在epoll集合中fd和相关状态之间的映射

When you add a fd to an epoll set, you can associate some state with it by using 'struct epoll_event.data'. When epoll reports an event on the fd, it, of course, returns the associated data to the user along with the event. This implies that the epoll set is maintaining a mapping between fds and associated state.When you add a fd to an epoll set, you can asso