From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 10.140.104.51 with SMTP id z48mr11753199qge.27.1464395160227;
        Fri, 27 May 2016 17:26:00 -0700 (PDT)
X-Received: by 10.157.40.228 with SMTP id s91mr259251ota.13.1464395160184;
 Fri, 27 May 2016 17:26:00 -0700 (PDT)
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!usenet.blueworldhosting.com!feeder01.blueworldhosting.com!border2.nntp.dca1.giganews.com!nntp.giganews.com!j92no27988qga.1!news-out.google.com!yi6ni921igc.0!nntp.google.com!q6no2065903igz.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Fri, 27 May 2016 17:25:59 -0700 (PDT)
In-Reply-To: <25c43463-47ca-4021-82ee-299e6a075faa@googlegroups.com>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com;
 posting-host=2601:191:8202:8510:5985:2c17:9409:aa9c;
 posting-account=fdRd8woAAADTIlxCu9FgvDrUK4wPzvy3
NNTP-Posting-Host: 2601:191:8202:8510:5985:2c17:9409:aa9c
References: <25c43463-47ca-4021-82ee-299e6a075faa@googlegroups.com>
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <2c0dfaf8-9344-4b9c-87b4-12de687687ce@googlegroups.com>
Subject: Re: Advice,  tasking and hardware
From: rieachus@comcast.net
Injection-Date: Sat, 28 May 2016 00:26:00 +0000
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Xref: news.eternal-september.org comp.lang.ada:30499
Date: 2016-05-27T17:25:59-07:00
List-Id: <comp.lang.ada>

I think there is a lot of well argued stuff here which is several months ah=
ead of where the OP is.

If you are writing a controller for a wall of instruments talking IEEE 488 =
or some such, and the goal is to write programs for automated testing, the =
first huge win with Ada is:

   In Ada you model the environment, not the problem set.

It is not unusual to have changes in the requirements for automated testing=
 (or just about any other field) at the last minute.  If you modeled the en=
vironment/solution space, a small (in text terms) change in the requirement=
s with require only a small change in the code.

Next, there are choices you have to make in doing your modelling.  Having a=
 package corresponding to each instrument, and putting a task inside iff yo=
u need to deal with state in the instrument.  Otherwise use a protected obj=
ect to insure sequential communications.

Some instruments have state, lots of state, others are nice and use a state=
less protocol.  To use POs, to control other hardware you definitely want t=
o do a classic semaphore.  Any thread/task talking to that device does a Se=
ize, and a Release when done.  For devices with state, you need a task that=
 tracks the state of the device.  Would it were so simple as that makes it =
sound.  The interface the package presents to the rest of the world is clea=
n, but your task may end up pages long to deal with error states and recove=
ry.

Finally how you deal with task termination will depend to a great deal on t=
he safety requirements.  There is a design pattern when you need each activ=
e device to have a call during shutdown and multiple shutdowns are the way =
to go.  Basically you have a shutdown package in the with list of packages =
with hidden tasks.  You create a new task which does what is necessary and =
it waits on a task entry provided by the shutdown package:

package Shutdown is
  task Suspend is
    entry Stop;
    entry Wait;
  end Suspend;
end Shutdown;
...
package body Shutdown is
begin
 task body Suspend is
 begin
   accept Stop;
   loop
     select
       accept Wait;
     or
       accept Stop; -- in case of multiple calls to Stop.
     or
       terminate;
     end select;
   end loop;
 end Shutdown;

  Now any package can call Shutdown.Suspend.Wait from a task that does the =
necessary shutdown actions.  (This is about the only situation where you wa=
nt to call abort in Ada.  You have a task which deals with normal operation=
s, and a emergency task which turns of the gas, high voltage, fuel, or what=
ever.)

   task Killer is
   end Killer  -- no entries.

   task body Killer is=20
   begin
     Shutdown.Suspend.Wait;
     select
       Local_Task.Shutdown;
     or delay XXX;  --- delay time will depend on physical system propertie=
s.
     end select;
     delay YYY;   --- delay time will depend on physical system properties.
     abort Local_Task;
     -- shut down everything controlled by Local_Task here.
   end Killer;=20