[Casimiro Noteví]

"Números empaquetados", ¿te refieres a big endian y little endian?

unsigned long blockLength  = *blockLengthPtr++ << 24;
blockLength |= *blockLengthPtr++ << 16;
blockLength |= *blockLengthPtr++ << 8;
blockLength |= *blockLengthPtr;

unsigned long dataLength  = *dataLengthPtr++ << 24;
dataLength |= *dataLengthPtr++ << 16;
dataLength |= *dataLengthPtr++ << 8;
dataLength |= *dataLengthPtr;