Re: Ada.Text_IO and protected objects

["fabio de francesco" <fmdf@tiscali.it>]

Dmitry A. Kazakov wrote:
> On 30 Apr 2005 11:07:54 +0200, Jacob Sparre Andersen wrote:
>
> > I am not sure if the appropriate way of handling access to
> > Standard_Output is by using a protected object to manage a queue
for
> > transferring data to the task doing the actual writing to
> > Standard_Output, or if the interaction should be through a direct
> > rendezvous with the task doing the actual writing to
Standard_Output.
>
> #3: It can be also done on the context of the same task using a
protected
> object implementing a mutex. A controlled object is then used for
safe
> mutex access.
>
> But a monitor task (the second variant you mention) is cleaner and
safer in
> my view. It might be slightly slower than mutexes, though that should
> depend on the compiler-platform.

Thank all of you. I have thought a while about the need to start a
third task to do the actual writing to Standard_Output as the second
variant mentions. I decided not to do it because the original problem
is to manage two different printer devices or output files, one at each
door. Otherwise there wouldn't be any need two have two different tasks
to emit tickets, only one would suffice. (I hope I am not missing
something...)

Anyway just to reason on the solution Dmitry and Jacob suggest, how
would you implement that one? If I have understood there would be the
need for a third task in charge to handle IO. I suppose it means that
the other two tasks have to emit "tickets" to a protected queue that
would be later read by the third task in order to print them. Is it
what you mean? How can we assure that all elements in queue are ordered
by increasing numbers if any of the previous two tasks add tickets to
this queue at random times? I think there must be something I didn't
catch. I am not even sure you were proposing a queue working in terms I
explained here. Can you make your solution clearer for me, please?

Regards,

fabio de francesco

>
> > There is a "dining philosophers" example available somewhere on the
> > web (written by Feldman, I think), where you probably can find a
good
> > example of how to solve the combination of tasking and IO.
>
> Dining philosophers is rather an example of why mutexes/semaphores
are
> dangerous (deadlock) and low-level and what could be done about that.
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de