Most elegant solution to Reader/Writer Problem with Objective-C?

[Lachlan Deck]

Hi all,

I'm doing a presentation for Uni tomorrow and I'm trying to whip up 
some examples.

What would be the most elegant solution in Objective-C to solving 
the Reader/Writer problem (i.e., allow multiple reader threads to 
run concurrently should there be no writer(s); allow at most one 
writer, blocking other threads) using, for example the below 
algorithm?

Thanks very much!

Any examples would be great, but one which does the following would 
be even better:

Readers:
a) concurrent readers activity (i.e., if no writers - go ahead. 
Incrementing/Decrementing count on entering and exiting of 
sections).
b) if (waiting writers) new readers join a new queue instance that 
corresponds to a single writer in the queue.

Writers:
c) at most one writer activity exclusive of any other threads 
(i.e., if no readers or writers - go ahead).
d) if active readers or active writer, this writer is placed as the 
key at the end of the queue (with any subsequent readers entering 
joining the list associated with this writer) and awakes when it is 
the at the head of the queue and there are no active readers. When 
it finishes, the waiting readers associated with it in the queue 
are signalled to proceed (when they finish the next writer goes and 
so we go on...)

So we have the following,
- A count of active readers
- A bool for an active writer
- A queue that maintains a list of waiting threads where:
  + Each entry in the queue represents a waiting writer (so the 
queue count is the waiting writer count (less 1 if activeWriter is 
true))
  + Each writer in the queue is associated (or mapped) with a list 
of waiting readers. i.e., as each readers 'enters', if there are 
waiting writers, it is added to a list with other waiting readers 
(associated with the last waiting writer in the queue).

Thanks muchly.

with regards,

--
Lachlan Deck
ldeck@tpg.com.au