1.设置

#include <stdio.h>

Options->GeneralOptions->LibraryConfiguration设置了semihosted 和via semihosting（swo）.
2. Printf不能被重定向到串口，否则将导致Terminal I/O无数据输出。
3. 打开Terminal I/O窗口，查看输出
view ->Terminal I/O

#ifdef KEIL
#pragma import(__use_no_semihosting)             
//标准库需要的支持函数                 
struct __FILE 
{ 
    int handle; 
}; 

FILE __stdout;       
//定义_sys_exit()以避免使用半主机模式    
void _sys_exit(int x) 
{ 
    x = x; 
} 
//重定义fputc函数 
int fputc(int ch, FILE *f)
{
    while((USART1->SR & 0X40)==0);//循环发送,直到发送完毕   
    USART1->DR = (u8) ch;      
    return ch;
}
#else //IAR
#ifdef __GNUC__
  /* With GCC/RAISONANCE, small printf (option LD Linker->Libraries->Small printf
     set to 'Yes') calls __io_putchar() */
  #define PUTCHAR_PROTOTYPE int __io_putchar(int ch)
#else
  #define PUTCHAR_PROTOTYPE int fputc(int ch, FILE *f)
#endif /* __GNUC__ */

PUTCHAR_PROTOTYPE
{
  /* Place your implementation of fputc here */
  /* e.g. write a character to the USART */
  USART_SendData(USART1, (uint8_t) ch);

  /* Loop until the end of transmission */
  while (USART_GetFlagStatus(USART1, USART_FLAG_TC) == RESET)
  {}

  return ch;
}

#endif