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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Uninterruptible (atomic) operation?
Date: 1996/12/11
Message-ID: <EACHUS.96Dec10204319@spectre.mitre.org>#1/1
X-Deja-AN: 203425796
references: <9612091326.AA04263@most> <E25tK3.3G7@world.std.com>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada
Date: 1996-12-11T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <E25tK3.3G7@world.std.com> bobduff@world.std.com (Robert A Duff) writes:

  > Pragma Atomic is essentially the same thing as Ada 83's pragma Shared.
  > I don't think it's what you want in this case.

  Actually this is the case where pragma Shared or pragma Atomic can
be used, if supported for the object type.  Wes just needs to lock the
value while it is being modified.  The trick is to have a lock which
all the potential modifiers respect, and to use pragma Atomic, and
Atomic reads and writes, so that the readers can trust the value they
see.

    (Potential) Writer:

       Acquire(lock);
       Temp := Atomic_Object;
       -- operations on Temp...
       Atomic_Object := Temp;
       Release(lock);

    Reader:

       Temp := Atomic_Object;
       -- use value in temp.

--

					Robert I. Eachus

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