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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Blocking protected ops (was: Tasks and C/C++ code)
Date: 1998/11/24
Message-ID: <EACHUS.98Nov24143823@spectre.mitre.org>#1/1
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Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-11-24T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <365AD980.5729FCD8@elca-matrix.ch> Mats Weber <Mats.Weber@elca-matrix.ch> writes:

 > Anyway, the question remains: is it legal for a protected body to call
 > some imported subprogram that could block (in the pthreads sense of the
 > term) ?

    You quoted the answer yourself, it is a bounded error per
9.5.1(8&16) seem to cover it completely.

 > Does someone know why the rule 9.5.1(8-18) is there at all ?

    To rigorously define the conditions where you should allow for
the possibility of deadlocks and other blocking problems.  Some
deadlocks (or worse, race conditions) can't be ruled out, because they
are inherent in the nature of the beast.  Subclause 9.5.1 limits the
number of programs that are potentially affected by this, in part by
strongly encouraging programmers not to do that!

    This is a perfect example.  Most compilers will generate code to
call gethostbyname within a protected subprogram, but most users will
be encouraged by the RM to do it right.  In this case really right is
probably to put the potentially offensive call in a task (or protected
object) that encapsulates all potentially locking OS calls.  If you
are on a symmetric multiprocessor, you may be able to have several
such guards.
--

					Robert I. Eachus

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