try a method/class like (untested)
Qt Code: 
  1. class InputProcessor : public QObject
  2. {
  3. Q_OBJECT
  4.  
  5. QIODevice *input_;
  6. qint64 blocksize_;
  7. QDataStream datastream_;
  8.  
  9. public:
  10.    InputProcessor(QIODevice *input, QObject *parent=0)
  11.    : QObject(parent)
  12.    , blocksize_(0)
  13.    , input_(input)
  14.    , datastream(input)
  15.    {
  16.       connect(input_, SIGNAL(readyRead()), SLOT(slotProcessInput()));
  17.    }
  18.  
  19. public slots:
  20. // connect tcp socket readyRead() here
  21. void slotProcessInput()
  22. {
  23.     // if we do not have a block size, wait for enough bytes
  24.     if (blocksize_==0)
  25.     { // wait for blocksize
  26.        if (input_->bytesAvailable() < sizeof(qint64))
  27.          return;
  28.        datastream_ >> blocksize_;
  29.     }
  30.  
  31.     // now we know the size of the transmitted block: wait for the data
  32.     if (input_->bytesAvailable() < blocksize_)
  33.       return; // we will be called again
  34.  
  35.    // all data here, read block:
  36.    // datastream_ >> ...;
  37.  
  38.    blocksize_ = 0; // wait for next block (size)
  39. }
  40.  
  41. };
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(I have not tried to compile that. Please post if it is buggy.)

HTH