Re: Size of 0..255 is not 8 bits?

[Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>]

Robert Dewar wrote:
> It is indeed a bug in a compiler if a Size clause for an object does not
> result in the object tkaing the indicated nnumber of bits. But perhaps
> you had better post the EXACT code that leads you to that conclusion.

Ok, here comes the exact code and the execution result of what
confuses me.

Environment: gnat-3.10p-i386-linux-bin
             Linux kernel 2.0.29
             libc.so.5.3.12.

---------------------------------------------------------------------
with Ada.Text_IO.Text_Streams, Ada.Text_IO, Ada.Integer_Text_IO;
use  Ada.Text_IO.Text_Streams, Ada.Text_IO, Ada.Integer_Text_IO;

procedure Strange2 is

   type Value is range 0..255;
   for Value'Size use 8;

   V: Value;
   for V'Size use 8;

begin
   for X in 1..5 loop
      Value'Read(Stream(Standard_Input), V);
      Put(Integer(V));
      New_Line;
   end loop;
end Strange2;
---------------------------------------------------------------------
$ echo "ABCDEFGHIJK" | ./strange2
         65
         67
         69
         71
         73
---------------------------------------------------------------------
PROBLEM: Although I have been very explicit about that I want V
to be 8 bits large, and although GNAT generated no errors or
warnings when compiling the above, Value'Read clearly eats 16-bit
chunks per call, but stores only the first 8 bits, which I consider
highly surprising.
I would have expected to see either an output like
         65
         66
         67
         68
         69
or to get a compiler error message telling me that the "use 8"
representation clause for "range 0..255" cannot be used. 
---------------------------------------------------------------------


And here is a related problem if I use streams on range 0..255
Arrays:

---------------------------------------------------------------------
with Ada.Text_IO.Text_Streams, Ada.Text_IO, Ada.Integer_Text_IO;
use  Ada.Text_IO.Text_Streams, Ada.Text_IO, Ada.Integer_Text_IO;

procedure Strange3 is

   type Value is range 0..255;
   for Value'Size use 8;

   type Value_Array is array(1..5) of Value;
   for Value_Array'Component_Size use 8;
   pragma Pack(Value_Array);

   VA: Value_Array;

begin
   Put(Value_Array'Size);
   Value_Array'Read(Stream(Standard_Input), VA);
   Put_Line("Read Success");
   for X in 1..5 loop
      Put(Integer(VA(X)));
      New_Line;
   end loop;
end Strange3;
---------------------------------------------------------------------
$ echo "ABCDEFGHIJK" | ./strange2
         40

raised CONSTRAINT_ERROR
---------------------------------------------------------------------
PROBLEM: Why do I get a CONSTRAINT_ERROR here?
---------------------------------------------------------------------

This looks like a GNAT 3.10p bug to me. If I replace "range 0..255"
with "mod 256", then everything works nicely.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>