Re: Advantages

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 02 Jul 2004 10:49:52 +1000, Brian May wrote:

>>>>>> "Randy" == Randy Brukardt <randy@rrsoftware.com> writes:
> 
>     Randy> That's correct. And it makes sense to allow I/O, as many
>     Randy> implementation don't in fact do anything blocking. But I
>     Randy> would expect that the implementation would treat things
>     Randy> that are *known* to be blocking (like a delay statement)
>     Randy> differently - there can be no legitimate reason for doing
>     Randy> that. In a ceiling locking implementation, blocking would
> 
> <hat mode="devils advocate">
> 
> You want to send a message to a hardware device. The requirements
> specify that one message should be sent to the device, followed by a
> fixed delay, then another message. During this entire period of time,
> exclusive access is required to the device, because other threads
> could otherwise interfere.
> 
> Lets also assume that sending the message is a blocking function that
> will block until either an acknowledgement or error is returned by the
> device.
> 
> What is the safest way of implementing this under Ada?
> 
> Three blocking statements, that require exclusive access to a
> resource.
> 
> You could have a protected type emulate a semaphore, but then we are
> back to using primitive operations (and related mistakes) that Ada was
> meant to avoid.
> 
> You could have these 3 statements run from a procedure within a
> protected type (along with procedures to do other things), but you not
> suppose to make calls that potentially block from a protected type.
> 
> Just curious as to the recommended way of dealing with this
> situation...

There is no recommended way, there are only ways.

Just to add to what Jeffrey Carter said. A protected action can be split
into "floating" pieces using the requeue statement.

For example, you can initiate I/O in one entry point, then requeue to
another which barrier will fire upon completion. You might argue that it is
rather a complicated way, especially if requeue with abort should be
supported and there is a large number of different states, but it works.
(It much resembles a finite state machine.)

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de