1、listen fd，有新連接請求，觸發EPOLLIN。
2、對端發送普通數據，觸發EPOLLIN。
3、帶外數據，只觸發EPOLLPRI。
4、對端正常關閉（程序里close()，shell下kill或ctr+c），觸發EPOLLIN和EPOLLRDHUP，但是不觸發EPOLLERR和EPOLLHUP。
關于這點，以前一直以為會觸發EPOLLERR或者EPOLLHUP
。
再man epoll_ctl看下后兩個事件的說明，這兩個應該是本端（server端）出錯才觸發的。
5、對端異常斷開連接（只測了拔網線），沒觸發任何事件。
附man：
EPOLLIN
The associated file is available for read(2) operations.
EPOLLOUT
The associated file is available for write(2) operations.
EPOLLRDHUP
Stream socket peer closed connection, or shut down writing half
of connection. (This flag is especially useful for writing sim-
ple code to detect peer shutdown when using Edge Triggered moni-
toring.)
EPOLLPRI
There is urgent data available for read(2) operations.
EPOLLERR
Error condition happened on the associated file descriptor.
epoll_wait(2) will always wait for this event; it is not neces-
sary to set it in events.
EPOLLHUP
Hang up happened on the associated file descriptor.
epoll_wait(2) will always wait for this event; it is not neces-
sary to set it in events.
EPOLLET
Sets the Edge Triggered behavior for the associated file
descriptor. The default behavior for epoll is Level Triggered.
See epoll(7) for more detailed information about Edge and Level
Triggered event distribution architectures.
EPOLLONESHOT (since Linux 2.6.2)
Sets the one-shot behavior for the associated file descriptor.
This means that after an event is pulled out with epoll_wait(2)
the associated file descriptor is internally disabled and no
other events will be reported by the epoll interface. The user
must call epoll_ctl() with EPOLL_CTL_MOD to re-enable the file
descriptor with a new event mask.