How do I use Direct_IO many times on the same opened file?

How do I use Direct_IO many times on the same opened file?

[Matthew Kennedy]

I need to read in a BMP file for image analysis. First I read the header
into a packed record, then I need to read in the palette based on the
information in the header (this goes into an array) and then, finally, I
read in the raw data.

My approach was to instantiate a BMP_Header_IO, a BMP_Palette_IO and a
BMP_Data_IO from the Direct_IO package.

This is the bare essentials of my original code... (errors follow after)

with Direct_IO, Interfaces, Interfaces.C, Text_IO, Mode_13h;
use  Interfaces, Interfaces.C, Mode_13h;

procedure BMP_Info is

   type Bmp_Header is
      record
         Identification          : Char_Array(1..2);
         -- etc ...
         Colours_Important       : Unsigned_32;
      end record;

   type Bmp_Palette_Entry is
      record
         Blue, Red, Green, Reserved : Unsigned_8;
      end record;

   type Bmp_Palette is array(0..Unsigned_8'Last) of Bmp_Palette_Entry;

   pragma Pack(Bmp_Header);
   pragma Pack(Bmp_Palette_Entry);
   pragma Pack(Bmp_Palette);

   procedure Put(Item : in Bmp_Header) is
      use Text_IO;
      B : Char := Item.Identification(1);
      M : Char := Item.Identification(2);
   begin
      Put_Line("Identification     : " & Char'Image(B) & "," &
Char'Image(M));
      -- etc ...
   end Put;

   package Bmp_Header_IO is new Direct_IO(Bmp_Header);
   use Bmp_Header_IO;

   package Bmp_Palette_IO is new Direct_IO(Bmp_Palette);
   use Bmp_Palette_IO;

   Header : Bmp_Header;
   Palette : Bmp_Palette;
   Bitmap_File : File_Type;
   Bmp_Not_Supported : exception;

begin
   Open(Bitmap_File, In_File, "C:/TEMP/TEST.BMP");
   Read(Bitmap_File, Header);
   if Header.Bits /= 8 then
      raise Bmp_Not_Supported;
   end if;
   Read(Bitmap_File, Palette);
   Close(Bitmap_File);

   Put(Specification.Header);

exception
   when Bmp_Not_Supported =>
      Set_Mode_Revert;
      Text_IO.Put_Line("This BMP file format is not supported.");
end BMP_Info;


These are the errors I experienced:

cd f:/gnu/
gnatmake bmp_info 
gcc -c bmp_info.adb
bmp_info.adb:62:18: "File_Type" is not visible
bmp_info.adb:62:18: multiple use clauses cause hiding
bmp_info.adb:62:18: hidden declaration at a-direio.ads:28, instance at
line 57
bmp_info.adb:62:18: hidden declaration at a-direio.ads:28, instance at
line 54
f:/gnu/bin/gnatmake.exe: "bmp_info.adb" compilation error

I sort-of understand why File_Type isn't visible but I would like to
hear from you if you can suggest a work-around or a different approach.

-- 
Matthew Kennedy
Student of Electronics Engineering
University of Southern Queensland, Australia
  "When will you realise your already there?" 
      - Marilyn Manson

Re: How do I use Direct_IO many times on the same opened file?

[Stephen Leake]

Matthew Kennedy asks about multiple use clauses hiding a type.

The short answer is to use a fully qualified name:

Bitmap_File : BMP_Palette_IO.File_Type;

However, this will not get you there; you cannot read the header from
one file object and then the pallette from another file object.

The solution is to use Ada Streams. I've got code (at home) that reads
BMP files, based on the Win32Ada binding. It compiles with gnat and
ObjectAda. Unfortunately, it's part of a (very slow) effort to write a
clean (and distributable) Win32 binding, so it's not a small package.

email me if you are interested; I could post it on my home web page if
there is enough interest.

If anyone else is interested in working on a clean, distributable Win32
binding, maybe we could collaborate. For me, the effort mainly serves as
a way to learn Ada 95 and Win32, but maybe it will be more broadly
useful. My approach is to implement enough of a system to write a card
game, then worry about the rest as I need it.
-- 
- Stephe
Stephen.Leake@gsfc.nasa.gov (work)
leakstan@erols.com (home)