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From: Ludovic Brenta <ludovic.brenta@insalien.org>
Newsgroups: comp.lang.ada
Subject: Re: File_Type
Date: 20 Apr 2004 08:08:26 GMT
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Jorge Suarez-Solis Rivaya asked:
> Is there anyway to use an Integer Type like a File_Type?,
> Is there anyway to asign a File_Type the value 3?

Yes, there is, on Unix and Unix-like operating systems; perhaps also
on others as well.  The way to achieve this is to write a thin binding
to the operating system's file management functions, like so:

package Low_Level_IO is
   type File_Type is new Integer;
   File_Error : exception;
   function Open (File_Name : in String) return File_Type;
   procedure Write (Into : in File_Type; What : in String);
end Low_Level_IO;

package body Low_Level_IO is
   function Open (File_Name : in String) return File_Type is
      function Internal (File_Name : in String,
                         Flags : in Integer) return File_Type;
      pragma Import (C, Internal, "open");
      Result : File_Type := Internal (File_Name & ASCII.NUL, 12);
   begin
      if Result = 0 then
         raise File_Error;
      end if;
      return Result;
   end Open;


   procedure Write (Into : in File_Type; What : in String) is
      function Internal (Into : in File_Type; What : in String;
                         Size : in Integer) return Integer;
      pragma Import (C, Internal, "write");
      Result : Integer := Internal (Into, What, What'Length);
   begin
      if Result /= What'Length then
         raise File_Error;
      end if;
   end Write;
end Low_Level_IO;


But, you have to ask yourself exactly why you are trying to use such
low-level code.  This is platform-dependent and error-prone.  I
encourage you to consider using Ada's IO libraries, or GNAT.OS_Lib,
which provide most of what you want in a portable manner.  Indeed, if
you look at the implementation of GNAT.OS_Lib, you will find something
roughly equivalent to what I wrote above.

You should try to avoid messing around with file descriptors if you
can.  It is better to use high-level abstractions; they are much
safer.

-- 
Ludovic Brenta.

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