务器端基于select I/O模型。为防止程序界面阻塞，有一个子线程用于不断接收socket并select其中的处理。客户端只有一个线程函数，不过其利用率很高，可用于远程目录交换，请求文件大小，创建若干线程来下载文件。文件传输有上传和下载,还有对等传输，这个项目中，传输文件具体指下载。-Client server based on select I/O model. Programming interfaces in order to prevent obstruction, there is a sub-thread for receiving socket and select which treatment. There is only one client thread function, but its utilization very high, can be used for remote directory exchange, request the file size, create a number of threads to download the file. File transfer are uploading and downloading, as well as peer-to-peer transmission, this project, the specific means to transfer files to download.