#include "MeshServer.hh"
#include <iostream>
#include "TableView.hh"
using namespace std;
FortuneServer::FortuneServer(QObject *parent)
: QTcpServer(parent)
{
}
void FortuneServer::procesServerString(string serverString){
emit procesServerStringToParent(serverString);
}
void FortuneServer::getStringToThread(string serverString){
emit getStringToThreadSignal(serverString);
}
void FortuneServer::incomingConnection(int socketDescriptor)
{
FortuneThread *serverthread = new FortuneThread(socketDescriptor, this);
//connect(&serverthread, SIGNAL(finished()), &serverthread, SLOT(deleteLater()));
QThread* thread = new QThread;
serverthread->moveToThread(thread);
connect(thread, SIGNAL(started()), serverthread, SLOT(getString()));
connect(serverthread, SIGNAL(fromThreadString(string)), this, SLOT(procesServerString(string)));
connect(this, SIGNAL(getStringToThreadSignal(string)), serverthread, SLOT(sendString(string)));
connect(serverthread, SIGNAL(finished()), thread, SLOT(quit()));
connect(serverthread, SIGNAL(finished()), serverthread, SLOT(deleteLater()));
connect(serverthread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();
}
FortuneThread::FortuneThread(int socketDescriptor, QObject *parent)
: QObject(parent), socketDescriptor(socketDescriptor)
{
}
void FortuneThread::getString()
{
if (!tcpSocket.setSocketDescriptor(socketDescriptor)) {
emit error(tcpSocket.error());
cout<<"socket error"<<endl;
return;
}
//in part
if(!tcpSocket.waitForReadyRead(10000)){
emit finished();
return;
}
int joj = tcpSocket.bytesAvailable();
char inbuffer[1024];
tcpSocket.read(inbuffer,1024);
string instring;
instring = inbuffer;
instring.resize(joj);
emit fromThreadString(instring);
}
void FortuneThread::sendString(string sendoutString)
{
//out part
char buffer[1024];
int buffer_len = 1024;
int bytecount;
memset(buffer, '\0', buffer_len);
string outstring = sendoutString;
int TempNumOne= (int)outstring.size();
for (int a=0;a<TempNumOne;a++)
{
buffer[a]=outstring[a];
}
QByteArray block;
block = buffer;
tcpSocket.write(block);
tcpSocket.disconnectFromHost();
tcpSocket.waitForDisconnected();
emit finished();
}
this is from parent:
//server start
QHostAddress adr;
adr.setAddress( QString("127.0.0.1") );
adr.toIPv4Address();
quint16 port = 1101;
if (!server.listen( adr, port)) {
QMessageBox::critical(this, tr("CR_bypasser"),
tr("Unable to start the server: %1.")
.arg(server.errorString()));
close();
return;
}
QString ipAddress;
ipAddress = server.serverAddress().toString();
statusLabel->setText(tr("The server is running on\n\nIP: %1\nport: %2\n\n"
"Run the Fortune Client example now.")
.arg(ipAddress).arg(server.serverPort()));
connect (&server, SIGNAL(procesServerStringToParent(string)), this, SLOT(procesServerString(string)));
connect (this, SIGNAL(StringToServer(string)), &server, SLOT(getStringToThread(string)));
edit: what I am trying to do:
I have a client (part of a game engine(Cryengine)) that I made to send string of ingame coordinates and some other things with a socket like its done in a link I gave before. This works ok. I get data on "127.0.0.1" port 1101. Now I just need this data to be evaluated in my own program, that has this TableView class, inside which I can collect coordinates I get from the string, callculate some data from coordinates and then return this new string back through the server to gameengine. In game I will click on objects get their coor., make a string out of that (containing coor,entityid, etc..), send this string to server, that returns callculated info from TableView. I just need this one way flow only one client that is sending strings. I am not sure about recv(hsock, buffer, buffer_len, 0), I guess node that is responsible for string sending in game will wait for return string? This is one of my first programs atm I am realy confused...

Answer 1

The code you present is exemplary of cargo cult coding: you do various unnecessary things, apparently in hopes of fixing the problem.
The Likely Crasher ...
There are tons of problems with the code, but I think the cause of the crash is this: tcpSocket.write(block) does not send out a zero-terminated string down the wire. The block is zero-terminated, but the assignment to a byte array does not add this zero termination to the size() of QByteArray. The following code prints 1, even though there is a zero terminating byte internally in the contents of the byte array.
QByteArray arr = "a";
qDebug("len=%d", arr.size());
The receiving code expects the zero termination, but never receives it. You then proceed to assign a non-zero-terminated buffer to std::string:
string instring;
instring = inbuffer;
instring.resize(joj);
The subsequent resize is cargo cult: you're trying to fix the problem after std::string & std::string::operator=(const char*) has already read past your buffer, in all likelihood.
Do not take this to mean that fixing just that is the right way to proceed. Not at all. The right way to proceed is to delete the code you wrote and do it right, without a ton of unnecessary incantations that don't help.
... and All The Other Problems
You've fallen into the trap of believing in magic, perpetuated endlessly in various forums.
The threads are not magical objects that you can just apply to any problem out there in hopes that they help. I don't know what makes people think that threads are magical, but the rule of thumb is: If someone tells you "oh, you should try threads", they are most likely wrong. If they tell that in relation to networking, they are pretty much never right, they are unhelpful, and they don't understand your problem at all (neither do you, it seems). More often than not, threads will not help unless you clearly understand your problem. Qt's networking system is asynchronous: it doesn't block the execution of your code, if you don't use the waitxxxx() functions. You shouldn't use them, by the way, so all is good here. No need for a bazillion threads.
So, it is completely unnecessary to start a new thread per each incoming connection. It will decrease the performance of your server -- especially if the server does simple processing because you add the overhead of context switching and thread creation/dismantling to each connection. You want less than 2 threads per each core in your system, so using QThread::idealThreadCount() for the number of threads in the pool would be a good starting point.
You are also depriving yourself of the benefit of threading since you use the networking thread only to receive the data, and you then send out a fromThreadString(string) signal. I presume that signal is sent to your application's main thread. Now that's just silly, because receiving a bunch of bytes from a network socket is downright trivial. Your threads don't do any work, all the work they do is wasted on their creation and removal.
The code below is a simple example of how one might correctly use the Qt APIs to implement a client-server system that distributes work across the physical cores in a round-robin fashion. It should perform quite well. The Fortune client example included in Qt is very unfortunate indeed, because it's precisely the wrong way to go about things.
What one will notice is:
It's not entirely trivial. Qt could be more helpful, but isn't.
Both the clients and the senders are moved into threads from a thread pool.
Disconnected clients are not deleted, but merely returned to a list of clients
kept by the tread pool. They are reused when a client is called for.
QThread is not derived from. QTcpServer is only derived to access the socket handle.
No functions whose name begins with wait() are used. Everything is handled asynchronously.
The ThreadPool keeps a looked-up QMetaMethod for the newConnection(int) slot of the Client. This is faster than using QMetaObject::invokeMethod() as it has to look things up every time.
A timer running in the main thread sets off a signal-slot chain by deleting the first sender. Each senders' deletion triggers the deletion of the next one. Eventually, the last sender sets off the quit() slot in the thread pool. The latter emits the finished() signal when all threads are indeed finished.
#include <QtCore/QCoreApplication>
#include <QtNetwork/QTcpServer>
#include <QtNetwork/QTcpSocket>
#include <QtCore/QQueue>
#include <QtCore/QThread>
#include <QtCore/QTimer>
#include <QtCore/QMetaMethod>
// Processes data on a socket connection
class Client : public QObject
{
Q_OBJECT
public:
Client(QObject* parent = 0) : QObject(parent), socket(new QTcpSocket(this))
{
connect(socket, SIGNAL(readyRead()), SLOT(newData()));
connect(socket, SIGNAL(stateChanged(QAbstractSocket::SocketState)),
SLOT(newState(QAbstractSocket::SocketState)));
qDebug("Client()");
}
~Client() { qDebug("~Client()"); }
signals:
void done();
public slots:
void newConnection(int descriptor) {
socket->setSocketDescriptor(descriptor);
}
private slots:
void newData() {
QByteArray data = socket->readAll();
if (0) qDebug("got %d bytes", data.size());
if (0) qDebug("got a string %s", data.constData());
// here we can process the data
}
void newState(QAbstractSocket::SocketState state) {
qDebug("client new state %d", state);
if (state == QAbstractSocket::UnconnectedState) { emit done(); }
}
protected:
QTcpSocket* socket;
int descriptor;
};
// Connects to a client and sends data to it
class Sender : public QObject
{
Q_OBJECT
public:
Sender(const QString & address, quint16 port, QObject * parent = 0) :
QObject(parent), socket(new QTcpSocket(this)),
bytesInFlight(0), maxBytesInFlight(65536*8)
{
connect(socket, SIGNAL(stateChanged(QAbstractSocket::SocketState)),
SLOT(newState(QAbstractSocket::SocketState)));
connect(socket, SIGNAL(bytesWritten(qint64)), SLOT(sentData(qint64)));
socket->connectToHost(address, port);
qDebug("Sender()");
}
~Sender() { qDebug("~Sender()"); }
protected:
// sends enough data to keep a maximum number of bytes in flight
void sendData() {
qint64 n = maxBytesInFlight - bytesInFlight;
if (n <= 0) return;
bytesInFlight += n;
socket->write(QByteArray(n, 44)); // 44 is the answer, after all
}
protected slots:
void sentData(qint64 n) {
bytesInFlight -= n;
Q_ASSERT(bytesInFlight >= 0);
sendData();
}
void newState(QAbstractSocket::SocketState state) {
qDebug("sender new state %d", state);
if (state == QAbstractSocket::ConnectedState) sendData();
}
protected:
QTcpSocket* socket;
qint64 bytesInFlight;
qint64 maxBytesInFlight;
};
// Keeps track of threads and client objects
class ThreadPool : public QTcpServer
{
Q_OBJECT
public:
ThreadPool(QObject* parent = 0) : QTcpServer(parent), nextThread(0) {
for (int i=0; i < QThread::idealThreadCount(); ++i) {
QThread * thread = new QThread(this);
connect(thread, SIGNAL(finished()), SLOT(threadDone()));
thread->start();
threads << thread;
}
const QMetaObject & mo = Client::staticMetaObject;
int idx = mo.indexOfMethod("newConnection(int)");
Q_ASSERT(idx>=0);
method = mo.method(idx);
}
void poolObject(QObject* obj) const {
if (nextThread >= threads.count()) nextThread = 0;
QThread* thread = threads.at(nextThread);
obj->moveToThread(thread);
}
protected:
void incomingConnection(int descriptor) {
Client * client;
if (threads.isEmpty()) return;
if (! clients.isEmpty()) {
client = clients.dequeue();
} else {
client = new Client();
connect(client, SIGNAL(done()), SLOT(clientDone()));
}
poolObject(client);
method.invoke(client, Q_ARG(int, descriptor));
}
signals:
void finished();
public slots:
void quit() {
foreach (QThread * thread, threads) { thread->quit(); }
}
private slots:
void clientDone() {
clients.removeAll(qobject_cast<Client*>(sender()));
}
void threadDone() {
QThread * thread = qobject_cast<QThread*>(sender());
if (threads.removeAll(thread)) delete thread;
if (threads.isEmpty()) emit finished();
}
private:
QList<QThread*> threads;
QQueue<Client*> clients;
QMetaMethod method;
mutable int nextThread;
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
ThreadPool server;
if (!server.listen(QHostAddress::Any, 1101)) qCritical("cannot establish a listening server");
const int senderCount = 10;
Sender *prevSender = 0, *firstSender = 0;
for (int i = 0; i < senderCount; ++ i) {
Sender * sender = new Sender("localhost", server.serverPort());
server.poolObject(sender);
if (!firstSender) firstSender = sender;
if (prevSender) sender->connect(prevSender, SIGNAL(destroyed()), SLOT(deleteLater()));
prevSender = sender;
}
QTimer::singleShot(3000, firstSender, SLOT(deleteLater())); // run for 3s
server.connect(prevSender, SIGNAL(destroyed()), SLOT(quit()));
qApp->connect(&server, SIGNAL(finished()), SLOT(quit()));
// Deletion chain: timeout deletes first sender, then subsequent senders are deleted,
// finally the last sender tells the thread pool to quit. Finally, the thread pool
// quits the application.
return a.exec();
}
#include "main.moc"
Given your explanation, you game engine starts up and creates a connection to some port on localhost. Your Qt program is supposed to accept that connection on port 1101, receive some strings, process them, then send them back.
The code is modified to accept the connection on a fixed port number. All of the data processing, including sending the response back, has to be done from the newData() slot. You can also pass that data off to a different thread, if your computations are very complex. By complex I mean tens of thousands of operations like additions and multiplications, or thousands of trig operations.
The Sender class is there just as an example. Your game engine does the sending, of course, so you don't need the Sender class.

Answer 2

I got my old "the wrong way to do it" code to work. I guess this part was where the error was:
//removed
tcpSocket.disconnectFromHost();
tcpSocket.waitForDisconnected();
emit finished();
...
#include <stdlib.h>
#include <QtNetwork>
#include "MeshServer.hh"
#include <iostream>
#include "TableView.hh"
using namespace std;
FortuneServer::FortuneServer(QObject *parent)
: QTcpServer(parent)
{
}
void FortuneServer::procesServerString(string serverString){
emit procesServerStringToParent(serverString);
}
void FortuneServer::getStringToThread(string serverString){
emit getStringToThreadSignal(serverString);
}
void FortuneServer::incomingConnection(int socketDescriptor)
{
FortuneThread *serverthread = new FortuneThread(socketDescriptor, this);
//connect(&serverthread, SIGNAL(finished()), &serverthread, SLOT(deleteLater()));
QThread* thread = new QThread;
serverthread->moveToThread(thread);
connect(serverthread, SIGNAL(fromThreadString(string)), this, SLOT(procesServerString(string)));
connect(this, SIGNAL(getStringToThreadSignal(string)), serverthread, SLOT(sendString(string)));
connect(serverthread, SIGNAL(finished()), thread, SLOT(quit()));
connect(serverthread, SIGNAL(finished()), serverthread, SLOT(deleteLater()));
connect(serverthread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();
}
FortuneThread::FortuneThread(int socketDescriptor, QObject *parent): QObject(parent), socketDescriptor(socketDescriptor)
{
if (!tcpSocket.setSocketDescriptor(socketDescriptor)) {
emit error(tcpSocket.error());
cout<<"socket error"<<endl;
emit finished();
return;
}
connect(&tcpSocket, SIGNAL(readyRead()), this, SLOT(getString()));
//connect(&tcpSocket, SIGNAL(disconnected()), this, SLOT(ondisconnected()));
}
void FortuneThread::getString()
{
int joj = tcpSocket.bytesAvailable();
if(joj==0){
tcpSocket.disconnectFromHost();
emit finished();
return;
}
char inbuffer[1024];
int buffer_len = 1024;
memset(inbuffer, '\0', buffer_len);
tcpSocket.read(inbuffer,1024);
string instring;
instring = inbuffer;
instring.resize(joj);
emit fromThreadString(instring);
}
void FortuneThread::sendString(string sendoutString)
{
char buffer2[1024];
int buffer_len = 1024;
memset(buffer2, '\0', buffer_len);
strcat(buffer2,sendoutString.c_str());
tcpSocket.write(buffer2,buffer_len);
}
void FortuneThread::ondisconnected()
{
emit finished();
}