zeromq nonblocking

// Virtual interface to be exposed by object that want to be notified
// about events on file descriptors.
 
struct i_poll_events
{
    virtual ~i_poll_events () {}
 
    // Called by I/O thread when file descriptor is ready for reading.
    virtual void in_event () = 0;
 
    // Called by I/O thread when file descriptor is ready for writing.
    virtual void out_event () = 0;
 
    // Called when timer expires.
    virtual void timer_event (int id_) = 0;
};

0mq 中实现（或部分实现）了这些接口的类包括： io_thread_t, reaper_t, zmq_connect_t, zmq_listener_t, zmq_engine_t等。