一、Poll什么？
Poll某个fd特定的event。
二、event的总类包括？
POLLIN – This bit must be set if the device can be read without blocking.
POLLRDNORM – This bit must be set if “normal” data is available for reading. A readable device returns (POLLIN | POLLRDNORM).
POLLRDBAND – This bit indicates that out-of-band data is available for reading from the device. It is currently used only in one place in the Linux kernel (the DECnet code) and is not generally applicable to device drivers.
POLLPRI – High-priority data (out-of-band) can be read without blocking. This bit causes select to report that an exception condition occurred on the file
