Post a Comment On: cbloom rants

If you have either event_count or condition_variable, it's pretty straightforward to get the other from the one you have. eventcount from condition_variable : # by Chris M Thomasson # # class eventcount { # public: # typedef unsigned long key_type; # # # private: # mutable rl::atomickey_type> m_count; # rl::mutex m_mutex; # rl::condition_variable m_cond; # # # void prv_signal(key_type key) { # if (key & 1) { # m_mutex.lock($); # while (! m_count($).compare_exchange_weak(key, (key + 2) & ~1, # rl::memory_order_seq_cst)); # m_mutex.unlock($); # m_cond.notify_all($); # } # } # # # public: # eventcount() { # m_count($).store(0, rl::memory_order_relaxed); # } # # # public: # key_type get() const { // aka prepare_wait # return m_count($).fetch_or(1, rl::memory_order_acquire); # } # # # void signal() { // aka notify_one # prv_signal(m_count($).fetch_add(0, rl::memory_order_seq_cst)); # } # # # void signal_relaxed() { # prv_signal(m_count($).load(rl::memory_order_relaxed)); # } # # # void wait(key_type cmp) { # m_mutex.lock($); # if ((m_count($).load(rl::memory_order_seq_cst) & ~1) == (cmp & ~1)) { # m_cond.wait(m_mutex, $); # } # m_mutex.unlock($); # } # }; # and condition variable from event count : by Dmitry V'jukov : 1. class condition_variable 2. { 3. eventcount ec_; 4. 5. public: 6. void wait(mutex& mtx) 7. { 8. int count = ec_.prepare_wait(); 9. mtx.unlock(); 10. ec_.wait(count); 11. mtx.lock(); 12. } 13. 14. void signal() 15. { 16. ec_.notify_one(); 17. } 18. 19. void broadcast() 20. { 21. ec_.notify_all(); 22. } 23. }; (note this is a simplified condition variable without all the POSIX compliance crud). In C++0x you have condition_variable at the stdlib level, so that is probably the best approach for the future. Unfortunately that future is still far away. On Pthreads you also have condition_variable (though a rather more complex one). Unfortunately, on Win32 (pre-Vista) you don't have condition_varaiable at all, so you have to build one of these from OS primitives. (BTW there are various sources for good condition_var implementations for Win32, such as boost::thread and Win32 pthreads by Alex Terekhov). ADDENDUM : really eventcount is the more primitive of the two; it's sort of a mistake that C++0x has provided "condition_var" as a primitive. They are not trying to provide a full set of OS-level thread control types (eg. they don't provide semaphore, event, what have you) - they are trying to provide the minimal basic set, and they chose condition_var. They should have done mutex and eventcount, as you can build everything from that. (actually there's something perhaps even more primitive that eventcount which is "waitset" which can be easily used to build any of the basic blocking thread control devices).