STM32 将 printf 重定向至 串口或自定义 printf 函数.

1. 实现串口发送字符串函数。

   /**
     * @brief USART1 send data
     * @param None
     * @retval None
     * @author shizj
     * @date  2019.07.10
     */
   inline void USART1_SendData(uint8_t data)
   {
     /* Loop until the end of transmission */
     while (USART_GetFlagStatus(USART1, USART_FLAG_TC) == RESET)
     {
       // __nop();
     }
    
     /* Write a character to the USART */
     USART_SendData(USART1, (uint16_t)data);  
   }
    
   /**
     * @brief USART1 send data
     * @param None
     * @retval None
     * @author shizj
     * @date  2019.07.10
     */
   void USART1_SendString(uint8_t* data, uint32_t size)
   {
     for(uint32_t i = 0; i < size; ++i)
     {
       USART1_SendData(*(data + i));
     }
   }
   

2. 自定义 printf 函数^[1]。

   /**
     * @brief Overridden cstd 'vprint' function
     * @param None
     * @retval None
     * @author shizj
     * @date  2019.07.10
     */
   void vprint(const char *fmt, va_list argp)
   {
     char string[200];
     if(0 < vsprintf(string, fmt, argp)) // build string
     {
       USART1_SendString((uint8_t*)string, strlen(string)); // send message via UART
     }
   }
   
   /**
     * @brief Custom my own printf function
     * @param None
     * @retval None
     * @author shizj
     * @date  2019.07.10
     */
   void UsartPrintf(const char *fmt, ...)
   {
     va_list argp;
     va_start(argp, fmt);
     vprint(fmt, argp);
     va_end(argp);
    }
   

   该方法同样出现在 STM32's official demos，官方样例很重要啊！

3. 使用原始 printf 函数，通过重载 putc 函数实现。

   #ifdef __GNUC__
     /* With GCC, 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
   {
     USART_SendData(USART1, (uint16_t)ch);
     return ch;
   }
   

   注意：该方法需要禁用 semihost：

   #pragma import(__use_no_semihosting)
   
   void _sys_exit(int x)
   {
     x = x;
   }
   
   struct __FILE
   {
     int handle;
   };
    
   FILE __stdout;
   

   否则 printf 会触发进入主机模式并尝试使用主机的 I/O 而导致程序卡住^[2]：

   Semihosting is a mechanism that enables code running on an ARM target to communicate and use the Input/Output facilities on a host computer that is running a debugger.

   Examples of these facilities include keyboard input, screen output, and disk I/O. For example, you can use this mechanism to enable functions in the C library, such as printf() and scanf(), to use the screen and keyboard of the host instead of having a screen and keyboard on the target system

参考链接

1. Bence Kaulics's answer - stackoverflow ↩

2. What is semihosting? - arm ↩