ok, i use it this way:

server send-method
Qt Code: 
  1. void Server::send_xml_data(QString message) {
  2.     blockSize = 0;
  3.     QString out_string = message;
  4.     QByteArray block;
  5.     QDataStream out_stream(&block, QIODevice::WriteOnly);
  6.     out_stream.setVersion(QDataStream::Qt_4_7);
  7.     out_stream << (qint16)0;
  8.     out_stream << out_string;
  9.     out_stream.device()->seek(0);
  10.     out_stream << quint16(block.size() - sizeof(quint16));
  11.     socket->write(block);
  12. }
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client-readyReadSlot-method:
Qt Code: 
  1. void Client::readyReadSlot()
  2. {
  3.     qDebug() << "readyReadSlot()";
  4.     QDataStream in(socket);
  5.     in.setVersion(QDataStream::Qt_4_7);
  6.     if (blockSize == 0) {
  7.         if (socket->bytesAvailable() < (int)sizeof(quint16))
  8.             return;
  9.         in >> blockSize;
  10.     }
  11.     if (socket->bytesAvailable() < blockSize)
  12.              return;
  13.  
  14.     QString result;
  15.     in >> result;
  16.     receive(result); //process my data
  17. }
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