References in Ada exceptions

References in Ada exceptions

[cis.ohio-state.edu!pacific.mps.ohio-state.edu!math.ohio-state.edu!cs.utex]

[ The following is a request for information by a member of my department.
  Please reply direct to my email address, and I will forward. mg.]

I seem to remember that the following is dangerous:

	for i in 1..10 loop
	 begin
	   ...
	 exception
		...
		put(i);
	 end;
	end loop;

The state of the variable i cannot be guaranteed in an exception 
handler.  We had a problem like this on the DEC Ada compiler, and
I claim it was not a compiler bug, but a known limitation of Ada 
validated compilers.  Is there a formal ruling on this?  We require 
a hard reference.  Please reply by email to mgatlin@atlas.nafb.trw.com.
Thanks.

Re: References in Ada exceptions

[pa.dec.com!e2big.mko.dec.com!jac.nuo.dec.com!digits.enet.dec.com!brett]

In article <3176.2c26e734@atlas.nafb.trw.com>, mgatlin@atlas.nafb.trw.com write
s...
>[ The following is a request for information by a member of my department.
>  Please reply direct to my email address, and I will forward. mg.]
> 
>I seem to remember that the following is dangerous:
> 
>	for i in 1..10 loop
>	 begin
>	   ...
>	 exception
>		...
>		put(i);
>	 end;
>	end loop;
> 
>The state of the variable i cannot be guaranteed in an exception 
>handler.  We had a problem like this on the DEC Ada compiler, and
>I claim it was not a compiler bug, but a known limitation of Ada 
>validated compilers.  Is there a formal ruling on this?  We require 
>a hard reference.  Please reply by email to mgatlin@atlas.nafb.trw.com.

If a DEC Ada product did this, we would definitely regard it as a bug.
Exceptions within the begin..exception...end should not interrupt operations
such as the loop increment done outside the frame.

I have sent personal mail asking for help reproducing the problem.

/Bevin
DEC Ada team

Re: References in Ada exceptions

[Brian Hanafee]

>I seem to remember that the following is dangerous:
>
>	for i in 1..10 loop
>	 begin
>	   ...
>	 exception
>		...
>		put(i);
>	 end;
>	end loop;

I can't think of any reason why this might be dangerous, but I was
struck by the similarity between this and the example code in LRM
11.6. Perhaps a memory of that example is causing the concern?

--
Brian Hanafee                         Advanced Decision Systems
bhanafee@ads.com                      1500 Plymouth Street
(415) 960-7300                        Mountain View, CA 94043-1230

Re: References in Ada exceptions

[news.intercon.com!panix!netnews!schonberg!dewar]

Well of course I is defined within the exception handler. This is a normal
consequence of fundamental rules of visibility etc. Thus no ruling is
needed or available, and there is no specific "hard reference" to point
to (after all you can't find a hard reference that says that there is no
problem in referencing an identifier starting in column 51!)

Re: References in Ada exceptions

[Michael Feldman]

In article <dewar.741011826@schonberg> dewar@schonberg.NYU.EDU (Robert Dewar) w
rites:
>Well of course I is defined within the exception handler. This is a normal
>consequence of fundamental rules of visibility etc. Thus no ruling is
>needed or available, and there is no specific "hard reference" to point
>to (after all you can't find a hard reference that says that there is no
>problem in referencing an identifier starting in column 51!)
>
OK, Robert, then let's get this perfectly clear: any compiler that
would put an unpredictable value in I, in that context, is simply
buggy, right? Is that a firm YES I heard all the way from New York?

I scratched my head on this when it was posted - nothing looked wrong
that would cause a strange value in I, but then Ada still surprises me now
and then.:-)

Mike Feldman
------------------------------------------------------------------------
Michael B. Feldman -  co-chair, SIGAda Education Committee
Professor, Dept. of Electrical Engineering and Computer Science
The George Washington University -  Washington, DC 20052 USA
202-994-5253 (voice) - 202-994-5296 (fax) - mfeldman@seas.gwu.edu (Internet)
"Pork is what those other guys get from the Government."
------------------------------------------------------------------------