Re: Size and pack

["Smark" <not.provided@all.com>]

"Adrian Hoe" <byhoe@greenlime.com> wrote in message
news:9ff447f2.0110100005.2503bb00@posting.google.com...
> 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.

As you indicated in another post, neither pragma pack nor a rep clause
gives you what you want.  This is because neither of these will violate
the alignment rules for a given type.  Unless you can use Streams_Io
to read and write (and you don't have to worry about what the data looks
like otherwise), you will have to do something ugly, like:

    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;
          L0      : Character;
          L1      : Character;
          L2      : Character;
          L3      : Character;
          End_Byte     : Character := Latin_1.ETX;
          LRC          : Character;
       end record;

    subtype Str4 is String(1..4);
    function Convert is new Unchecked_Conversion
                (Source => Str4, Target => Interfaces.C.Long);

    Log_Num_Str : Str4 :=
         Rx_Header_Data.L0
         & Rx_Header_Data.L1
         & Rx_Header_Data.L2
         & Rx_Header_Data.L3;

   Log_Num : Interfaces.C.Long := Convert(Log_Num_Str);

Markku