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From: "James S. Rogers" <jimmaureenrogers@worldnet.att.net>
Subject: Re: Streams and Concurrency
Date: 1998/12/30
Message-ID: <76dn7b$a35@bgtnsc03.worldnet.att.net>#1/1
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References: <76c3tv$acs@bgtnsc02.worldnet.att.net>
 <76cat4$2ldc$1@news.gate.net>
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Newsgroups: comp.lang.ada
Date: 1998-12-30T00:00:00+00:00
List-Id: <comp.lang.ada>

Following is a modest example of what I mean about
encapsulating a stream in a protected object.

The example uses Ada.Streams.Stream_Io for simplicity.  You can
reasonably substitute your own stream implementation as needed.

Note that the file name for the log file is passed as a discriminant to
the protected type.

I believe this solution solves the interleaving problem.  The read and
write operations will be atomic.  The write operation will not complete
until all 'write operations for the tagged type complete.  The key is
that the task calling the protected write and read operations is
ignorant of the use of a stream.  No stream operations occur outside
the protection of the protected type.

--------------------------------------------------------------------------
-- Simple Example of using streams in a protected object for access by
-- multiple tasks.
--------------------------------------------------------------------------
   package Type_Hierarchy is
      type Base_Type is tagged private;

   -- Add dispatching operations to the Base_type

      type First_Child is new Base_Type with private;

   -- Add or override operations for First_Child

   private

      type Base_Type is tagged record
            Date : String(1..10);
            Time : String(1..8);
         end record;

      type First_Child is new Base_type with record
            Voltage : Float;
         end record;
   end Type_Hierarchy;


   with Type_Hierarchy;
   use Type_Hierarchy;
   with Ada.Steams.Stream_Io;

   package Multi_Task_Logging is

      protected type Log_Stream (Log_Name : String) is
         Procedure Write(Item : in Base_Type'Class);
         Entry Read(Item : out Base_Type'Class);
      private
         File : Ada.Streams.Stream_Io.File_Type;
         Log_Access : Ada.Streams.Stream_Io.Stream_Access :=
            Ada.Streams.Stream_Io.Stream(File => File);
      end Log_Stream;
   end Multi_Tasking_Logging;

   package body Multi_Tasking_Logging is

      protected body Log_Stream is
         Procedure Write(Item : in Base_Type'Class) is
         begin
            if not Ada.Streams.Stream_Io.Is_Open(File) then
               Ada.Streams.Stream_Io.Open(File => File,
                                          Name => Log_Name,
                                          Mode =>
Ada.Streams.Stream_Io.Append_File);
            else
               Ada.Streams.Stream_Io.Reset(File => File,
                                           Mode =>
Ada.Streams.Stream_Io.Append_File);
            end if;
            Base_Type'Class'Write(Stream => Log_Access, Item => Item);
         end Write;

         Entry Read(Item : out Base_Type'Class) when not
         Ada.Streams.Stream_Io.End_of_File(File) is
         begin
            if not Ada.Streams.Stream_Io.is_Open(File) then
               Ada.Streams.Stream_Io.Open(File => File,
                                          Name => Log_Name,
                                          Mode =>
Ada.Streams.Stream_Io.In_File);
            else
               Ada.Streams.Stream_Io.Reset(File => File,
                                           Mode =>
Ada.Streams.Stream_Io.In_File);
            end if;
            Base_Type'Class'Read( Stream => Log_Access, Item => Item);
         end Read;
      end Log_Stream;

   End Multi_Tasking_Logging;

-----------------------------------

Jim Rogers
Colorado Springs, Colorado USA