EPOLLPRI when does this case happen?

Question

When I set my epoll option, I can see EPOLLPRI.

It expalins there is urgent "read" required.

When does this actually occur?

Is there any way to send in that PRI Mode?

Answer 1

EPOLLPRI in epoll(7) as well as POLLPRI in poll(2) is used to receive these urgent data.

Sometimes it's necessary to send high-priority (urgent) data over a connection that may have unread low-priority data at the other end. For example, a user interface process may be interpreting commands and sending them on to another process through a stream connection. The user interface may have filled the stream with as yet unprocessed requests when the user types a command to cancel all outstanding requests. Rather than have the high-priority data wait to be processed after the low-priority data, you can have it sent as out-of-band (OOB) data or urgent data.

A Socket-based IPC Tutorial

To send OOB specify MSG_OOB flag in send(2).