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.
* }: _4 T: ]: w! W/ ~  K$ Y下面是ARM Compiler6文档中对printf的介绍:

The printf family consists of _printf(), printf(), _fprintf(), fprintf(), vprintf(), and vfprintf().
" m- i+ Z" M3 i" U( ^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.
1 D4 {! N  N  I! y7 \: VThe standard output functions of the form _printf(…) are equivalent to:
. z, T5 y( R! N4 Z+ Ifprintf(& __stdout, …)where __stdout has type __FILE.

6 h" r* C. g$ J6 h$ t) @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.

! _* ^2 \* Z8 y6 k7 W. n" cIf 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.

  l% q; |$ T! ]5 p重新定义__FILE, fputc(), ferror(), __stdout, 就可以使用printf()家族的各种函数了.
: z( f4 e! ~7 a' b5 E3 E
STM32中printf重定向
  p5 z1 i1 v8 u; v$ ?5 T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>

0 [1 c5 [9 X2 X0 U2 c' e: E) W

1. #include <stdio.h>
   0 ]3 ^, C$ {6 p  j& u( \$ U
2. struct __FILE
   . n7 c+ N& O# N' i' _( ?# I6 R; v
3. {
   
4.   int handle;
   
5.   /* Whatever you require here. If the only file you are using is */
   & Z- O2 K' i/ w1 ]; |
6.   /* standard output using printf() for debugging, no file handling */
   
7.   /* is required. */
   8 b6 m# ~% @, I, c5 }6 D
8. };
   
9. /* FILE is typedef’d in stdio.h. */
   
10. FILE __stdout;
    , j& S3 T, g+ T" I6 D+ _* Q
11. int fputc(int ch, FILE *f)
    3 I( U8 h7 ]& k+ D# U
12. {
    
13.   /* Your implementation of fputc(). */
    
14.   return ch;
    0 z- m* @* A% J* x
15. }
    
16. int ferror(FILE *f)
    
17. {
    
18.   /* Your implementation of ferror(). */
    
19.   return 0;
    
20. }

复制代码

+ e0 y5 Y; G! f- c/ f- \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.

. i% Q( ~! Q) U* W

1. #include <stdio.h>
   
2. struct __FILE
   
3. {
   
4.   int handle;
   * v, ?4 F9 }" @% `- _
5.   /* Whatever you require here. If the only file you are using is */
   
6.   /* standard output using printf() for debugging, no file handling */
   8 `5 Z8 E2 J2 a5 I1 R
7.   /* is required. */
   5 ~0 `* e6 z& |: D) ^4 u9 s
8. };
   ( Z/ }6 \0 X) n, j( R. p, |; J: g
9. /* FILE is typedef’d in stdio.h. */
   : S7 a. I) h9 V9 a, m, Y4 i
10. FILE __stdout;
    7 i; K1 [& S% e, r8 h# O
11. int fputc(int ch, FILE *f)
    
12. {
    
13.   /* Your implementation of fputc(). */
    
14.   HAL_UART_Transmit( &huart3, ( uint8_t* )&ch, 1, 0xFFFF );
    ( l1 r' Q& ]( x- M- C/ \
15.   return ch;
    ( p3 [0 Y, Y; |0 m! ]6 ?$ ]5 l
16. }
    
17. int ferror(FILE *f)
    
18. {
    
19.   /* Your implementation of ferror(). */
    
20.   HAL_UART_Transmit( &huart3, ( uint8_t* )"Printf Error\n", 13, 0xFFFF );// 可选
    
21.   return 0;
    ! x4 v  \3 [3 j9 x1 `# r+ q
22. }

复制代码

9 K# P9 `& T2 j+ [7 r+ x/ s1 z写个简单的程序试一试:
2 q6 H  }; b' r: E" Q2 V

1. int main(void)
   0 A4 O4 h* t8 ~' k! Z/ J8 w
2. {
   * `. V! b9 z* \2 }& c  Q( W5 ?
3.     MPU_Config();
   : `5 W% Y/ B' k
4.     SCB_EnableICache();
   
5.     SCB_EnableDCache();
   9 n; d" Q# \% L1 A) u  x6 z" u
6.     HAL_Init();
   . E* A- i/ j! H8 v9 D2 k/ z" [
7.     SystemClock_Config();
   
8.     MX_GPIO_Init();
   0 S( P1 P) U/ I
9.     MX_ETH_Init();
   ) W# f+ h* l3 X. A( m# S% q
10.     MX_USART3_UART_Init();
    $ r. @0 r, p0 f: R
11.     /* USER CODE BEGIN 2 */
    
12.         double d = 1.234;
    
13.         int i = 60;
    
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);
    4 [) A) V3 Z5 y/ a! x0 k$ A1 I
18. 
    
19.     while (1){}
    
20. }

复制代码

$ i4 k  [0 j5 w6 c4 ^0 }( z输出结果:
( b1 A! v9 u- L- n: g

+ x& q  r' x8 ?  {$ E0 H0 b8 k5 Y, o' [可以开始愉快的使用printf()了.
: I+ B- X/ y6 P- j# G& {
( C/ K$ ^2 G3 k3 x& D5 w

4 j- R& w* |9 J* O& ~7 n