Re: [boost] [thread] countdown_latch

Subject: Re: [boost] [thread] countdown_latch
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-04-21 06:54:23

On Apr 21, 2013, at 4:27 AM, Michael Marcin <mike.marcin_at_[hidden]> wrote:

> I recently ran across the need to spawn a thread and wait for it to finish its setup before continuing.
> 
> The accepted answer seems to be using a mutex and condition variable to achieve this.

That works, and is the appropriate mechanism using the available tools in Boost.Thread.

> However that work clutters up the code quite a bit with the implementation details.

Agreed

> I came across Java's CountDownLatch which does basically the same work but bundles it up into a tidy package.
> 
> It seems to be a fairly trivial but useful abstraction.
> 
> implementation:
> http://codepad.org/E8kd2Eb8
> 
> usage:
> 
> class widget
> {
> public:
>    widget()
>        : latch( 1 )
>        , thread_( [&]{ thread_func(); } )
>    {
>        latch.wait();
>    }
> 
> private:
>    void setup();
>    void run();
> 
>    void thread_func()
>    {
>        setup();
>        latch.count_down();
>        run();
>    }
> 
>    countdown_latch latch;
>    std::thread thread_;
> };

That sounds like a barrier: http://pubs.opengroup.org/onlinepubs/009695299/functions/pthread_barrier_wait.html

I'd prefer to create a barrier class and, in your example, it would release waiting threads when two are blocked behind it. IOW, you'd create a barrier for two threads and both thread_proc() and the constructor would wait() on the barrier. Once both threads have called wait(), they are both released. (I plan to present that at C++ Now, this year.)

Your idea is sound, but I'd prefer following the pthreads naming to that of Java.

___
Rob

(Sent from my portable computation engine)