Re: tasks, protected types and entries

["Pat Rogers" <progers@classwide.com>]

"Richard Riehle" <richard@adaworks.com> wrote in message
news:3C94F235.80C7166A@adaworks.com...
> "chris.danx" wrote:
>
> > What is the difference between an entry and a procedure?
>
> This is one of the most important and least often address issue
> in Ada.  In my classroom, after we have started our unit on
> tasking, I have my students recite, in unison,
>
>                "An entry has a queue."

Unless the Real-Time Annex is used, with the Ceiling_Locking policy.  Then there
is no need for a queue.

>
> That is so fundamental, and so overlooked.   I believe it is a
> starting point for understanding all the associated mechanisms.
> The properties of the queue itself, and behavior of its enclosing
> task or protected object can be understood only when one
> grasps this simple idea.    Yet I encounter programmers who
> have been using Ada for years who still don't know this.
>
> > ... a procedure will always execute regardless of any
> > condition, but that an entry can have a guard to allow/prevent its execution
> > if a given condition is satisfied.
>
> There are no guards on subprograms (procedures and functions).  If there
> were, they would be called pre-conditions.    In fact, protected entry
> barriers, and task entry guards, are simply a variation-on-the-theme of
> pre-conditions.
>
> Richard Riehle
>