线程同步与异步套接字编程，套接字在两种模式下执行操作，阻塞和非阻塞。在阻塞模式下，在操作完成前，执行操作的函数会一直等待下去，不会立即返回程序(将控制权交还给程序)。而在非阻塞模式下，函数无论如何都会立即返回。

-Threaded synchronous and asynchronous socket programming, socket perform the operation in two modes, blocking and non-blocking. In blocking mode before the operation is complete, the function will perform the operation wait forever, will not immediately return to the program (the control back to the program). And in non-blocking mode, the function returns immediately anyway.