Re: Another problem with stream reading.

[Erik Sigra <sigra@home.se>]

tisdagen den 26 mars 2002 18.54 skrev du:
> Just shooting from the hip here, I think the problem is that
> you gave the compiler an impossible assignment. By that, I
> mean that you've declared type Byte to have integer range
> 0..255 and at the same time said its size has to be 8 bits.
> There is no room for that range and a sign bit (since it is
> integer here) to exist in 8 bits (you need 9). As a result
> of lying to HAL, HAL decided to disregard your statement
> "for Byte'Size use 8" and used 16 instead.

If you look carefully at my code you can se that i did NOT write "Integer 
range", just "range". And there is certainly no need for a sign bit for the 
range 0 .. 255. If I lie to the compiler, it does certainly not disregard it. 
It complais loudly. If I for example say "for Byte'Size use 7;", it replies
"size for "Byte" too small, minimum allowed is 8"


> If you want signed numbers, you need to fix the range. If
> you want unsigned numbers, then you need a modulo type.

I should not need a modulo type for unsigned numbers. See "Programming in Ada 
95" by John Barnes, 2nd edition, page 552:

	"	For example we can specify the amount of storage to be allocated for
	objects of a type, for the storage pool of an access type and for the working
	storage of a task type. This is done by an attribute definition clause. Thus

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

	indicates that objects of type Byte should occupy only 8 bits. The size of
	individual objects can also be specified."


> Erik Sigra wrote:
> > Now I have the program
> >
> > with Ada;                   use Ada;
> > with Text_IO;               use Text_IO;
> > with Ada.Streams;           use Streams;
> > with Ada.Streams.Stream_IO; use Stream_IO;
> >
> > procedure Streamtest is
> >    The_File : Stream_IO.File_Type;
> > begin
> >    Open (The_File, In_File, "data");
> >    declare
> >       The_Stream : Stream_Access := Stream (The_File);
> >       type Byte is range 0 .. 255;
> >       for Byte'Size use 8;
> >       B : Byte;
> >    begin
> >       while not End_Of_File (The_File) loop
> >          Byte'Read (The_Stream, B);
> >          Put_Line ("Read B = " & B'Img);
> >       end loop;
> >    end;
> > end Streamtest;
> >
> >
> > The data file contains
> > "����" (hexadecimal "ff fe fd fc") (decimal "255 254 253 252). The output
> > of the program is:
> > Read B =  255
> > Read B =  253
> >
> >
> > The problem is that it reads 2 bytes instead of 1 and thus skips each
> > second byte. Why?