［blocking network call］
阻塞的网络调用：
1、gethostbyname（）： does not return until it has succeeded or failed in resolving www.xxx.com
2、 connect（）： does not return until it has connected
3、 recv（）： calls do not return until they have received data or a close
4、 send（）： call does not return until it has at least flushed its output to the kernel’s write buffers
