message: Add the expected header field lengths in comments
diff: @@ -120,12 +120,12 @@ ULong64_t CharArr2uint64(const char* str);
  struct BaseDataTopic
  {
  static const UInt_t fgkMagicNumber;
  - UInt_t fMagicNumber;
  - UInt_t fHeaderSize;
  - UInt_t fFlags;
  - UInt_t fBaseHeaderVersion;
  - ULong64_t fHeaderDescription;
  - ULong64_t fHeaderSerialization;
  + UInt_t fMagicNumber; // 4 bytes
  + UInt_t fHeaderSize; // 4 bytes
  + UInt_t fFlags; // 4 bytes
  + UInt_t fBaseHeaderVersion; // 4 bytes
  + ULong64_t fHeaderDescription; // 8 bytes
  + ULong64_t fHeaderSerialization; // 8 bytes
  BaseDataTopic();
  BaseDataTopic(UInt_t size, ULong64_t desc, ULong64_t seri);
  static BaseDataTopic* Get(void* buf) {
  @@ -140,12 +140,12 @@ struct DataTopic : public BaseDataTopic
  {
  static const ULong64_t fgkDataTopicDescription;
  static const UInt_t fgkTopicSerialization;
  - ULong64_t fDataDescription[2];
  - UInt_t fDataOrigin;
  - UInt_t fReserved;
  - ULong64_t fDataSerialization;
  - ULong64_t fSpecification; /// data specification of the data block
  - ULong64_t fPayloadSize;
  + ULong64_t fDataDescription[2]; // 2*8 bytes
  + UInt_t fDataOrigin; // 4 bytes
  + UInt_t fReserved; // 4 bytes
  + ULong64_t fDataSerialization; // 8 bytes
  + ULong64_t fSpecification; // 8 bytes data specification of the data block
  + ULong64_t fPayloadSize; // 8 bytes
  //ctor
  DataTopic()
  