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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: GNAT for MS Visual Studio
Date: Sat, 06 Oct 2007 13:32:47 -0700
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Date: 2007-10-06T13:32:47-07:00
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On 6 Pa , 20:30, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:

> >    type Whatever is tagged record
> >       ...
> >    end record;
>
> > You can definitely read an object of type Whatever from multiple tasks.
> > You don't have to use a protected object.
>
> > You can also read the memory pointed to any access type from multiple
> > tasks. You don't have to use a protected object.
>
> AFAIK, ARM is silent about this. The most close reference is 9(8):
>
> "In addition, tasks can communicate indirectly by reading and updating
> (unprotected) shared variables, presuming the access is properly
> synchronized through some other kind of task interaction."

Note that introducing some causal ordering (a.k.a. signalling) is
enough to make it safe.

Consider a main task that populates the shared object *and then*
starts N tasks that concurrently read the shared data (the main task
does not interfere with the data anymore). There is a causal
relationship between populating and reading which is guaranteed by
task activation. See 9.10/4. The tasks can safely read the data.

In other words, it is perfectly imaginable to set up a set of tasks
that concurrently read the same data without any additional hassle
like protected objects. So - Pascal is right that this is in principle
possible.
At the same time this is explicitly forbidden for containers.

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com