网络IO之阻塞、非阻塞、同步、异步总结

参考资料：

http://www.open-open.com/doc/view/cbb2c3363c3b49ceb5812220a9c42e42

http://blog.csdn.net/historyasamirror/article/details/5778378

http://www.zhihu.com/question/19732473

http://www.ibm.com/developerworks/cn/linux/l-async/