Re: Question about the need for requeue as described in Rationale

[progers@acm.org]

In <DuAyGJ.ACx@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:
>In article <31E16C28.3BE2@csehp3.mdc.com>,
>James A. Squire <m193884@CSEHP3.MDC.COM> wrote:
>>How is this different from barrier conditions on protected entries and
>>procedures?
>
>It's not different.  The guards of an accept statement cannot reference
>parameters of the entry call.  The barrier of an entry body cannot
>reference the parameters of the entry call.
>
>This illustrates the need for requeue.  (Well, not *need*; exactly, I
>guess it illustrates the *usefulness* of requeue.)
>
>Suppose you want to write a protected entry that accepts the caller
>based on some condition, and that condition is based on the parameters
>of the call, and also on the current state of the protected object.
>Well, Ada doesn't support that, directly, so requeue gives you a way out
>-- you can accept the call unconditionally, and then write some code
>that checks all the relevant stuff, and either requeues-for-later, or
>else does-it-now.

That is an understatement, IMHO.  When one looks at the code required to
be safe from aborts and and cancellations -- which requires agent tasks --
requeue looks more like a necessity. True, that is overstatement, since 
it can be done without it, but realistically, no.


pat
---------------
Patrick Rogers
progers@acm.org