boost::shared_mutex

shared_mutex.hpp

 class upgrade_mutex
    {
      typedef boost::mutex              mutex_t;
      typedef boost::condition_variable cond_t;
      typedef unsigned                count_t;

      mutex_t mut_;
      cond_t  gate1_;
      cond_t  gate2_;
      count_t state_;

      static const unsigned write_entered_ = 1U << (sizeof(count_t)*CHAR_BIT - 1);
      static const unsigned upgradable_entered_ = write_entered_ >> 1;
      static const unsigned n_readers_ = ~(write_entered_ | upgradable_entered_);
      }