Re: Controlling endian-ness?

[anon@anon.org (anon)]

For using Unsigned_32. Why not just override the built-in Write attributes 
routines for Unsigned_32 by re-writing the attributes routines. This may at 
most require a new package with a few extra attributes routines that may 
be needed. Also, this will also allow you to use "Shift_Left/Shift_Right" 
directly with Unsigned_32.  

So in your program you could use:

   Data : Unsigned_32 ;
  ...
   Unsigned_32'Write ( Stream_Pointer, Data ) ;


In GNAT you can not add new attributes, but you can override the built-in 
versions with a new set of attributes routines. 



In <4897b7f5$0$19705$4d3efbfe@news.sover.net>, "Peter C. Chapin" <pcc482719@gmail.com> writes:
>I'm trying to read/write 32 bit quantities from/to a binary file. The 
>file has a format defined by a specification that is outside my control. 
>Some of the quantities are stored in the file in big endian order. 
>However, my machine is a little endian machine. The file in question 
>also contains various values that are most naturally represented with 
>different data types. Thus I'm looking at Stream_IO as a way to deal 
>with it.
>
>As an experiment I wrote a small program that defines a 32 bit unsigned 
>integer type and then overrides the 'Write attribute to write the value 
>in big endian form. Here is what I have:
>
>with Ada.Streams;              use Ada.Streams;
>with Ada.Streams.Stream_IO;    use Ada.Streams.Stream_IO;
>with Interfaces;               use Interfaces;
>
>procedure Stream_Check is
>    pragma Assert(Stream_Element'Size = 8);
>
>    type Word is mod 2**32;
>    procedure Word_Write
>      (Stream : access Root_Stream_Type'Class; Item : in Word);
>    for Word'Size  use 32;
>    for Word'Write use Word_Write;
>
>    procedure Word_Write
>      (Stream : access Root_Stream_Type'Class; Item : in Word) is
>       Buffer    : Stream_Element_Array(0..3);
>       Workspace : Unsigned_32 := Unsigned_32(Word);  -- ERROR HERE!
>    begin
>       Buffer(0) :=
>           Stream_Element(Shift_Right((Workspace and 16#FF000000#), 24));
>       Buffer(1) :=
>           Stream_Element(Shift_Right((Workspace and 16#00FF0000#), 16));
>       Buffer(2) :=
>           Stream_Element(Shift_Right((Workspace and 16#0000FF00#),  8));
>       Buffer(3) :=
>           Stream_Element(Shift_Right((Workspace and 16#000000FF#),  0));
>       Write(Stream.all, Buffer);
>    end Word_Write;
>
>    Output_File    : File_Type;
>    Stream_Pointer : Stream_Access;
>
>    W : Word := 16#0000FFFF#;
>begin
>    Create(Output_File, Out_File, "test.bin");
>    Stream_Pointer := Stream(Output_File);
>    Word'Write(Stream_Pointer, W);
>    Close(Output_File);
>end Stream_Check;
>
>I'm using GNAT GPL 2008. It produces an error on the indicated line 
>saying, "invalid use of subtype mark in expression or call." Apparently 
>it doesn't like me trying to convert a Word to an Unsigned_32, but I 
>don't understand why (am I doing that conversion right?).
>
>I want to use Unsigned_32 so that I can use Shift_Right. I can't 
>override the 'Write attribute for Unsigned_32 directly because it's in a 
>different package (right?).
>
>Overall I have a feeling that there is probably a much better way to do 
>this.
>
>Peter