threads and sleep?

[Jp Calderone]

On 14 Jul 2005 05:10:38 -0700, Paul Rubin <"http://phr.cx"@nospam.invalid> wrote:
>Andreas Kostyrka <andreas at kostyrka.org> writes:
>> Basically the current state of art in "threading" programming doesn't
>> include a safe model. General threading programming is unsafe at the
>> moment, and there's nothing to do about that. It requires the developer
>> to carefully add any needed locking by hand.
>
>So how does Java do it?  Declaring some objects and functions to be
>synchronized seems to be enough, I thought.

Multithreaded Java programs have thread-related bugs in them too.  So it doesn't seem to be enough.  Like Python's model, Java's is mostly about ensuring internal interpreter state doesn't get horribly corrupted.  It doesn't do anything for application-level state.  For example, the following (Python, because it's way too early to write Java, but a straight port to Java would be broken in exactly the same way) program is not threadsafe:

things = []

def twiddle(thing):
    if thing in things:
        print 'got one'
        things.remove(thing)
    else:
        print 'missing one'
        things.append(thing)

The global list will never become corrupted.  stdout will always be fine (although perhaps a little weird).  The objects being appended to and removed from the list are perfectly safe.

But the program will double append items to the list sometimes, and raise ValueErrors from the list.remove() call sometimes.

Java's model isn't really too far from the traditional one.  It's a tiny bit safer, perhaps, but that's all.  For something different, take a look at Erlang's mechanism (this has been ported to Python, although I have heard nothing of the result since its release announcement, I wonder how it's doing?)

Hope this helps,

Jp