前面遗留的两个问题：

1.一个已连接UDP套接字能且仅能与一个对端交换数据报，那么客户端发送广播的时候如何防止recvfrom方法阻塞；

2.服务端忙的时候，已连接的UDP套接字也会被阻塞。

方法一：设置超时

UNP 14.2

There are three ways to place a timeout on an I/O operation involving a socket:

1.Callalarm, which generates theSIGALRMsignal when the specified time hasexpired. This involves signal handling, which can differ from oneimplementation to the next, and it may interfere with other existing calls toalarmin the process.

2.Block waiting for I/O inselect, which has a time limit built-in, instead ofblocking in a call toread orwrite.

3.Use the newerSO_RCVTIMEO andSO_SNDTIMEO socket options. The problemwith this approach is that not all im