什么是“紧急数据”？
The man page of epoll_ctl() says about EPOLLPRI:
There is urgent data available for
read(2) operations.
How exactly is "urgent data" defined and who decides which data has priority?
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TCP具有发送带外数据的功能，也称为紧急数据。通常，TCP中的数据是基于流的；也就是说，接收方读取数据的顺序与发送方写入数据的顺序完全相同。发送方可能决定发送紧急数据，这可以跳过该流。
然而，它有几个实现问题，并且很少有协议或程序使用它（ telnet 是我所知道的唯一一个）。本质上它是一个遗迹，在现代程序中没有使用。
TCP has a feature for sending out-of-band data, also known as urgent data. Normally, data in TCP is stream based; that is, the receiver reads data in the exact same order that the sender wrote the data. The sender may decide to send urgent data, which can skip the stream.
However, it has several implementation problems and very, very few protocols or programs use it (telnet is the only one I'm aware of). Essentially it's a relic and not used in modern programs.
TCP 数据包可以包含标记为“紧急”的数据。这是 OOB 数据，与普通数据流分开。例如，请参阅有关此内容的维基百科文章。正如文章还指出的那样，它并不常用，实现也各不相同，依赖它可能是愚蠢的。
A TCP packet can contain data marked as 'urgent'. This is OOB data, separate from the normal data stream. See, for example, the wikipedia article on this. As the article also notes, it's not commonly used, implementations vary, and relying on it would probably be foolish.