1、listen fd，有新连接请求，触发EPOLLIN。
2、对端发送普通数据，触发EPOLLIN。
3、带外数据，只触发EPOLLPRI。
4、对端正常关闭（程序里close()，shell下kill或ctr+c），触发EPOLLIN和EPOLLRDHUP，但是不触发EPOLLERR和EPOLLHUP。
关于这点，以前一直以为会触发EPOLLERR或者EPOLLHUP
。
再man epoll_ctl看下后两个事件的说明，这两个应该是本端（server端）出错才触发的。
5、对端异常断开连接（只测了拔网线），没触发任何事件。
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.