QTcpSocket client, write problem...

[J.Hilk]

@SPlatten I don't know if QJsonObject is registered with the meta system by default, if not, you'll have to do that yourself, or the connect won't work.

But you should get compile/runtime warnings if thats the case

[JonB]

@SPlatten
A while back in another of your post threads you were using explicit Qt::DirectConnections, and I said then (and I think others did too), why are you writing this, it will just lead to trouble.

So why do you ever put Qt::DirectConnection into your connect()s, I just don't get it, it keeps leading to this kind of problem?

[SPlatten]

@JonB , they are gone now.

[SPlatten]

@J-Hilk , I do register the type.

[JonB]

@SPlatten said in QTcpSocket client, write problem...:

@JonB , they are gone now.

:) May I suggest you never put them in? The default for connect() is DirectConnection, and Qt only changes that to Queued when it sees it's going across threads. At which point as I understand it you need it to do that, so why interfere? :)

Might I also suggest you consider simplifying your qDebugMsgHandler()? That's an awful lot of code to potentially go wrong, when you are down in a low-level message handler! And if it does you will lose the original message, as you have seen. At minimum: factor all your stuff out to a separate function, so that qDebugMsgHandler() is lean & clean. And wrap that with a try handler. If it goes doolally, allow the original qDebugMsgHandler() to not fall over but to issue some message in a simple fashion. That way you have some protection for not fouling up and still getting access to the original fault message. Making sure one's low-level error/message handlers do not themselves introduce errors is an important goal.

[SPlatten]

@JonB , thank you, the debugHandler works fine and quite sure there is nothing wrong with it, I accept your comments and in the final release it will be disabled altogether.

[kshegunov]

@SPlatten, fix the reason for the warning, so you don't get the warning. In other words, don't enable/disable sockets from different threads.

@JonB said in QTcpSocket client, write problem...:

The fact that QMessageLogger::warning is calling abort() does not look right.

He's running with fatal warnings (as suggested in earlier threads), so it's exactly right!

[SPlatten]

I think I spoke to soon I still have the problem:

QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread

I've modified all my QObject::connect's, ensuring that I am not using Qt::DirectConnection. I'm still getting this message. I've been careful that I am not calling write from any thread, I always emit a write signal which connects to a slot called onWrite which actually does the::

qint64 int64Written = mpsckClient->write(arybytMsg);

[KroMignon]

@SPlatten said in QTcpSocket client, write problem...:

I think I spoke to soon I still have the problem:
QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread

As an experimented developer, you should know that multithreading can solve problems but always introduce new problems ;)

When using QObject based classes, you should always ensure that you are using them in the thread in which they have be created / moved.
So, are you sure qint64 int64Written = mpsckClient->write(arybytMsg); is done in the right thread (mpsckClient->thread() == QThread::currentThread()).

[SPlatten]

@KroMignon I thought that one of the benefits of using signals and slots is that you can be assured that they are thread safe?

[KroMignon]

@SPlatten said in QTcpSocket client, write problem...:

I thought that one of the benefits of using signals and slots is that you can be assured that they are thread safe?

Yes it is, but it depends how you are doing it.

QObject::connect(srcObject, SIGNAL(), destObject, SLOT()) will ensure SLOT() will run in destObject work thread, when signal is emitted.

EDIT:
but
QObject::connect(srcObject, SIGNAL(), destObject, SLOT(), Qt::DirectConnection) will run SLOT() in thread from which signal has been emitted.

[SPlatten]

@KroMignon Thank you, rapidly learning this.

[KroMignon]

@SPlatten said in QTcpSocket client, write problem...:

Thank you, rapidly learning this.

Other things you should take care off when you are doing multithreading, is to set parent for class member to ensure that when you are moving the class instance to another thread, all member will follow.

To better explain what I mean, suppose following example:

class TestClass : public QObject
{
    Q_OBJECT
public:
    explicit TestClass(const QHostAddress &address, quint16 portNum, QHostAddress QObject * parent=nullptr)
         : QObject(parent), mSocket(new QTcpSocket(this))
   {
   }
private:
    QTcpSocket* mSocket;
};

class TestClass2 : public QObject
{
    Q_OBJECT
public:
    explicit TestClass2(const QHostAddress &address, quint16 portNum, QHostAddress QObject * parent=nullptr)
         : QObject(parent), mSocket(new QTcpSocket())
   {
   }
private:
    QTcpSocket* mSocket;
};

And somewhere in code:

auto sock = new TestClass(QHostAddess::LocalHost, 888);
auto sock2 = new TestClass2(QHostAddess::LocalHost, 889);
auto t = new QThread();
sock->moveToThread(t);
sock2->moveToThread(t);
t->start();

now:

• sock and sock->mSocket are moved to thread t.
• sock2 is move the thead t but sock2->mSocket still lives in thread in which it have been created

[SPlatten]

I feel like I am going around and round in circles and just not progressing. I have one process which listens for data on port 8123. It also starts another process which connects to the first process on the host IP and port 8123.

The processes connect without problem, I am trying to write data from the second process to the first. I emit a signal from the socket that is used to connectToHost on the first process, I have a slot that connects to the emitted write signal:

void clsMsgSender::onWrite(QJsonObject objJSON) {    
    if ( mpsckClient == nullptr ) {
        return;
    }
    QAbstractSocket::SocketState sckState = eGetSockState();

    if ( sckState != QAbstractSocket::ConnectedState ) {
#if defined(DEBUG_SOCKETS)
        qdbg() << "Not connected!";
#endif
        return;
    }
    QMutexLocker lock(&mMutex);
#if defined(DEBUG_SOCKETS)
    QString strMsg(QJsonDocument(objJSON).toJson(QJsonDocument::Compact));
    qdbg() << QString("clsMsgSender::onWrite, to: %1:%2, data: %3")
                .arg(mpsckClient->peerAddress().toString(), QString::number(mpsckClient->peerPort()), strMsg);
#endif
    clsJSON::addCommonJSONflds(objJSON, nullptr, mpModule);
    //Insert a unique message ID into the message
    objJSON.insert(clsJSON::mscszMsgID, QString::number(clsMsgSender::ulnglngGetUniqueMsgID()));
    //Associate this TCP socket with the output data stream
    QByteArray arybytMsg;
    arybytMsg = QJsonDocument(objJSON).toJson(QJsonDocument::Compact);
    //Write message
    qint64 int64Written = mpsckClient->write(arybytMsg);
#if defined(DEBUG_SOCKETS)
    qdbg() << QString("clsMsgSender::onWrite, written:%1")
                .arg(int64Written);
#endif
    if ( int64Written > 0 ) {
    //Remove the item from the queue
        mqueMsgsOut.dequeue();
    }
    //No longer busy
    mblnBusy = false;
}

When I emit the write signal I take a JSON packet from a queue, the item is only removed from the queue when the write completes. I can see using the Qt Creator debugger that in the onWrite slot the socket write is successful and returns the number of bytes written. However I see no evidence that the first application is receiving the data. What could be the cause?

[KroMignon]

@SPlatten said in QTcpSocket client, write problem...:

However I see no evidence that the first application is receiving the data. What could be the cause?

TCP sockets are stream interfaces, and they use buffers (reception/transmission) to avoid sending too much small packets.
To force data transfer you could do:

// write to transmission buffer
qint64 int64Written = mpsckClient->write(arybytMsg);
// force transmission
mpsckClient->flush();

[SPlatten]

@KroMignon , I'm wondering if sockets is the best mechanism for the purpose I am trying to use them for.

The main process will launch X number of processes, each process will provide a specific purpose.

Messages sent from the main process will issue requests to the other processes and the other processes will send the results of these requests back to the main. I am using JSON for packaging data messages between all applications.

I'm now looking at QSharedMemory, but I don't know if I can use QSharedMemory for this kind of communication?

[JonB]

@SPlatten
You can use shared memory if you prefer. But I think you'll end up with the same sort of thread issues as you're having with TCP.

[KroMignon]

@SPlatten said in QTcpSocket client, write problem...:

I'm now looking at QSharedMemory, but I don't know if I can use QSharedMemory for this kind of communication?

If you don't want to lose all done work, I would suggest you to use QLocalServer and QLocalSocket which are kind of TCP socket but which is a more aggressive transmission rule.
Data are sent almost directly, on entering event loop.
And you don't have to handle shared lock for shared memory access.

[SPlatten]

@KroMignon , thank you, I will look into QLocalServer and QLocalSocket, it sounds like a better fit as my intention is that all processes will run on the same system.

[SGaist]

Hi,

As already written in one of your other threads: QTcpSocket shall be created in the thread they are going to be used in. They cannot be moved around. You can pass around the descriptor but not the QTcpSocket itself. See the threaded QTcpServer/QTcpClient examples.