Re: Ada Protected Object Tutorial #1

[Richard D Riehle <laoXhai@ix.netcom.com>]

In article <848rus$s6v$1@nnrp1.deja.com>,
	Robert Dewar <robert_dewar@my-deja.com> wrote:

>A semaphore is a particular synchronization mechanism with
>advantages and disadvantages.

Lots of disadvantages.  More than commonly noted.  This is 
especially true when using languages other than Ada or when
using certain embedded R/T OS.   I ran across a very nice
exposition of this issue last week while browsing through
a new book, "An Embedded Systems Primer," my David Simon. 
This, by the way, is a good book for an introductory course
on embedded systems programming, although there is no mention
of Ada.  

>A protected object is a language mechanism for CREATING
>synchrnonization mechanisms.

Agreed.  Is it always the best choice?  Not sure.  When should
one use the semaphores provided by an embedded O/S, or the hardware
semaphores available in some environments?   

>Indeed a very standard example of the use of protected types
>is to implement a standard semaphore.

Yes, this is a common example and it sometimes leads the designer
away from the more robust option of a fully encapsulated set of
values.  

If one is determined to implement a semaphore, a protected object is a
better choice than a task.   However, Burns and Wellings in "Concurrency 
in Ada" note that  full encapsulation is the real value of a protected
resource, and express caution about low-level mechanisms such as
semaphores versus the reliability of a "monitor" mechanism a la the
protected object.   

Richard Riehle