嵌入式linux中 功能：完成串口读写操作，这里设定从串口读取消息时使用select函数，发送消息的程序不需要使用select函数，只发送“hello”消息由接收端接收-Embedded linux in function: the completion of serial read and write operations, set here to read messages from the serial port using the select function to send a message, does not need to use the select function, just send a " hello"　message received by the receiver