Re: protected objects and external call

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jean-Pierre Rosen" <rosen@adalog.fr> writes:

> "Evangelista Sami" <evangeli@cnam.fr> a �crit dans le message de news:5f59677c.0401220851.3c2922b0@posting.google.com...
> > hi all
> >
> > in RM-9.5.1 we see there is a difference between external and internal
> > protected calls. Let's consider the following code
> > ----------------------------------
> > protected type Prot is
> >    procedure P;
> > end Prot;
> > My_Prots : array (1..10) of Prot;
> >
> > protected body Prot is
> >
> >    procedure P is
> >    begin
> >       .....
> >       My_Prots(5).P;
> >       ....
> >    end;
> >
> > end Prot;
> > ----------------------------------
> >
> > Is the call "My_Prots(5).P" always considered as an external call or
> > is it an internal call if the object on which is executed the
> > procedure is My_Prots(5) ?
> >
> It is an external call (in the general case, you cannot know
> statically whether it's truly external or not).

Right.

> This means that you'll try to grab the lock of the PO again, and this
> can lead to dead-lock.

Right.

> This is why it is considered a potentially blocking operation, and as
> such disallowed in a protected operation.

No, I don't think that's quite right.  Entry calls are potentially
blocking, and you're not supposed to call them from inside a protected
procedure.  (Actually, it's a bounded error, and some compilers allow
it.)  But a protected procedure can call another protected procedure
with an external call.  It's up to the programmer to make sure it's not
the same protected object.  (You also have to obey the priority
ceiling rules.)

In other words, merely grabbing the lock of a PO is *not* considered
blocking or potentially blocking in Ada, and no queue-ing need be
involved.

- Bob