It seems that writes/reads to regular files can't not be made non-blocking. I found the following references for support:
from The Linux Programming Interface: A Linux and UNIX System Programming Handbook:
from Advanced Programming in the UNIX Environment 2nd Ed:
from http://www.remlab.net/op/nonblock.shtml:
When memory is adequately available, reads/writes is performed through kernel buffering.
简单来说，对于普通的磁盘文件来说，可读可写条件总是成立的（不像socket缓冲区空了就没法读，缓冲区满了就没法写），所以设置非阻塞模式并没有意义。
问题在于，虽然系统并不认为读／写普通文件会阻塞，可是实际上由于磁盘可能比较繁忙或者比较慢等因素，导致读／写事件延迟很长，同样带来阻塞的问题，
但这种问题却根本无法避免！