说明：  Libevent 是用于编写高速可移植非阻塞 IO 应用的库，其设计目标是： 可移植性：使用 libevent 编写的程序应该可以在 libevent 支持的所有平台上工作。即使 没有好的方式进行非阻塞 IO，libevent 也应该支持一般的方式，让程序可以在受限的环 境中运行。 速度：libevent 尝试使用每个平台上最高速的非阻塞 IO 实现，并且不引入太多的额外 开销。 可扩展性：libevent 被设计为程序即使需要上万个活动套接字的时候也可以良好工作。 方便：无论何时，最自然的使用 libevent 编写程序的方式应该是稳定的、可移植的
(Libevent is a library for writing high-speed, portable, non-blocking IO applications designed to: Portability: Programs written with libevent should work on all platforms supported by libevent. Even if there is no good way to do non-blocking IO, libevent should support the general way to allow programs to run in a restricted environment. Speed: libevent attempts to use the highest speed non-blocking IO implementation on each platform without introducing too much overhead. Extensibility: libevent is designed to work well even when the program requires tens of thousands of active sockets. Convenience: Whenever possible, the most natural way to write programs using libevent is to be stable and portable.)