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From: billwolf%hazel.cs.clemson.edu@hubcap.clemson.edu (William Thomas Wolfe, 2847 )
Newsgroups: comp.lang.ada
Subject: Re: Re: "limitations" of conditional compilation
Message-ID: <7448@hubcap.clemson.edu>
Date: 14 Dec 89 20:56:29 GMT
References: <920015@hpcladb.HP.COM>
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Reply-To: billwolf%hazel.cs.clemson.edu@hubcap.clemson.edu
List-Id: <comp.lang.ada>

>From article <920015@hpcladb.HP.COM>, by defaria@hpcladb.HP.COM (Andy DeFaria):
> I'm developing such a binding as we speak.  One problem that I have, and
> haven't been able to workaround, is that of different representation clauses.
> This OS binding is designed to be portable to multiple architectures.  The
> data alignment of the different machines are different so I need to:
> 
%          Machine 1                                Machine 2
% ----------------------------------   ----------------------------------------
% for RECORD_1 use                     for RECORD_1 use
%    record                               record
%       FIELD_1 at 0 range 0 .. 15;          FIELD_1 at 0 range 0 .. 15;
%       FIELD_2 at 2 range 0 .. 15;          FIELD_2 at 4 range 0 .. 15;
%       ...                                  ...
%    end record;                          end record;
> 
> As you can see Machine 2 need to be aligned at the 4 byte boundary.  Since the
> interface code I am going to is C (Unix system calls) I'm stuck the these
> record layouts and alignments.  I have thought using some sort of constant to
> determine where the next alignment would be but it would make the coding of
> the for clause exceeding difficult.  

   Hmmm... how about:

      ALIGNMENT_LENGTH : constant := 2;   -- or 4, or whatever...

      for RECORD_1 use
         record
            FIELD_1 at 0 * ALIGNMENT_LENGTH range 0..15;
            FIELD_2 at 1 * ALIGNMENT_LENGTH range 0..15;
            ...
         end record;

   The general form of the "at" expression seems to qualify as a
   static_simple_expression (by 4.9.7 & 4.4.2, plus a similar 
   example is given in 13.4.9), and the coding of the for clause 
   would seem to be relatively straightforward.


   Bill Wolfe, wtwolfe@hubcap.clemson.edu