records containing variable length arrays [long]

[Mats Karlssohn <mats@mida.se>]

I'm currently wrestling with an interesting problem. I feel that this
have probably been discussed previously, but I couldn't find anything
on it neither on AdaPower nor on the cla. archives on google.

I would like to declare a record type containing two (possibly more)
arrays of variable lengths, the length of each array is another element
of the record. I also need to specify an exact representation for the
record since it is a message that I cant change the format of.

A piece of code to illustrate (please not that this is NOT compilable)

package Communication is
   -- Basic types
   type Byte is mod 2**8;
   for Byte'size use 8;

   type Word is mod 2**16;
   for Word'size use 16;


   -- Variable length buffer
   type Buffer is array(Byte range <>) of Byte;
   pragma Pack(Buffer);

   -- Declaring the variable length elements is one major headache
   -- note that at least Output_Length may legally be 0
   type Message is
      record
         Magic           : Word;
         Operation       : Word;
         Status          : Word;
         Response_Length : Byte;
         Output_Length   : Byte;
         Response_Data   : Buffer(0..(Response_Length - 1));
         Output_Data     : Buffer(0..(Output_Length   - 1));
         CRC             : Word;
      end record;

   -- the other big problem is to get the correct representation
   for Message use
      record
          Magic           at 0 range 0..15;
          Operation       at 2 range 0..15;
          Status          at 4 range 0..15;
          Response_Length at 4 range 0.. 7;
          Output_Length   at 5 range 0.. 7;
          Response_Data   at 6 range 0..(Response_Length * Byte'size - 1);
          Response_Data   at (6 + Response_Length) range 0..(Output_Length * Byte'size - 1);
          CRC             at (6 + Response_Length + Output_Length) range 0..15;
      end record;

end Communication;

The compiler (Gnat 3.13p on Linux) says:
$ gnatgcc -c -g -gnata -gnatf communication.ads
communication.ads:23:39: component "Response_Length" cannot be used before end of record declaration
communication.ads:24:39: component "Output_Length" cannot be used before end of record declaration
communication.ads:34:11: component "Response_Length" overlaps "Status" at line 33
communication.ads:35:11: component "Output_Length" overlaps "Status" at line 33
communication.ads:36:42: "Response_Length" is undefined
communication.ads:37:35: "Response_Length" is undefined
communication.ads:37:62: "Output_Length" is undefined
communication.ads:38:35: "Response_Length" is undefined
communication.ads:38:53: "Output_Length" is undefined
$

I have currently solved this by overlaying the record with a large
enough array of Byte, and then calculating what portions of that I
must use Unchecked_Conversion on to find the data I need.
This works BUT:
* It is ugly and I would like to do this as as much of a schoolexample
  that I can possibly do.
* Performance may be a bottleneck (probably not in the particular case
  above) so I would really like to avoid copying any of the elements.


Any suggestions ?

Thanks for any help!

-- 
Mats Karlssohn, developer                         mailto:mats@mida.se  
Mida Systemutveckling AB                          http://www.mida.se
Box 64, S-732 22 ARBOGA, SWEDEN
Phone: +46-(0)589-89808   Fax: +46-(0)589-89809