这意味着，对于每个注册，选择器调用必须执行两个模式切换。
This means that for every event registration the selector call has to make two mode switches.
对于您想要注册的每个表，都会创建一个更改数据(CD)表和CD索引。
For each table you want to register, there will be a change data (CD) table and CD index created.
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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