QTcpSocket: reading and writing

Question

I know some similar questions may have been asked already, but the answers to those I found covered very specific problems and I still haven't figured it out.

In my program I'm creating a QObject (called QPeer) that uses a QTcpSocket to communicate with another such object over a network. QPeer has a slot that accepts a QByteArray with data (sendData(QByteArray)). The entire contents of that array are considered to be one 'message' and they are written to the socket. I want to do the following: every time a message is written, I want the receiving QPeer to emit its signal dataReceived(QByteArray) exactly once, that QByteArray containing the entire message. (NOTE: all signals/slots, both private ones connecting the QPeer with its socket and the public ones such as sendData(QByteArray) are serialized by using Qt::QueuedConnection whenever necessary.)

I use the signal QTcpSocket::readyRead() for asynchronous reading from the socket. Now I know I can't just call QTcpSocket::write() once in sendData and then assume that for every write I do, the QTcpSocket on the other side produces exactly one readyRead signal. So what should I do?

This is my idea, please tell me if this will work:

WRITING:

void QPeer::sendData(QByteArray data)
{
    // TODO: write data.size() as raw int of exactly 4 bytes to socket
    const char *bytes = data.constData();
    int bytesWritten = 0;
    while (bytesWritten < data.size())
        bytesWritten += _socket->write(bytes + bytesWritten);
}

READING:

now I want the read function (connected to QTcpSocket::readyRead()) to use the header (the 4 byte int specifying the length of the message) and then read that amount of bytes; next emit dataReceived with exactly those bytes. I'm having serious trouble trying to do this. For example: what to do if readyRead is emitted and I can read the header of a message, but not the amount of bytes specified? Or what if a header has only been received partially?

1. How do I correctly write the header (4 byte int) to the socket?

2. How do I correctly implement the read function so that it does what I want?

Any tips are welcome. Thanks!

Answer 1

I worked on a project that does what you expect, see here the solution that I developed to our problems, simplified to be easier to understand:

Edited, added support to the server deal with multiple clients.

Client.h:

#include <QtCore>
#include <QtNetwork>

class Client : public QObject
{
    Q_OBJECT
public:
    explicit Client(QObject *parent = 0);

public slots:
    bool connectToHost(QString host);
    bool writeData(QByteArray data);

private:
    QTcpSocket *socket;
};

Client.cpp:

#include "client.h"

static inline QByteArray IntToArray(qint32 source);

Client::Client(QObject *parent) : QObject(parent)
{
    socket = new QTcpSocket(this);
}

bool Client::connectToHost(QString host)
{
    socket->connectToHost(host, 1024);
    return socket->waitForConnected();
}

bool Client::writeData(QByteArray data)
{
    if(socket->state() == QAbstractSocket::ConnectedState)
    {
        socket->write(IntToArray(data.size())); //write size of data
        socket->write(data); //write the data itself
        return socket->waitForBytesWritten();
    }
    else
        return false;
}

QByteArray IntToArray(qint32 source) //Use qint32 to ensure that the number have 4 bytes
{
    //Avoid use of cast, this is the Qt way to serialize objects
    QByteArray temp;
    QDataStream data(&temp, QIODevice::ReadWrite);
    data << source;
    return temp;
}

Server.h:

#include <QtCore>
#include <QtNetwork>

class Server : public QObject
{
    Q_OBJECT
public:
    explicit Server(QObject *parent = 0);

signals:
    void dataReceived(QByteArray);

private slots:
    void newConnection();
    void disconnected();
    void readyRead();

private:
    QTcpServer *server;
    QHash<QTcpSocket*, QByteArray*> buffers; //We need a buffer to store data until block has completely received
    QHash<QTcpSocket*, qint32*> sizes; //We need to store the size to verify if a block has received completely
};

Server.cpp:

#include "server.h"

static inline qint32 ArrayToInt(QByteArray source);

Server::Server(QObject *parent) : QObject(parent)
{
    server = new QTcpServer(this);
    connect(server, SIGNAL(newConnection()), SLOT(newConnection()));
    qDebug() << "Listening:" << server->listen(QHostAddress::Any, 1024);
}

void Server::newConnection()
{
    while (server->hasPendingConnections())
    {
        QTcpSocket *socket = server->nextPendingConnection();
        connect(socket, SIGNAL(readyRead()), SLOT(readyRead()));
        connect(socket, SIGNAL(disconnected()), SLOT(disconnected()));
        QByteArray *buffer = new QByteArray();
        qint32 *s = new qint32(0);
        buffers.insert(socket, buffer);
        sizes.insert(socket, s);
    }
}

void Server::disconnected()
{
    QTcpSocket *socket = static_cast<QTcpSocket*>(sender());
    QByteArray *buffer = buffers.value(socket);
    qint32 *s = sizes.value(socket);
    socket->deleteLater();
    delete buffer;
    delete s;
}

void Server::readyRead()
{
    QTcpSocket *socket = static_cast<QTcpSocket*>(sender());
    QByteArray *buffer = buffers.value(socket);
    qint32 *s = sizes.value(socket);
    qint32 size = *s;
    while (socket->bytesAvailable() > 0)
    {
        buffer->append(socket->readAll());
        while ((size == 0 && buffer->size() >= 4) || (size > 0 && buffer->size() >= size)) //While can process data, process it
        {
            if (size == 0 && buffer->size() >= 4) //if size of data has received completely, then store it on our global variable
            {
                size = ArrayToInt(buffer->mid(0, 4));
                *s = size;
                buffer->remove(0, 4);
            }
            if (size > 0 && buffer->size() >= size) // If data has received completely, then emit our SIGNAL with the data
            {
                QByteArray data = buffer->mid(0, size);
                buffer->remove(0, size);
                size = 0;
                *s = size;
                emit dataReceived(data);
            }
        }
    }
}

qint32 ArrayToInt(QByteArray source)
{
    qint32 temp;
    QDataStream data(&source, QIODevice::ReadWrite);
    data >> temp;
    return temp;
}

Note: Do not use this method to transfer large files because with this method the entire contents of the message is put inside the memory before sent and this causes a high memory usage. And because 32 bits signed INT has max value to 2,147,483,647, if your input data has a value higher than that in bytes it won't work. Take care.

Answer 2

As you said, you need to wait that your header is entirely sent, before reading it, and then read the good number of bytes and emit a signal for data availability.

Here an example (untested) :

//header file

class Peer {
//[...]
protected:
   bool m_headerRead; //initialize to false
   unsigned int m_size_of_data_to_read;
//[...]
};

//source file
void QPeer::sendData(QByteArray data)
{
  int size = data.size();
  _socket->write((const char*) &size, sizeof(int);
  //use directly QIODevice::write(QByteArray)
  _socket->write(data);
}

void QPeer::readData()
{
    int bytes = _socket->bytesAvailable();
    bool contains_enough_data = true;

    while (contains_enough_data) {
       if (! m_headerRead && _socket->bytesAvailable() >= sizeof(int)) {
         //read header only and update m_size_of_data_to_read
         m_headerRead = true;
        } else if (m_headerRead && _socket->bytesAvailable >= m_size_of_data_to_read) {
          //read data here
          m_headerRead = false;
          emit dataAvailable();
       } else {
           contains_enough_data = false; //wait that further data arrived
       }
    }
}