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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: Size and pack
Date: Wed, 10 Oct 2001 09:27:08 +0000 (UTC)
Organization: IKS GmbH Jena
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Date: 2001-10-10T09:27:08+00:00
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* Adrian Hoe wrote:
>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;

Five Bytes so far.

>         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.

As expected.

>Why this happens?

Log_Num is aligned to use a single 64bit RAM access instead of bursting two
32bit accesses. Pragma Pack favors size over speed.

>Can I use representation clause instead of pragma pack?

Yes.

>If both methods work, which is the best approach and why?

Depends on your needs:
  - If you only need the small size, let the compiler choose the
    representation. (-gnatR shows the compilers decision)
      for rx_header_data_packed use record       
         start_byte   at  0 range  0 ..  7;
         splitter     at  1 range  0 ..  7;
         command_byte at  2 range  0 ..  7;
         pad_byte_1   at  3 range  0 ..  7;
         log_num      at  4 range  0 .. 31;  -- Changed
         pad_byte_2   at  8 range  0 ..  7;  -- Changed
         end_byte     at  9 range  0 ..  7;
         lrc          at 10 range  0 ..  7;
      end record;
  - If you need a special layout (i.e. for interfacing),
    use record representation clauses.