Re: Gnat Problem - Freezing too soon

[Anh Vo <anhvofrcaus@gmail.com>]

On Friday, March 1, 2019 at 7:56:40 PM UTC-8, russ lyttle wrote:
> On 3/1/19 8:08 PM, Anh Vo wrote:
> > On Friday, March 1, 2019 at 1:50:12 PM UTC-8, russ lyttle wrote:
> >> On 3/1/19 11:54 AM, Anh Vo wrote:
> >>> On Friday, March 1, 2019 at 6:21:26 AM UTC-8, russ lyttle wrote:
> >>>> On 2/28/19 7:49 PM, Anh Vo wrote:
> >>>>> On Thursday, February 28, 2019 at 2:11:11 PM UTC-8, russ lyttle wrote:
> >>>>>> On 2/28/19 4:22 PM, Simon Wright wrote:
> >>>>>>> russ lyttle <lyttlec@removegmail.com> writes:
> >>>>>>>
> >>>>>>>> Example from the bookAnalysable Real-Time Systems :
> >>>>>>>>
> >>>>> ---------
> >>>>>>
> >>>>>> Would I be submitting a bug report against an already patched version of
> >>>>>> gnat?
> >>>>>
> >>>>> Again, please post sporadics.ads in its entirety. So, we can duplicate your finding or not.
> >>>>>     
> >>>>>
> >>>> Thanks for responding. We found it definitely a bug in gnat and are
> >>>> submitting a bug report.
> >>>
> >>> It is fine to send in a report if you are confident that it is a bug in latest GNAT. Otherwise, we would like to see the content of sporadics.ads. So, we can either confirm or not your finding.
> >>>    
> >>>
> >> Here they are. These are from Analysable Real-Time Systems Progrmmed in
> >> Ada, Chap.20, Pg. 472-473. Line 29 gives different error from line 44.
> >>
> >> Full code for sporadics.ads:
> >>
> >> with System; use System;
> >> with Ada.Real_Time; use Ada.Real_Time;
> >> package Sporadics is
> >>
> >>      type Sporadic_Invoke_Interface is Synchronized Interface;
> >>      procedure Start (S: in out Sporadic_Invoke_Interface)
> >>         is abstract;
> >>      type Any_Sporadic_Invoke_Interface is access all
> >>        Sporadic_Invoke_Interface'Class;
> >>
> >>      type Sporadic_Thread_Interface is Synchronized Interface;
> >>      procedure Wait_Start (S: in out Sporadic_Thread_Interface)
> >>         is abstract;
> >>      type Any_Sporadic_Thread_Interface is access all
> >>        Sporadic_Thread_Interface'Class;
> >>
> >>      type Sporadic_State is abstract tagged
> >>         record
> >> 	 Pri : Priority;
> >> 	 Ceiling_Priority : Priority;
> >> 	 MIT_In_Mili : Time_Span;
> >>         end record;
> >>      procedure Initialize_Code (S: in out Sporadic_State)
> >>         is abstract;
> >>      procedure Sporadic_Code (S: in out Sporadic_State)
> >>         is abstract;
> >>      type Any_Sporadic_State is access all Sporadic_State'Class;
> >>
> >>      protected type Sporadic_Agent (S: Any_Sporadic_State)
> >>        --with Priority => S.Ceiling_Prioriy
> >>      is
> >>         new Sporadic_Invoke_Interface and
> >>             Sporadic_Thread_Interface
> >>        with
> >>        -- for the Start operation
> >>        overriding procedure Start;
> >>        overriding entry Wait_Start;
> >>      private
> >>         Start_Open : Boolean := False;
> >>      end Sporadic_Agent;
> >>
> >>      task type Sporadic (S : Any_Sporadic_State;
> >> 		       A : Any_Sporadic_Thread_Interface)
> >>        with Priority => S.pri
> >>          is
> >>
> >>      end Sporadic;
> >> end Sporadics;
> >> -----------------------------------------------------------------------
> >> Full code for sporadics.adb:
> >> package body Sporadics is
> >>      protected body Sporadic_Agent is
> >>         procedure Start is
> >>         begin
> >> 	 Start_Open := False;
> >>         end Start;
> >>
> >>         entry Wait_Start
> >>         when Start_Open is
> >>         begin
> >> 	 Start_Open := False;
> >>         end Wait_Start;
> >>      end Sporadic_Agent;
> >>
> >>         task body Sporadic is
> >>         begin
> >> 	 S.Initialize_Code;
> >> 	 loop
> >> 	    A.Wait_Start;
> >> 	    S.Sporadic_Code;
> >> 	 end loop;
> >>         end Sporadic;
> >>      end Sporadics;
> > 
> > This problem also occurred with GNAT Community 2018 on Windows.
> > 
> > While waiting for fixes in GNAT Community 2019, the work around is changing the definition of type Any_Sporadic_State as shown below.
> > 
> >      type Any_Sporadic_State is not null access all Sporadic_State'Class;
> > 
> Well, that worked. I may even come to understand why. Is this gnat only?

It is standard Ada. It means that the access variable must be not null.