说明：  FTP服务器示例 实现客户发出请求下载服务端文件的功能 服务器能同时响应多个客户的请求 编译方式: cc TcpSer.c -o TcpSer -lsocket
(example FTP server for customers download service request issued by the document-server functionality can also respond to the request of several customers compiler approach : TcpSer.c cc-o TcpSer- lsocket)