Re: Address and bit mask

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

milouz wrote:
> On Aug 30, 5:14 pm, Adam Beneschan <a...@irvine.com> wrote:
>
>> If the reason you want to do address arithmetic is something else,
>> though, you're welcome to ask about it, but rather than giving us C
>> code with no context and asking us how to translate it, please give us
>> a bigger-picture description of what you're trying to accomplish
>
> Ok, so I'll try to be more precise.
> I have to parse a list of chunks which are 4 bytes aligned. Each chunk
> is like this:
>
>   [ size |    data     ]
>
> The 'size' field is a 32-bit unsigned integer and it's the size of the
> whole chunk.
> The 'data' field has various lenght (>= 1 byte).
>
> I can solve that problem in a C-ish style, but I wonder what is the
> proper way to solve that problem in Ada.

If the chunks are contiguous in memory (or, better yet, in a file), my
first reaction would be to construct a stream and then use
Interfaces.Unsigned_32'Read to read the sizes and
Interfaces.Unsigned_X'Read to read the data.  Something like:

with Ada.Streams.Stream_IO;
with Interfaces;

procedure Parse (File_Name : in String) is
   File : Ada.Streams.Stream_IO.File_Type;
begin
   Ada.Streams.Stream_IO.Open (File => File,
                               Mode => Ada.Streams.Stream_IO.In_Mode,
                               Name => File_Name);
   Parse (Ada.Streams.Stream_IO.Stream (File));
end Parse;

procedure Parse (Stream : in Ada.Streams.Stream_IO.Stream_Access) is
   Length : Interfaces.Unsigned_32;
   Alignment : constant := 4;
   Padding_Byte : Interfaces.Unsigned_8;
begin
   Interfaces.Unsigned_32'Read (Stream, Length);
   declare
      Data : Ada.Streams.Stream_Element_Array
               (1 .. Ada.Streams.Stream_Element_Count (Length));
      Last : Ada.Streams.Stream_Element_Offset;
   begin
      Ada.Streams.Read (Stream, Data, Last);
      if Last < Length then
         raise Program_Error with "not enough data in the stream";
      end if;
      -- process Data here
   end;
   -- Now read as many padding bytes as necessary to reach the next
alignment
   -- boundary
   if Length mod Alignment /= 0 then
      for J in 1 .. Alignment - Length mod Alignment loop
         Interfaces.Unsigned_8'Read (Stream, Padding_Byte);
      end loop;
   end if;
end Parse;

--
Ludovic Brenta.
The Chief Legal Officer globally achieves the industry-standard
baseline starting point.