Re: problem solved, but I don't know why (was: GNAT implementation bug)

[Tucker Taft <stt@averstar.com>]

Steve Arnold wrote:
> 
> It looks like I solved the problem, but I still have no idea why there was a
> problem in the first place (being a newbie, it seems like it *should've*
> worked).
> 
> When I replaced the following (in Remove):
> 
>        if List.Count = 1 then
>           Clear(List) ;
>        elsif List.Cursor = List.Head then...
> 
> with the actual code from Clear:
> 
>        if List.Count = 1 then
>           List := (Count => 0, Traversing => False, others => null) ;
>        elsif List.Cursor = List.Head then...
> 
> it started working correctly.  But I still have at least two questions (on
> this particular issue, anyway ;)
> 
> 1) Why did this code behave differently when running under Linux than when
> running under windoze?  (obvious answer: windoze is brain-dead)

It could be a bug in the gcc back end for Linux, or it
could be a problem with a small default stack size.

> 
> 2) Why did my trivial change fix it?

You "hand-inlined" one function, which conceivably might lessen
the stack requirements.  Or it is a codegenerator bug, and you
made a small change which happened to eliminate the combination
of constructs which hit the latent bug.

> 
> In other parts of these packages, I call functions/procedures that were
> defined in the same package, and it always worked...

It sounds like a bug in the run-time system, the code generator,
or a too-small default stack size.  I wouldn't try to figure
out what *you* did wrong.  It is probably someone else's fault.

> ... Just what is going on here?  When is it safe to do
> this (if ever) and when is it not safe?

You seem to have been unlucky enough to bump into a compiler or run-time
bug.

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA