at91编译错误之semihosting

MDK编译at91sam9261-ek的basic-fatfs-project，出现错误：

.\at91sam9261-sdram\at91sam9261-sdram.axf: Error: L6915E: Library reports error: __use_no_semihosting_swi was requested, but _sys_open was referenced

解决办法：

在retarget.c文件中，做如下定义：

struct __FILE { int handle;} ;

FILE __stdout;
FILE __stdin;
FILE __stderr;

重新编译即可。