It cannot respond to a filedescriptor immediately when the data becomes ready.
当一个文件描述符上数据准备好时，它无法立即响应。
2
These events can be things like timers, signals, data ready for reading on a filedescriptor, and so on.
这些事件可以是诸如计时器，信号，文件描述符上预备读取的数据，等等。
3
Monitoring applications may read event data from deleted monitor files while the filedescriptor for the monitor file is still open but may not block for further event occurrences.