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From: mheaney@ni.net (Matthew Heaney)
Subject: How to implement I/O using Streams
Date: 1997/04/01
Message-ID: <mheaney-ya023680000104972123200001@news.ni.net>#1/1
X-Deja-AN: 230041792
Organization: Estormza Software
Newsgroups: comp.lang.ada
Date: 1997-04-01T00:00:00+00:00
List-Id: <comp.lang.ada>


The package Ada.Streams.Stream_IO is example of using a stream to do
(heterogeneous) file I/O.  I want to do network I/O - using BSD Sockets,
say - and use a streams interface to do it.

How do I do this?  The example in RM95 shows how a file object gives the
client a pointer to a stream (through which the I/O is performed), but is
silent about how the stream and file connect to each other.  How does the
stream know about the file? 

If I have a socket descriptor, how can I use a stream to do the I/O over
the socket?

How do I implement the Read and Write operations of my type that derives
from Root_Stream?

declare
   sd : int;  -- or perhaps I'll use a higher-level abstraction
   The_Stream : Stream_Access;
   The_Data : My_Record;
begin
   <create socket and connect to server>

   The_Stream := <get a stream connected to the socket descriptor sd>; -- ???

   My_Record'Write (The_Stream, The_Data);  -- more or less done for me
end;

Does anyone know how the implement that middle line?  Has anyone
implemented a streams interface, ie used something other than
Ada.Streams.Stream_IO or Ada.Text_IO.Text_Streams?

Here's a trial attempt:

type My_Stream (sd : access int) is
   new Root_Stream_Type with null record;

procedure Read (Stream ...);
-- reads from the socket descriptor discriminent

procedure Write (Stream ...);
-- writes to the socket descriptor discriminent

Suppose I built a higher-level abstraction.  Would this work:

   type Socket_File is limited private;

   type Stream_Access is access all Root_Stream_Type'Class;

   function Stream (File : Socket_File) return Stream_Access;
...
private
 
   type My_Stream (File : access Socket_File) is
      new Root_Stream_Type with null record;

   type Socket_File is limited
      record
         sd : int;
         Stream : aliased My_Stream (Socket_File'Access);
      end record;

   procedure Read (Stream : in out My_Stream...);
   procedure Write (Stream : in out My_Stream ...);

end;

Well, is that the way?  Is that how the language designers intended a
stream to be implemented?

Thank you much!
Matt

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271