-- From John Barnes, "Programming in Ada 95".


package body rw is
  V : integer;  -- V is the data

  task Control is
    entry Start;
    entry Stop;
    entry Write(X: in integer);
   end;

  task body Control is
      Readers: Integer := 0; -- number of active readers
      begin
        accept Write(X: in integer) do -- Write initial value of V;
           V := X;
        end Write;
      loop
       select
         accept Start do   
           Readers := Readers + 1; 
         end Start;
       or 
        accept Stop do
           Readers := Readers - 1;
         end Stop;
       or
        when Readers = 0 =>
           accept Write(X: in integer) do
              V := X;
            end Write;
       end select;
    end loop;
end Control;

procedure Read(X: out Integer) is
  begin Control.Start;
        X := V;
        Control.Stop;
  end Read;

procedure Write(X: in Integer) is
   begin Control.Write(X);
   end Write;

end rw;