title: Why is epoll thread-safe but not libevent
body: <p>I am a bit confused about the difference between epoll and libevent.
  Libevent is just an encapsulation of epoll.</p>
  <p>Since epoll is considered to be thread-safe (e.g. <a href="https://stackoverflow.com/questions/7058737/is-epoll-thread-safe">Is epoll thread-safe?</a> <a href="https://stackoverflow.com/questions/42442052/is-epoll-ctl-safe-to-be-called-simultaneously-from-multiple-threads">Is epoll_ctl() safe to be called simultaneously from multiple threads?</a>), why does libevent become thread-unsafe?</p>
  <p>Which part of libevent breaks its thread-safety?</p>