Re: records containing variable length arrays [long]

[Mats Karlssohn <mats@mida.se>]

Jeffrey Carter wrote:
> 
> Mats Karlssohn wrote:
%<
> The basic concept for what you're trying to do is a discriminated record
> type:
> 
> type Bounded_String (Max_Length : Positive) is record
>    Current_Length : Natural := 0;
>    Value          : String (1 .. Max_Length) := (others => '*');
> end record;

Yes, that's right.

> >    -- the other big problem is to get the correct representation
> >    for Message use
> >       record
%<
> >           Status          at 4 range 0..15;
> >           Response_Length at 4 range 0.. 7;

This is a typo... "Response_Length at 5 range 0..7;" is the correct
representation statement, this also means that all the following
components move up one byte.

%<
> However, you have additional problems, such as overlaying your lengths,
> which would be your discriminants, with Status. I presume that you
> receive your message as a simple Buffer. In that case you might be able
> to work something along these lines

Yes, I receive a C pointer (converted to System.Address) and an integer
Length from a buffer manager for the shared memory where thit beast
lives. I forgot to mention that I really only neet to read this data
structure, I will NOT need to assign to it.

> type Message (Response_Length : Byte; Output_Length : Byte) is record
>    Magic         : Word;
>    Operation     : Word;
>    Response_Data : Buffer (1 .. Response_Length);
>    Output_Data   : Buffer (1 .. Output_Length);
>    CRC           : Word;
> end record;
> 
> I have specified the Buffers starting at one because Ada does not allow
> an expression containing a discriminant.

That's OK but not ideal... I can handle that in the access functions.
The spec. says that it is indexed from 0 but I'll just have to take care
of it.

The problem comes when Output_Length becomes 0, a value which it can
legally have. In that case the Output_Data buffer is 0 bytes long and
hence disapears from the message. This is kind of an almost error, not
the normal use, but still, allowed.
Any suggestions on that one ?


> for Message use record
>    Magic           at 0 range 0..15;
>    Operation       at 2 range 0..15;
>    Response_Length at 4 range 0.. 7;
>    Output_Length   at 5 range 0.. 7;
> end record;
> pragma Pack (Message);

Hmmm... yes, I think I see. I think that I havn't done my homework, I
really didn't think of trying to include the discriminants in the
representation.

> We will have to ensure that the compiler will lay out the Buffers and
> CRC properly, but that seems the most likely result.

Ahh... this seems doable, I'll look into it some more this evening.

%<
> Your only issue now is to reconstruct Status from the 2 discriminants.
> This is reasonably simple and left as an exercise for the reader.

And not an issue since I mistyped....


TNX 1e6

-- 
Mats Karlssohn, developer                         mailto:mats@mida.se  
Mida Systemutveckling AB                          http://www.mida.se
Box 64, S-732 22 ARBOGA, SWEDEN
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