Re: Advantages

["Randy Brukardt" <randy@rrsoftware.com>]

"Brian May" <bam@snoopy.apana.org.au> wrote in message
news:sa47jtse93b.fsf@snoopy.apana.org.au...
...
> The implication of what you say is you can't have a deadlock in Ada
> across different packages, that will compile, if you use protected
> objects in this manner to access variables
>
> Not true!

<Long example omitted>

All your example proves is that GNAT doesn't catch the error in this
program. It is a bounded error to use a blocking operation in protected
action (see 9.5.1(8-17)). It's defined to either be detected (and raise
Program_Error) or otherwise "may result in deadlock".

So, if you write code that's defined to deadlock, it isn't particularly
surprising that it deadlocks.

The reason that this is a bounded error is that it can't in general be
detected at compile-time, and there were those that thought the overhead of
detecting at run-time was too high. (Janus/Ada always detects it, so your
program would just raise Program_Error. But Janus/Ada wasn't designed for
hard real-time systems.) I'm pretty sure the intent was that it would be
detected if the cost wasn't too severe - there was no intent to allow such
things because of the havoc that could occur in the run-time.

Since GNAT detected the error at compile-time (thus the warning), there
couldn't be any run-time overhead for detecting the error. It should
therefore have raised Program_Error (allowing a known deadlock condition
seems like a very bad thing). But perhaps there is some customer of ACT's
that likes living dangerously? Or, perhaps the case just never came up,
because people rarely try to do the obviously bad...

                                   Randy.