可用的运算符列表随即显示出来。
在顶层是打开的file对象，它由进程的文件描述符列表引用。
At the top is the open file object, which is referenced by a process's file descriptor list.
select调用返回一个文件描述符列表，对于每个文件描述符至少注册了一个事件。
A select call returns a list of file descriptors for which at least one registered event has occurred.
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