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From: dennison@telepath.com
Subject: Re: Tasks and C/C++ code.
Date: 1998/11/10
Message-ID: <729ndu$jfo$1@nnrp1.dejanews.com>#1/1
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Date: 1998-11-10T00:00:00+00:00
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In article <364702E5.F6987321@hiwaay.net>,
  bdorough@phaseiv.com wrote:
>     I am working on a project where we need to call the same C function
> from multiple tasks.  I am wondering how to do this safely.  Can I make
> a generic with my C pragma defined there are do I need to define the
> prama in the task type so each task will have its own copy of the C
> function.

If the C function truly isn't safe to call from multiple threads (iaw: relies
on global data, "static" local declarations, or calls other non-thread safe
routines) then you have to ensure that calls to it happen from only one
thread. The safest way to do it is to make *one* server task responsible for
calling the routine and returning its results. Client tasks rendezvous with
the server task to request a call to the unsafe C routine. This is the method
used in at least one set of X/Motif Ada bindings.

A generic won't do any good, because you end up with multiple bindings to the
*same* C routine, not multiple copies of the C routine itself. A task type
won't work for roughly the same reason, unless you only allow *one* object of
that type. Even then you could have problems with object caching and
thread-local data.

--
T.E.D.

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