No, it's still wrong, you are discarding previous content of the buffer.
This is correct (by the way, there are numerous threads in this forum where I explain this particular situation, how come you didn't encounter any of the threads while using our search?):
Qt Code: 
  1. QByteArray buffer; // global or member variable
  2. void Cls::onSocketReadyRead() {
  3.   buffer.append(socket->readAll());
  4.   tryProcessData();
  5. }
  6.  
  7. void Cls::tryProcessData() {
  8.   forever {
  9.     // for the sake of the example I'm assuming my record is always 24 bytes long
  10.     if(buffer.size()<24) return;
  11.     QByteArray record = buffer.left(24);
  12.     processRecord(record);
  13.   } 
  14. }
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The above code has one small flaw introduced for purpose so that you need to understand completely what the code does before actually using it.