how can a child thread notify a parent thread its status?

[MRAB]

Christian Heimes wrote:
> scriptlearner at gmail.com wrote:
>> My parent thread keeps a counter for the number of free child workers
>> (say 100) and initializes some child threads and call child.start().
>> Once the number of free child workers reach 0, the parent thread will
>> wait until some at least one child thread finishes and then it will
>> initialize another child thread.
>> My question is, how can a child thread notify the parent that it's
>> done so that the parent can call join() on it?  I am not sure how a
>> child thread can send a signal to its parent while it may not even
>> know anything about it's parent.  Can you guys please provide some
>> suggestions?  Some code samples will be nice.  Thanks.
> 
> You are using the wrong approach here. There is a much easier solution
> to your problem although it's not obvious in the first place. In this
> approach the worker threads don't have to notify the parent thread that
> they are ready.
> 
> The main thread creates a queue and a bunch of worker threads that are
> pulling tasks from the queue. As long as the queue is empty all worker
> threads block and do nothing. When a task is put into the queue a random
> worker thread acquires the task, does it job and sleeps as soon as its
> ready. That way you can reuse a thread over and over again without
> creating a new worker threads.
> 
> When you need to stop the threads you put a kill object into the queue
> that tells the thread to shut down instead of blocking on the queue.
> 
The extra trick is that when a thread gets the kill object it puts it
back before terminating so that another thread will also get it.