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From: Simon Wright <simon@pushface.org>
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Subject: Re: Gnat Problem - Freezing too soon
Date: Thu, 28 Feb 2019 21:22:37 +0000
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Content-Type: text/plain

russ lyttle <lyttlec@removegmail.com> writes:

> Example from the bookAnalysable Real-Time Systems :
>
> 1  package Environment_Monitor.Ch4_Status is --Protected
> 2    protected type Methane_Agent (Ceiling : Priority)
> 3     with Priority => Ceiling
> 4       is new CH4_Safety
> 5    with
> 6      overriding function Check_Safe return Methane_Status;
> 7      overriding procedure Set_Safety (M : Methane_Status);
> 8    private
> 9       Current_Status : Methane_Status := Motor_Unsafe;
> 10   end Methane_Agent;
> 11 end Environment_Monitor.Ch4_Status;
>
> Gnat complains that Methane_Agent is frozen at line 2, and line 3 is
> trying to change an attribute after being frozen.

The resolution may well be to upgrade your compiler. GNAT GPL 2017 and
FSF GCC 8 are both happy with this compilable demonstrator:


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Content-Type: text/plain
Content-Disposition: inline
Content-Description: demo

with System;
with Ada.Text_IO;
procedure Russ is

   package P is

      type Parent is synchronized interface;
      function Is_Safe (P : Parent) return Boolean is abstract;
      procedure Set_Safety (P : in out Parent; To : Boolean) is abstract;

      protected type Child (Ceiling : System.Priority)
      with Priority => Ceiling
      is new Parent with
         overriding
         function Is_Safe return Boolean;
         overriding
         procedure Set_Safety (To : Boolean);
      private
         Safety : Boolean := False;
      end Child;

   end P;

   package body P is

      protected body Child is
         function Is_Safe return Boolean is (Safety);
         procedure Set_Safety (To : Boolean) is
         begin
            Safety := To;
         end Set_Safety;
      end Child;

   end P;

   A_Child : P.Child (Ceiling => System.Priority'Last);

begin
   Ada.Text_IO.Put_Line (A_Child.Is_Safe'Img);
   A_Child.Set_Safety (True);
   Ada.Text_IO.Put_Line (A_Child.Is_Safe'Img);
end Russ;

--=-=-=--