Re: [boost] Request for interest in the new Synchro library

24 Feb 2009

      ----- Original Message ----- 
From: "Dmitry Goncharov" <dgoncharov@unison.com>
To: <boost@lists.boost.org>
Sent: Thursday, February 19, 2009 8:37 AM
Subject: Re: [boost] Request for interest in the new Synchro library
...
vicente.botet wrote:
...
...
What type of semaphore are you planning to have in your library?
Hi,
currently Synchro provides the same semaphore as Boost.Interprocess (counting semaphores), except that it uses Threads resources on a inter-threads context, and Interprocess resources on a inter-process context.
The library could provide binary semaphores (the difference is minimal).Could you talk us about your use case, and why a mutex cannot be used in your context?
I need to block a thread until either a timeout expires or some other 
thread wakes this one up. This is really what 
condition_variable::timed_wait() is for, right?
Why can't condition_variable::timed_wait() be used? It can. It has a 
drawback, though.
After the condvar was signaled it has to lock a mutex. This means there 
is an unbound timeout between the time condvar was signaled and the time 
condition_variable::timed_wait() returns.
A binary_semaphore::timed_wait() can be used for the same purpose. The 
advantage is it doesn't have to lock a mutex.
Hi,

the binary_semaphore you describe must poll with test_and_set. I have no such implementation, but as far as a portable test_and_set is available this can be implemented in the library. 

Waiting for that what do you think of using a mutex as a binary_semaphore:

template <typename ScopeTag=multi_threaded_tag>
class basic_binary_semaphore
{
    typedef synchronization_family<ScopeTag> Sync;
    typedef typename Sync::mutex_type lockable_type;
    lockable_type mtx_;
public:
    BOOST_COPY_CONSTRUCTOR_DELETE(basic_binary_semaphore) /*< disable copy construction >*/
    BOOST_COPY_ASSIGNEMENT_DELETE(basic_binary_semaphore) /*< disable copy asignement >*/

    inline basic_binary_semaphore() {        mtx_.unlock();    };

    inline void post()    {        mtx_.unlock();    }

    inline void wait() {        mtx.lock();    }

    inline bool try_wait() {        return mtx.try_lock();    }

    inline bool try_wait_until(const system_time &abs_time) {
        return mtx.try_lock_until();
    }

    template<typename TimeDuration>
    inline bool try_wait_for(const TimeDuration &rel_time) {
        return try_wait_until(get_system_time()+rel_time);
    }

    inline void wait_until(const system_time &abs_time) {
        if (!try_wait_until(abs_time)) throw timeout_exception();
    }

    template<typename TimeDuration>
    inline void wait_for(const TimeDuration &rel_time) {
        if (!try_wait_until(rel_time)) throw timeout_exception();
    }        
};

Vicente