Size and pack

[byhoe@greenlime.com (Adrian Hoe)]

Hello,

I have the following declaration:

   type Rx_Header_Data is
      record
         Start_Byte   : Character := Latin_1.STX;
         Splitter     : Character;
         Command_Byte : Character;
         Pad_Byte_1   : Character;
         Pad_Byte_2   : Character;
         Log_Num      : Interfaces.C.Long;
         End_Byte     : Character := Latin_1.ETX;
         LRC          : Character;
      end record;

It supposed to be 11 bytes long but Rx_Header_Data'Size reports 14
bytes. 'Size will report 11 bytes when I add the following line:

pragma pack (Rx_Header_Data);

I have found that Log_Num : Interfaces.C.Long takes up 7 bytes without
pragma pack.

Why this happens? Can I use representation clause instead of pragma
pack? If both methods work, which is the best approach and why?

I use GNAT 3.13p on Linux.

Thank you.