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From: Jeffrey Carter <jeffrey.carter@boeing.com>
Subject: Re: records containing variable length arrays [long]
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Date: 2001-06-08T22:28:34+00:00
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Jeffrey Carter wrote:
> 
> Mats Karlssohn wrote:
> >
> > I ended up declaring the record in the way that was discussed earlier.
> > Then I declare an access type to the record and use an instance of
> > System.Address_To_Access_Conversions to convert the C pointer to an
> > Ada pointer. I'll probably wrap another layer around the buffer
> > manager and let that glue perform the address to access conversion.
> 
> You can probably avoid the conversion of the pointer. Given
> 
> type Message (...) is record ...;
> for Message use record ...;
> 
> and something like
> 
> void get (buffer* p, int* len);

This is probably wrong, but my C is bad, so I tend to get things wrong.
What is needed here is the equivalent of Ada's "P : out Ptr_To_Byte",
which on second look is probably

byte** p

> 
> you can define
> 
> type Message_Ptr is access all Message;
> pragma Convention (C, Message_Ptr);
> 
> procedure Get (Ptr : out Message_Ptr; Length : out Interfaces.C.Int);
> pragma Import (C, Get, "get");
> 
> and use the access value you get from Get directly.
> 
> > Given this declaration and representation:
> >
> >    type Message(Response_Length : Byte; Output_Length : Byte) is
> >       record
> >          Magic           : Word;
> >          Operation       : Word;
> >          Status          : Word;
> >          Response_Data   : Buffer(1..Response_Length);
> >          Output_Data     : Buffer(1..Output_Length);
> >          CRC             : Word;
> >       end record;
> >
> >    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 6 range 0.. 7;
> >           Output_Length   at 7 range 0.. 7;
> >       end record;
> >
> >    pragma Pack(Message);
> >
> > Is there a way to explicitly put at least the CRC element into the
> > representation clause ? My experiments haven't gotten me anywhere
> > on this part.
> 
> Only if it comes before the variable-length part of the record. The
> values used in a record representation clause have to be static, which
> means known at compile time.
> 
> --
> Jeffrey Carter


-- 
Jeffrey Carter