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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Constraint checking of actuals passed to Attributes
Date: 2000/05/17
Message-ID: <wccitwd1fs2.fsf@world.std.com>#1/1
X-Deja-AN: 624522311
Sender: bobduff@world.std.com (Robert A Duff)
References: <391250A8.99D1585C@hotmail.com> <39171B69.2F983487@averstar.com>
 <8f93lm$1es$1@nnrp1.deja.com> <wcc4s8721fj.fsf@world.std.com>
 <8f9snr$vbr$1@nnrp1.deja.com> <wccitwnmi4r.fsf@world.std.com>
 <yecitwn9kmr.fsf@king.cts.com> <wccr9bak1zc.fsf@world.std.com>
 <x7vpuqp2jpa.fsf@pogner.demon.co.uk>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 2000-05-17T00:00:00+00:00
List-Id: <comp.lang.ada>

Simon Wright <simon@pogner.demon.co.uk> writes:

> Robert A Duff <bobduff@world.std.com> writes:
> 
> > In the run-time model the AverStar compiler chooses, the compiler can
> > prove validity quite often.  For example, an 'in' parameter of an
> > integer type is always valid, because it was constraint checked at the
> > call site.
> 
> Presumably this isn't true if the subprogram was exported ..

You mean exported as in pragma Export?  It's true even in that case.
Eg:

    procedure P(X: Positive) is
	S: String(1..10);
    begin
        S(X) := 'x';
    end P;

Inside P, we assume X is in Positive.  On the assignment, the generated
code will check that X <= 10, but will *not* check that X >= 1.  If you
call P from C code, it's your responsibility to pass the correct stuff.
If you pass in a negative value, you will overwrite an arbitrary memory
location.

This is always true: you always lose all checking (compile time and run
time) at the interface between Ada and some foreign language.

- Bob