error: #20: identifier "FILE" is undefined

问题：

..\SYSTEM\usart\usart.c(26): error:  #20: identifier "FILE" is undefined

  FILE __stdout;       
..\SYSTEM\usart\usart.c(33): error:  #20: identifier "FILE" is undefined

  int fputc(int ch, FILE *f)

解决：

把FILE再usart1.h中重新定义typedef struct __FILE FILE;