libevent API 提供了一种机制：当一个特殊事件在一个文件描述符上发生或者一个事

先设定的时间到达的时候执行一个callback 函数，另外，callback 也支持由信号或者规则的

timeout 引发的callback.-libevent API provides a mechanism for : When a special event in a file descr iptors occurred or a pre-set time when the arrival of the implementation of a ca llback function, addition, callback also supported by signals or rules timeout triggered the callback.