QTcpSocket connection problem

[valdemar593]

Hello everyone. I'm trying to write a chat using QTcpSocket and QTcpServer, but strange things are happening. In the first example QTcpServer emits newConnection(), but QTcpSocket remains in connecting state without emiting error(...). In the second example I receive SIGSEGV when trying connectToHost() . What is more, FortuneClient/Server (single threaded) example runs just fine. In both examples network sessiong is not required. Being discouraged I examined the Fortune code and discovored that it is just the same concerning establishing connection. Here are both examples.

FIRST

//CLIENT
ChatClient::ChatClient(QObject *parent)
    : QObject(parent) {
    tcpSocket = new QTcpSocket(this);
    QNetworkConfigurationManager manager;
    if (QNetworkConfigurationManager::NetworkSessionRequired
        & manager.capabilities()) {
        qDebug() << "Network session required";
    }
    connect(tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
            this, SLOT(error(QAbstractSocket::SocketError)));
    connect(tcpSocket, SIGNAL(connected()),
            this, SLOT(requestForID()));
    connect(tcpSocket, SIGNAL(readyRead()),
            this, SLOT(receiveMessage()));
    tcpSocket->connectToHost("192.168.0.100", PORT);
}
...
void ChatClient::error(QAbstractSocket::SocketError error) {
    qDebug() << "Socket error" << error;
}
 
void ChatClient::requestForID() {
    qDebug() << "Connected, requesting for ID";
    QByteArray segment;
    QDataStream out(&segment, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_7);
    out << (quint16)0 << ID;
    out.device()->seek(0);
    out << (quint16)(segment.size() - sizeof(quint16));
    tcpSocket->write(segment);
}
 
//SERVER
ChatServer::ChatServer(QObject *parent)
    : QObject(parent) {
    tcpServer = new QTcpServer(this);
    if (!tcpServer->listen(/*QHostAddress::Any, PORT*/)) {
        qDebug() << "Unable to start the server"
                 << tcpServer->errorString();
    }
    qDebug() << "Server port" << tcpServer->serverPort();
    connect(tcpServer, SIGNAL(newConnection()),
            this, SLOT(processConnection()));
}
 
void ChatServer::processConnection() {
    qDebug() << "Incoming connection";
    QTcpSocket *clientSocket = tcpServer->nextPendingConnection();
    /*connect(clientSocket, SIGNAL(readyRead()),
            this, SLOT(readData()));
    readData(clientSocket);
    connect(clientSocket, SIGNAL(disconnected()),
            clientSocket, SLOT(deleteLater()));*/
    QByteArray segment;
    QDataStream out(&segment, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_7);
    out << (quint16)0 << (quint16)Message
        << "Successfully connected";
    out.device()->seek(0);
    out << (quint16)(segment.size() - sizeof(quint16));
    clientSocket->write(segment);
    clientSocket->disconnectFromHost();
}

//CLIENT
ChatClient::ChatClient(QObject *parent)
    : QObject(parent) {
    tcpSocket = new QTcpSocket(this);
    QNetworkConfigurationManager manager;
    if (QNetworkConfigurationManager::NetworkSessionRequired
        & manager.capabilities()) {
        qDebug() << "Network session required";
    }
    connect(tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
            this, SLOT(error(QAbstractSocket::SocketError)));
    connect(tcpSocket, SIGNAL(connected()),
            this, SLOT(requestForID()));
    connect(tcpSocket, SIGNAL(readyRead()),
            this, SLOT(receiveMessage()));
    tcpSocket->connectToHost("192.168.0.100", PORT);
}
...
void ChatClient::error(QAbstractSocket::SocketError error) {
    qDebug() << "Socket error" << error;
}

void ChatClient::requestForID() {
    qDebug() << "Connected, requesting for ID";
    QByteArray segment;
    QDataStream out(&segment, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_7);
    out << (quint16)0 << ID;
    out.device()->seek(0);
    out << (quint16)(segment.size() - sizeof(quint16));
    tcpSocket->write(segment);
}

//SERVER
ChatServer::ChatServer(QObject *parent)
    : QObject(parent) {
    tcpServer = new QTcpServer(this);
    if (!tcpServer->listen(/*QHostAddress::Any, PORT*/)) {
        qDebug() << "Unable to start the server"
                 << tcpServer->errorString();
    }
    qDebug() << "Server port" << tcpServer->serverPort();
    connect(tcpServer, SIGNAL(newConnection()),
            this, SLOT(processConnection()));
}

void ChatServer::processConnection() {
    qDebug() << "Incoming connection";
    QTcpSocket *clientSocket = tcpServer->nextPendingConnection();
    /*connect(clientSocket, SIGNAL(readyRead()),
            this, SLOT(readData()));
    readData(clientSocket);
    connect(clientSocket, SIGNAL(disconnected()),
            clientSocket, SLOT(deleteLater()));*/
    QByteArray segment;
    QDataStream out(&segment, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_7);
    out << (quint16)0 << (quint16)Message
        << "Successfully connected";
    out.device()->seek(0);
    out << (quint16)(segment.size() - sizeof(quint16));
    clientSocket->write(segment);
    clientSocket->disconnectFromHost();
}

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SECOND

//CLIENT
Client::Client() {
    QTcpSocket *clientSocket = new QTcpSocket(this);
    connect(clientSocket, SIGNAL(connected()),
            this, SLOT(connectionEstablished()));
    connect(clientSocket, SIGNAL(error(QAbstractSocket::SocketError)),
            this, SLOT(error()));
}
 
void Client::establishConnection() {
// throws SIGSEGV
    clientSocket->connectToHost(QHostAddress("192.168.0.100"), 6178);
}
 
void Client::connectionEstablished() {
    qDebug() << "Connection established";
}
 
void Client::error() {
    qDebug() << clientSocket->errorString();
}
 
//SERVER
Server::Server() {
    QNetworkConfigurationManager manager;
    if (manager.capabilities() &
        QNetworkConfigurationManager::NetworkSessionRequired) {
        qDebug() << "Network session required";
    } else {
        sessionOpened();
    }
    connect(server, SIGNAL(newConnection()),
            this, SLOT(processConnection()));
}
 
void Server::processConnection() {
    qDebug() << "Incoming connection";
}
 
void Server::sessionOpened() {
    server = new QTcpServer(this);
    if (!server->listen(QHostAddress::Any, 6178)) {
        qDebug() << QString("Unable to start the server: %1")
                .arg(server->errorString());
        return;
    }
    qDebug() << "The server is running...";
}

//CLIENT
Client::Client() {
    QTcpSocket *clientSocket = new QTcpSocket(this);
    connect(clientSocket, SIGNAL(connected()),
            this, SLOT(connectionEstablished()));
    connect(clientSocket, SIGNAL(error(QAbstractSocket::SocketError)),
            this, SLOT(error()));
}

void Client::establishConnection() {
// throws SIGSEGV
    clientSocket->connectToHost(QHostAddress("192.168.0.100"), 6178);
}

void Client::connectionEstablished() {
    qDebug() << "Connection established";
}

void Client::error() {
    qDebug() << clientSocket->errorString();
}

//SERVER
Server::Server() {
    QNetworkConfigurationManager manager;
    if (manager.capabilities() &
        QNetworkConfigurationManager::NetworkSessionRequired) {
        qDebug() << "Network session required";
    } else {
        sessionOpened();
    }
    connect(server, SIGNAL(newConnection()),
            this, SLOT(processConnection()));
}

void Server::processConnection() {
    qDebug() << "Incoming connection";
}

void Server::sessionOpened() {
    server = new QTcpServer(this);
    if (!server->listen(QHostAddress::Any, 6178)) {
        qDebug() << QString("Unable to start the server: %1")
                .arg(server->errorString());
        return;
    }
    qDebug() << "The server is running...";
}

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Any ideas?

[kosasker]

Use QThread and Mutex. I solved with them. I want to send it but my code is C based.

[valdemar593]

Please explain why I should use QMutex and QThread in those examples? Should I implement tcp connection in a seperate thread?

[kosasker]

Oh.. its just my opinion
No not. Just inherit your class from QThread like this

class logthread : public QThread
{
    Q_OBJECT
public:
    explicit logthread(QObject *parent = 0);
    ~logthread();
..........

class logthread : public QThread
{
    Q_OBJECT
public:
    explicit logthread(QObject *parent = 0);
    ~logthread();
..........

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and use mutex only when your application sends data

QMutex mutex;
...........
.........
.........
mutex.lock();
 send_function();
mutex.unlock();

QMutex mutex;
...........
.........
.........
mutex.lock();
 send_function();
mutex.unlock();

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Its a very basic example but my code is running clearly. For more information just read the docs.

[valdemar593]

Thanks I'll try right now. Should I lock tcpSocket->connectToHost()? But why FortuneClient/Server doesn't use mutex and doesn't inherit QThread and works fine?

[wysota]

There is no reason to use threads and mutexes here.

[valdemar593]

Yes that hasn't solved the problem. But still I'm confused because Fortune example works and my doesn't. At the same time I do not find differencies except that Fortune iterates over network interfaces to choose one and I choose manually. Even then both IP addresses are the same. May be I should post the full code? It is not long.

[wysota]

Post the debugger backtrace after a crash.

[valdemar593]

//
int idx = obj->metaObject()->indexOfMethod(sig.constData());
//
return invokeMethod(obj, member, type, QGenericReturnArgument(), val0, val1, val2, val3, val4, val5, val6, val7, val8, val9);
//
QMetaObject::invokeMethod(this, "connectToHostImplementation",
Qt:: DirectConnection,
Q_ARG(QString, hostName),
Q_ARG(quint16, port),
Q_ARG(OpenMode, openMode));
//
connectToHost(address.toString(), port, openMode);
//
clientSocket->connectToHost(QHostAddress("192.168.0.100"), 6178);

please note that I talk about two different projets, where in the first one QTcpSocket remains in connecting state while in the second QTcpSocket::connectToHost() produces SIGSEGV.

[wysota]

Please post the backtrace as requested.

[valdemar593]

Sorry I misunderstand what is backtrace?

[wysota]

http://en.wikipedia.org/wiki/Call_stack

If you are using Qt Creator, the backtrace is in the "stack" view when you're in the Debug mode.

[valdemar593]

gdb

#0  0x00402b73 in QMetaObject::invokeMethod (obj=0x7e9ff4, 
    member=0x22a33f "connectToHostImplementation", type=Qt::DirectConnection, 
    ret=..., val0=..., val1=..., val2=..., val3=..., val4=..., val5=..., val6=..., 
    val7=..., val8=..., val9=...)
    at /var/tmp/qt-src/src/corelib/kernel/qmetaobject.cpp:1135
#1  0x001efe73 in invokeMethod (this=0x7e9ff4, hostName=..., port=6178, openMode=...)
    at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs.h:408
#2  QAbstractSocket::connectToHost (this=0x7e9ff4, hostName=..., port=6178, 
    openMode=...) at /var/tmp/qt-src/src/network/socket/qabstractsocket.cpp:1304
#3  0x001f1835 in QAbstractSocket::connectToHost (this=0x7e9ff4, address=..., 
    port=6178, openMode=...)
    at /var/tmp/qt-src/src/network/socket/qabstractsocket.cpp:1417
#4  0x080494e4 in Client::establishConnection (this=0xbffff1fc)
    at ../NetTestClient/client.cpp:13
#5  0x08049201 in main (argc=1, argv=0xbffff2d4) at ../NetTestClient/main.cpp:8

#0  0x00402b73 in QMetaObject::invokeMethod (obj=0x7e9ff4, 
    member=0x22a33f "connectToHostImplementation", type=Qt::DirectConnection, 
    ret=..., val0=..., val1=..., val2=..., val3=..., val4=..., val5=..., val6=..., 
    val7=..., val8=..., val9=...)
    at /var/tmp/qt-src/src/corelib/kernel/qmetaobject.cpp:1135
#1  0x001efe73 in invokeMethod (this=0x7e9ff4, hostName=..., port=6178, openMode=...)
    at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs.h:408
#2  QAbstractSocket::connectToHost (this=0x7e9ff4, hostName=..., port=6178, 
    openMode=...) at /var/tmp/qt-src/src/network/socket/qabstractsocket.cpp:1304
#3  0x001f1835 in QAbstractSocket::connectToHost (this=0x7e9ff4, address=..., 
    port=6178, openMode=...)
    at /var/tmp/qt-src/src/network/socket/qabstractsocket.cpp:1417
#4  0x080494e4 in Client::establishConnection (this=0xbffff1fc)
    at ../NetTestClient/client.cpp:13
#5  0x08049201 in main (argc=1, argv=0xbffff2d4) at ../NetTestClient/main.cpp:8

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qt creator

0 QHashData::detach_helper2 qhash.cpp 206 0x001a18fa
1 QHash<QObject*, QHash<QEvent::Type, int> >::detach_helper qhash.h 582 0x002e69eb
2 detach qhash.h 299 0x002dbc23
3 operator[] qhash.h 737 0x002dbc23
4 QStateMachinePrivate::registerEventTransition qstatemachine.cpp 1598 0x002dbc23
5 invokeMethod qobjectdefs.h 408 0x00a25e73
6 QAbstractSocket::connectToHost qabstractsocket.cpp 1304 0x00a25e73
7 QAbstractSocket::connectToHost qabstractsocket.cpp 1417 0x00a27835
8 Client::establishConnection client.cpp 13 0x080494e4
9 main main.cpp 8 0x08049201

[wysota]

What does the main() function look like?

[valdemar593]

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Client client;
    client.establishConnection();
    return a.exec();
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Client client;
    client.establishConnection();
    return a.exec();
}

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[wysota]

Ah.... I missed it. You are creating a local variable called "clientSocket" in your constructor that shadows the class member called "clientSocket" thus it remains uninitialized and a dangling pointer causes a crash.

[valdemar593]

Oh my god =))
Thanks a lot man!
But what about the first example? Everything seems to be correct there, but the socket stays in connecting state all the time while the server emits newConnection()?

[wysota]

I don't know, I'd need to see code that's more complete than this.