printf重定向介绍
在学C语言的时候，会经常用标准库<stdio.h>中printf(), scanf()这两个函数, 来实现数据的输入与输出. 格式化输入输出的功能还是挺强大的, 那已经有了现成的轮子, 就要寻思一下如何进一步在单片机上使用它.
下面是ARM Compiler6文档中对printf的介绍:
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3 B/ i* z3 z2 X7 k" @The printf family consists of _printf(), printf(), _fprintf(), fprintf(), vprintf(), and vfprintf().
All these functions use __FILE opaquely and depend only on the functions fputc() and ferror(). The functions _printf() and _fprintf() are identical to printf() and fprintf() except that they cannot format floating-point values.
2 ^5 v1 W% ~- k, r; vThe standard output functions of the form _printf(…) are equivalent to:
fprintf(& __stdout, …)where __stdout has type __FILE.

) o; D/ p$ b. W4 q! @/ Y大致是说printf家族有这么些函数<_printf(), printf(), _fprintf(), fprintf(), vprintf(), and vfprintf()>, 这些函数都隐式的使用__FILE<文件流>, 并且只依靠fputc(), ferror()这两个函数.
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If you define your own version of __FILE, your own fputc() and ferror() functions, and the __stdout object, you can use all of the printf() family, fwrite(), fputs(), puts() and the C++ object std::cout unchanged from the library.
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0 h6 _# T6 b/ [3 a+ y4 Y重新定义__FILE, fputc(), ferror(), __stdout, 就可以使用printf()家族的各种函数了.

* |5 m4 `7 E! ^8 u  `0 g4 mSTM32中printf重定向
下面是官方给出的重映射模板:
<遇到不会的先看看文档往往能得到最直接的答案>
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1. #include <stdio.h>
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2. struct __FILE
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3. {
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4.   int handle;
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5.   /* Whatever you require here. If the only file you are using is */
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6.   /* standard output using printf() for debugging, no file handling */
   
7.   /* is required. */
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8. };
   
9. /* FILE is typedef’d in stdio.h. */
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10. FILE __stdout;
    
11. int fputc(int ch, FILE *f)
    
12. {
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13.   /* Your implementation of fputc(). */
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14.   return ch;
    
15. }
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16. int ferror(FILE *f)
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17. {
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18.   /* Your implementation of ferror(). */
    
19.   return 0;
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20. }

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要将printf()重定向到STM32中的串口, 只需要重新实现 fputc() 函数即可, 在fputc()中通过串口发送一个char. 在上一次的工程上进行修改, 将printf()重定向到串口3.
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1. #include <stdio.h>
   
2. struct __FILE
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3. {
   
4.   int handle;
   
5.   /* Whatever you require here. If the only file you are using is */
   
6.   /* standard output using printf() for debugging, no file handling */
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7.   /* is required. */
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8. };
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9. /* FILE is typedef’d in stdio.h. */
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10. FILE __stdout;
    
11. int fputc(int ch, FILE *f)
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12. {
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13.   /* Your implementation of fputc(). */
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14.   HAL_UART_Transmit( &huart3, ( uint8_t* )&ch, 1, 0xFFFF );
    
15.   return ch;
    
16. }
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17. int ferror(FILE *f)
    
18. {
    
19.   /* Your implementation of ferror(). */
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20.   HAL_UART_Transmit( &huart3, ( uint8_t* )"Printf Error\n", 13, 0xFFFF );// 可选
    
21.   return 0;
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22. }

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写个简单的程序试一试:

1. int main(void)
   
2. {
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3.     MPU_Config();
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4.     SCB_EnableICache();
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5.     SCB_EnableDCache();
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6.     HAL_Init();
   
7.     SystemClock_Config();
   
8.     MX_GPIO_Init();
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9.     MX_ETH_Init();
   
10.     MX_USART3_UART_Init();
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11.     /* USER CODE BEGIN 2 */
    
12.         double d = 1.234;
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13.         int i = 60;
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14.         char str[] = "Do not be lazy.\n";
    
15.         printf("Hello World\n");
    
16.         printf("d = %lf\ni = %d\n", d, i);
    
17.         printf("this is a string : %s", str);
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18. 
    
19.     while (1){}
    
20. }

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输出结果:

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" M6 D& z1 C. I+ p# X7 d可以开始愉快的使用printf()了.
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