Controlling endian-ness?

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

"Peter C. Chapin" <pcc482719@gmail.com> wrote in message
news:4897b7f5$0$19705$4d3efbfe@news.sover.net...
> 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

I don't know if this will be useful to you, but here is the technique I used
for reversing the byte order on a 32 bit integer. Actually I was going for
"network byte order" of a float (s_float is a 32 bit float, u_long is a
32bit unsigned integer).

TYPE aByte IS MOD 256;
FOR aByte'SIZE USE 8;
TYPE aByteArray IS ARRAY( Positive RANGE <> ) OF aByte;

FUNCTION nltohf( value : u_long ) RETURN s_float IS
TYPE aFourBytes IS NEW aByteArray(1..4);
FUNCTION Conv IS
NEW Ada.Unchecked_Conversion( aFourBytes, s_float );
FUNCTION Conv IS
NEW Ada.Unchecked_Conversion( u_long, aFourBytes );
temp : aFourBytes := Conv( value );
BEGIN
RETURN Conv( aFourBytes'( temp(4), temp(3), temp(2), temp(1) ) );
END nltohf;

It isn't endian neutral (or particularly pretty), but it got the job done.

Regards,
Steve

I didn't spot it myself first, but it's easy:

> * * 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!
|should be Item here.

christoph.grein@eurocopter.com wrote:

> I didn't spot it myself first, but it's easy:
>
>> 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!
> |should be Item here.

Of course! Thanks... and sorry about consuming bandwidth for something
so silly. Now I know why I usually suffix type names with "_Type". If I
had called it "Word_Type" I probably would not have made the error in
the first place. I guess that's what I get for trying to write something
quick and dirty!

Peter

Steve wrote:

> It isn't endian neutral (or particularly pretty), but it got the job done.

Hmm. Unchecked_Conversion, huh? That's an approach I didn't consider.

Peter

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

anon wrote:

> 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 ) ;

I actually tried something like this but the compiler told me that I
couldn't redefine attributes for that type (or something like that). I
think the problem is that since the type is defined in another package
it is already frozen or some such. It could also be that I did it wrong.

Peter

On Aug 5, 3:14 pm, "Peter C. Chapin" <pcc482...@gmail.com> wrote:
> anon wrote:
> > 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 ) ;
>
> I actually tried something like this but the compiler told me that I
> couldn't redefine attributes for that type (or something like that). I
> think the problem is that since the type is defined in another package
> it is already frozen or some such. It could also be that I did it wrong.

Yes. You can't apply any "for T'attribute use..." clause to something
defined in another package. (It sounds to me like this is what you
tried... if I guessed wrong, sorry.) You might consider defining your
own type (perhaps *derived* from Unsigned_32) and using your new type
when declaring a variable or component that needs to be written using
a special routine.

type My_Unsigned_32 is new Unsigned_32;
procedure My_Write
(Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in My_Unsigned_32);
for My_Unsigned_32'Write use My_Write;

-- Adam
-- Adam

Adam Beneschan wrote:

> type My_Unsigned_32 is new Unsigned_32;
> procedure My_Write
> (Stream : not null access Ada.Streams.Root_Stream_Type'Class;
> Item : in My_Unsigned_32);
> for My_Unsigned_32'Write use My_Write;

You're right that works. I actually thought I tried using a derived type
before, but evidently I didn't do it right. Anyway thanks for the
suggestion.

Peter

-- File: newprog.adb
--
-- An example of how to Override an built-in Attribute:
-- Based on examples from RM 13.3 ( 84 ) and 13.13.2 ( 40 ) ;
--
with Interfaces ;
use Interfaces ;

with Ada.Text_IO ;
use Ada.Text_IO ;

with Ada.Streams.Stream_IO;

procedure NewProg is

-----------------------------------------------------------

type Unsigned_32 is new Interfaces.Unsigned_32 ;

--
-- Define New Attribute Specifications
--
procedure newWrite
( Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in Unsigned_32 ) ;
-- Performs Overriding of attribute
for Unsigned_32'Write use newWrite ;

-----------------------------------------------------------

--
-- Define New Attribute Body
--
procedure newWrite
( Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in Unsigned_32 ) is
begin
--
-- Replace with your "Write code"
--
-- test message only, just to prove that we are using this Attribute
--
Put_Line ( "New Write Attribute" ) ;
end newWrite ;

-----------------------------------------------------------

Data_File : Ada.Streams.Stream_IO.File_Type;
Data_Stream : Ada.Streams.Stream_IO.Stream_Access;
The_Filename : constant String := "Test.Dat" ;

Test_Data : Unsigned_32 := 9 ;

begin

Ada.Streams.Stream_IO.Create ( Data_File,
Ada.Streams.Stream_IO.Out_File,
The_Filename ) ;

Data_Stream := Ada.Streams.Stream_IO.Stream ( Data_File ) ;


Put_Line ( "Testing: New Atrribute: Write" ) ;
New_Line ;

Unsigned_32'Write ( Data_Stream, Test_Data ) ;

Ada.Streams.Stream_IO.Close ( Data_File ) ;

end NewProg ;

In <4898d0ca$0$19680$4d3efbfe@news.sover.net>, "Peter C. Chapin" <pcc482719@gmail.com> writes:
>anon wrote:
>
>> 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 ) ;
>
>I actually tried something like this but the compiler told me that I
>couldn't redefine attributes for that type (or something like that). I
>think the problem is that since the type is defined in another package
>it is already frozen or some such. It could also be that I did it wrong.
>
>Peter