Ok, I used this:

Qt Code: 
  1. QCryptographicHash hash(QCryptographicHash::Md5);
  2.     QFile in("/dev/cdrom");
  3.  
  4.     if (in.open(QIODevice::ReadOnly)) {
  5.         char buf[2048];
  6.         int bytesRead;
  7.         qint64 overallBytesRead = 0;
  8.  
  9.         while ((bytesRead = in.read(buf, 2048)) > 0) {
  10.             overallBytesRead += bytesRead;
  11.             hash.addData(buf, 2048);
  12.         }
  13.  
  14.         in.close();
  15.         qDebug() << "overall bytes read:" << overallBytesRead;
  16.         qDebug() << hash.result().toHex();
  17.     }
  18.     else {
  19.         qDebug() << "Failed to open device!";
  20.     }
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After completed, overallBytesRead is "8738865152", but the file size of image is "8738846720". He reads more bytes as on dvd is! I'm confused!