各位我找解决办法了
#pragma import(__use_no_semihosting)
_sys_exit(int x)
{
x = x;
}
struct __FILE
{
int handle;
/* Whatever you require here. If the only file you are using is */
/* standard output using printf() for debugging, no file handling */
/* is required. */
};
/* FILE is typedef’ d in stdio.h. */
FILE __stdout;
加在USART。c就好了