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From: james@sandy.mcci-arl-va.com (Scott James)
Subject: Asynchronous communication between protected types
Date: 1998/07/21
Message-ID: <6p2cvs$u2$1@client2.news.psi.net>#1/1
X-Deja-AN: 373534272
Organization: MCCI, Arlington VA
Newsgroups: comp.lang.ada
Date: 1998-07-21T00:00:00+00:00
List-Id: <comp.lang.ada>

I'm trying to connect concurrent objects via an asynchronous
message. I was hoping to do this entirely with protected objects,
however, cannot seem to do so. The difficulty I have is that even if I
put the receiver into a loop (waiting on a send entry) it seems that
*some* task has to start the receiver, so I will still need one (Ada)
task per (active) receiver.

About the best I can come up is the following code.
This is acceptable for my purposes but I was curious if I
could trick this into working using only protected types.
(and without the distribution annex's Asynchronous Pragma
either :-)

(some code snippets borrowed from _Concurrency in Ada (95)_)
-----------------------------------------------------------

with Ada.Text_IO;
procedure Test_Persistent is

   type Messages is (Run, Die);

   protected type Persistent_Signals is
      procedure Send (Message: Messages);
      entry Wait (Message: out Messages);
   private
      Message: Messages;
      Signal_Arrived: Boolean:=False;
   end Persistent_Signals;

   protected body Persistent_Signals is

      procedure Send (Message: Messages) is
      begin
         Persistent_Signals.Message:=Message;
         Signal_Arrived:=True;
      end Send;

      entry Wait (Message: out messages) when Signal_Arrived is
      begin
         Signal_Arrived:=False;
         Message:=Persistent_Signals.Message;
      end Wait;

   end Persistent_Signals;

   type Access_To_Persistent_Signals is access Persistent_Signals;

   task type Processes is
      entry Initialize (With_Signal: Access_To_Persistent_Signals);
   end Processes;

   task body Processes is
      Message: Messages;
      Signal: Access_To_Persistent_Signals;

   begin
      accept Initialize (With_Signal: Access_To_Persistent_Signals) do
         Signal:=With_Signal;
      end Initialize;
      loop
         Signal.Wait (Message);
         case Message is
            when Run =>
               Ada.Text_IO.Put_Line ("I'm running");
            when Die =>
               exit;
         end case;
      end loop;
   end Processes;

   Process: Processes;
   Signal: Access_To_Persistent_Signals;

begin
   Signal:=new Persistent_Signals;
   Process.Initialize (Signal);
   for I in 1 .. 4 loop
      Signal.Send (Run);
      delay 0.5;
   end loop;
   Signal.Send (Die);
end Test_Persistent;

-- 
 Dr. Scott James        james@mcci-arl-va.com