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From: chad.rmeiners@gmail.com (Chad R. Meiners)
Newsgroups: comp.lang.ada
Subject: Re: Synchronizing on multiple distinict resources
Date: 19 Aug 2004 15:18:44 -0700
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message news:<1f4ugyz3ig9rc$.785e2wqt1qyg$.dlg@40tude.net>...
> >> 1. multiple protected actions / rendezvous (to queue to two chopsticks)
> > 
> > Perhaps something like 
> > 
> > procedure Eat(Left, Right : in out Chop_Stick) is
> >   Synchronize Left and Right;  -- Exclusive access until end of
> > procedure
> > begin
> >   ...
> > end Eat;
> 
> One could allow "free" entry points with normal syntax:
> 
> protected type Chop_Stick is
>    procedure Release; -- A "member" procedure
> private
>    Free : Boolean;
> end Chop_Stick;
>    -- This is not Ada
> entry Seize (Left, Right : in out Chop_Stick);
>    -- Not a "member", but still a primitive operation
> 
> entry Seize (Left, Right : in out Chop_Stick)
>    when Left.Free and Right.Free is
>       -- Dispatching parameters are allowed to appear in barriers
> begin
>    Left.Free  := False;
>    Right.Free := False;
> end Seize;


I think that the when clause isn't even needed since it would need
exclusive access to both left and right.   The question becomes
whether this syntax breaks the encapsulation of the protected types.

For example:

protected type Signal is
  entry Wait;
  procedure Signal;
private
  entry Release;
  On : boolean := false;
end

entry Wait(One, Two : Signal);

protected body Signal is
  entry Wait when not On is
  begin
    requeue Release;
  end Wait;
  procedure Signal is
  begin
    On := true;
  end Signal;
  entry Release when On is
  begin
    On := Release'Count > 0;
  end Release;
end Signal;

entry Wait(One, Two : Signal) when One.On and Two.On is
begin
  null;
end Wait;