Re: Problem with -gnatt

[Stephen Leake <stephen_leake@acm.org>]

Pascal Obry <pascal@obry.org> writes:

> Stephen Leake <stephen_leake@acm.org> writes:
> 
> > > Note that protected operations should only execute "for a short time."  
> > 
> > I wish Robert Dewar were here. I suspect he would say something like
> > "Rubbish!".
> 
> Well I don't know what Robert would say, but for sure you can't use IO inside
> a protected object routine.

Hmm. If by "IO" you mean "an operation that does input output to some
hardware", then that's not true; the LRM says nothing about hardware.
And a protected type is a good way to serialize access to hardware.

On the other hand, if by "IO" you mean "a potentially blocking
operation that does input output to some hardware", then true, the LRM
say "you can't do blocking operations in protected operations".

However, Robert Dewar has called that requirement "rubish" as well,
since it is impossible to enforce in general.

Calling Ada.Text_IO.Put_Line from a protected object "works" in some
implmentations (GNAT on MS Windows), not in others (GNAT on LynxOS).
It is _not_ portable, since it is forbidden by the language, since
Ada.Text_IO.Put_Line is _defined_ _by the language_ to be a
"potentially blocking operation".

> > "a short time" is not definable in any meaningful _standard_ way.
> > It is _totally_ up to the user to decide what that means.
> 
> Ok.
> 
> > If your application can afford to wait an hour for a protected object,
> > then an hour is "a short time". 
> 
> An hour without a single IO ?

An hour without a blocking operation. I'm computing PI, or filtering
for ET messages, or something. And I'm on a really slow machine. Ada must
work in a _very_ wide range of circumstances!

> So "for a short time" is certainly something (even if not formal) we
> have to keep in mind.

No, "blocking operations" is what we have to keep in mind.

-- 
-- Stephe
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