Re: Ada Protected Object Tutorial #1

[kaz@ashi.footprints.net (Kaz Kylheku)]

On Wed, 15 Dec 1999 22:27:17 -0700, James S. Rogers
<jimmaureenrogers@worldnet.att.net> wrote:
>This is the first of hopefully several examples of uses for Ada Protected
>Objects.

[ snip ]

>Protected Procedures:
>
>Protected procedures are typically used to alter the value of one or more
>data
>components in a protected object. The parameters of a protected procedure
>may have the "IN" mode, like functions. They may also have "OUT" mode,
>indicating that the value is write only, or "IN OUT" mode indicating that
>the
>parameter value can be read and updated. Only one task at a time may
>execute a protected procedure. Protected functions cannot access the
>protected object while it is being accessed by a protected procedure.

What if a protected procedure of an object calls another
protected procedure? Presumably this is allowed, which means that some
recursive lock has to be used for the implementation, or else the compiler has
to generate code that passes around secret ``already locked'' flags into
procedures. 

How does the user of the protected object compose atomic operations?

This approach to design is generally wasteful. It's more reasonable to have
public methods which always lock and unlock, and unprotected methods which
assume that an object is already locked. It's also useful for an object
to expose lock and unlock methods, and expose unprotected operations, so that
the user of the object can compose a sequence of operations into an indivisible
block.
 
>Protected Entries:
>
>Protected entries are just like protected functions with one small
>difference.
>Protected entries block untils a boundary condition becomes true. For
>example, you would use a protected entry to read from a protected
>bounded queue. The entry would block as long as the queue was empty.

What if you want to block for something in the middle of the critical region?
I assume that you can call a protected entry from within a protected procedure.

Could you expand on what flexibilities there exist within this framework?