Re: new_line in a put_line

["Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca>]

Jeffrey Carter wrote:
> Stephen Leake wrote:
>> Assuming Lock is a controlled type, that releases the lock in
>> Finalize, this [using a semaphore] is an excellent design.
> 
...
> Of course, that had more complex requirements than what is being 
> discussed here. Apparently the OP doesn't mind if the tasks are blocked 
> during output, so simply rendezvousing with a task that does the output 
> would solve the problem and be simpler and clearer than the semaphore + 
> protected type approach.

The Rendezvous has the benefit that the request to perform output
is queued. This results in greater efficiency on most platforms
because it avoids having threads wake up and test to see if they
won out on the condition variable set (protected object).

It also guarantees "order". With the protected object approach a
"Johnnie come lately" task could gain access while already blocked
tasks are waiting to gain access -- destroying the waiting line
sequence. This may create some confusing to read "output".

However, the mutex does have one advantage in this scenario. If you
have to perform multiple calls to Text_IO then it makes more sense
to lock, perform I/O and then unlock. For example:

Lock;
New_Line;
Put_line("Hello World");
Unlock;

You could implement Lock and Unlock using the Rendevous however.
-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg
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