cannot read correct len from socket

[saman_artorious]

I am sending a QByteArray from server to client. the client needs to extract the first four bytes for length of packet and then read the rest of the packet. The unexpected value I get from length of packet though, is a very large value;
what I do is:

int len;
char *data;
 
client.read((char*)len, 4); // read the first integer for the length
client.read(data, len);

int len;
char *data;

client.read((char*)len, 4); // read the first integer for the length
client.read(data, len);

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can anyone guess what the problem is?

[Jonny174]

Try client.read((char*)&len, 4);

---

Added after 4 minutes:

global int blockSize = 0;
...
 
while (clientConnection->state() != 0)
{
  clientConnection->waitForReadyRead(1);
 
  if (clientConnection->bytesAvailable() > 0)
  {
    if (blockSize == 0)
    {
      if (clientConnection->bytesAvailable() < sizeof( int ))
        continue;
 
      clientConnection->read( (char *)&blockSize, sizeof( int ) );
    }
 
    clientConnection->waitForReadyRead(1);
 
    if (clientConnection->bytesAvailable() < blockSize)
      continue;
 
    char *msg = new char[ blockSize ];
    memset( msg, 0, blockSize );
 
    clientConnection->read( &msg[0], blockSize );
    ...
    blockSize = 0;
  }
}

global int blockSize = 0;
...

while (clientConnection->state() != 0)
{
  clientConnection->waitForReadyRead(1);

  if (clientConnection->bytesAvailable() > 0)
  {
    if (blockSize == 0)
    {
      if (clientConnection->bytesAvailable() < sizeof( int ))
        continue;

      clientConnection->read( (char *)&blockSize, sizeof( int ) );
    }

    clientConnection->waitForReadyRead(1);

    if (clientConnection->bytesAvailable() < blockSize)
      continue;

    char *msg = new char[ blockSize ];
    memset( msg, 0, blockSize );
    
    clientConnection->read( &msg[0], blockSize );
    ...
    blockSize = 0;
  }
}

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[saman_artorious]

it doesn't help. it gives terrible large numbers.

is the byte array is "47dannie", reading 4 bytes would I think, read 47da, next time it would read nnie, converting them into integers.

int len;
char *data;
client.read((char*)&len, 4); 
client.read(&data[0], len);

int len;
char *data;
client.read((char*)&len, 4); 
client.read(&data[0], len);

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this is not correct.

[Jonny174]

void MyServer::slotReadClient()
{
  QTcpSocket* pClientSocket = (QTcpSocket *)sender();
  QDataStream in( pClientSocket );
 
  in.setVersion( QDataStream::Qt_4_7 );
 
  for (;;)
  {
    if (!m_nNextBlockSize)
    {
      if (pClientSocket->bytesAvailable() < sizeof( quint16 ))
        break;
 
      in >> m_nNextBlockSize;
    }
 
    if (pClientSocket->bytesAvailable() < m_nNextBlockSize)
      break;
 
    QTime time;
    QString str;
 
    in >> time >> str;
 
    QString strMessage = time.toString() + " " + "Client has sended - " + str;
    m_ptxt->append( strMessage );
 
    m_nNextBlockSize = 0;
 
    sendToClient( pClientSocket, "Server Response: Received \"" + str + "\"" );
  }
}
 
// ----------------------------------------------------------------------
void MyServer::sendToClient( QTcpSocket *pSocket, const QString &str )
{
  QByteArray arrBlock;
  QDataStream out( &arrBlock, QIODevice::WriteOnly );
 
  out.setVersion( QDataStream::Qt_4_7 );
  out << quint16(0) << QTime::currentTime() << str;
 
  out.device()->seek(0);
  out << quint16( arrBlock.size() - sizeof( quint16 ) );
 
  pSocket->write( arrBlock );
}

void MyServer::slotReadClient()
{
  QTcpSocket* pClientSocket = (QTcpSocket *)sender();
  QDataStream in( pClientSocket );

  in.setVersion( QDataStream::Qt_4_7 );

  for (;;)
  {
    if (!m_nNextBlockSize)
    {
      if (pClientSocket->bytesAvailable() < sizeof( quint16 ))
        break;

      in >> m_nNextBlockSize;
    }

    if (pClientSocket->bytesAvailable() < m_nNextBlockSize)
      break;

    QTime time;
    QString str;

    in >> time >> str;

    QString strMessage = time.toString() + " " + "Client has sended - " + str;
    m_ptxt->append( strMessage );

    m_nNextBlockSize = 0;

    sendToClient( pClientSocket, "Server Response: Received \"" + str + "\"" );
  }
}

// ----------------------------------------------------------------------
void MyServer::sendToClient( QTcpSocket *pSocket, const QString &str )
{
  QByteArray arrBlock;
  QDataStream out( &arrBlock, QIODevice::WriteOnly );

  out.setVersion( QDataStream::Qt_4_7 );
  out << quint16(0) << QTime::currentTime() << str;

  out.device()->seek(0);
  out << quint16( arrBlock.size() - sizeof( quint16 ) );

  pSocket->write( arrBlock );
}

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[saman_artorious]

I am not asking for the way we deal with code in qtcpsocket.
i am asking why the above code which is quite logical in C, does not work out! and how to correct it.

[nix]

What is the value of the big value you get?

[Jonny174]

X3. Maybe not all the data had to come. Shaw all read code.

[saman_artorious]

What is the value of the big value you get?

output:
newConnection
1632843572
879649133
1835094839
926183009
1634558291
1396126830
1851878753
1632843572
879649133
1835094839
926183009

the strange part is when you readLine() and use qbytearray, it contains both the digit and the string "47Saman". Though, in this case, nope

[nix]

Ok, according to the example you gave, your length seems to have a 2 octet size, if the size is fixed try this

QByteArray lenBA(2,'0');
QByteArray data;
 
client.read(lenBA.data(), 2); // read the first integer for the length
short int length  = lenBA.toShort(); // check the conversion is ok here
data.reserve(length);
 
client.read(data.data(), length  );

QByteArray lenBA(2,'0');
QByteArray data;

client.read(lenBA.data(), 2); // read the first integer for the length
short int length  = lenBA.toShort(); // check the conversion is ok here
data.reserve(length);

client.read(data.data(), length  );

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1632843572 = 0x61533734 and give in ascii : "aS74" look realy close to "47Sa" if you considering byte order issue from brutal conversion from char * to int.

[Lesiok]

the strange part is when you readLine() and use qbytearray, it contains both the digit and the string "47Saman". Though, in this case, nope

As You see the length of data is transported as characters not as binary int. So You can't read read it as binary data. You must read it as string and convert to int. Another problem : what if data length is bigger then 99 - it is 3 digits.

[saman_artorious]

what if data length is bigger then 99 - it is 3 digits.

Yes, the length of data is random and may vary. I cannot find a way to deal with this.

[Lesiok]

Yes, the length of data is random and may vary. I cannot find a way to deal with this.

This is very simple. Always send fixed number of digits with leading zeros. In example if You want to send 4 digits You should send "0047Saman".
In fact, the problem is the definition of the protocol.

[wysota]

I am sending a QByteArray from server to client. the client needs to extract the first four bytes for length of packet and then read the rest of the packet. The unexpected value I get from length of packet though, is a very large value;
what I do is:

int len;
char *data;
 
client.read((char*)len, 4); // read the first integer for the length
client.read(data, len);

int len;
char *data;

client.read((char*)len, 4); // read the first integer for the length
client.read(data, len);

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can anyone guess what the problem is?

How were the first four bytes written to the socket by the server?

[saman_artorious]

How were the first four bytes written to the socket by the server?

By the Client, it was written as such:

void    tcpClient::sendCommand(QString com)
{
    QByteArray data;
 
    data.append(com);      // "47Saman"
 
    if(clientSocket->write (data) == -1)
    {
        qDebug("sendCommand(QString) Failed");
    }
}

void    tcpClient::sendCommand(QString com)
{
    QByteArray data;

    data.append(com);      // "47Saman"

    if(clientSocket->write (data) == -1)
    {
        qDebug("sendCommand(QString) Failed");
    }
}

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[wysota]

If you are writing "47Saman" to the socket, what do you expect to receive as first four bytes on the other end of the communication?

[saman_artorious]

If you are writing "47Saman" to the socket, what do you expect to receive as first four bytes on the other end of the communication?

Wysota, client is sending me a jpeg file. in the server program, I used readAll() to read it all and pixmap it to a label. But, the image does not appear in the label (though the widget background flashes). To solve this, I was told that they would send the size of the jpeg file along with its content. Hence, I need to read the length first, and then realize how many bytes exactly to read.

void MainWindow::readPixmap(QByteArray data)
{
    QPixmap pm;
 
    pm.loadFromData(data);
 
    ui->label->setPixmap(pm);
}

void MainWindow::readPixmap(QByteArray data)
{
    QPixmap pm;

    pm.loadFromData(data);

    ui->label->setPixmap(pm);
}

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[wysota]

Please answer my question.

[saman_artorious]

Please answer my question.

A string of chars.

[wysota]

Then why are you trying to read them as an integer?

[saman_artorious]

Then why are you trying to read them as an integer?

because I have no idea what the length of the integer might be as a character, 2 digits or 5 digits!
that's why I try to read 4 bytes = sizeof(char)