C语言嵌入式实现支持浮点输出的printf示例详解

typedef   char  * va_list;                  //将char*别名为va_list;
#define   _INTSIZEOF(n)   ((sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1))  
#define   va_start(ap,v)   (ap = (va_list)&v + _INTSIZEOF(v))
#define   va_arg(ap,t)     (*(t*)((ap += _INTSIZEOF(t)) -  _INTSIZEOF(t)))
#define   va_end(ap)      (ap = (va_list)0)

typedef __builtin_va_list       __gnuc_va_list;
typedef __gnuc_va_list          va_list;
#define va_start(v,l)           __builtin_va_start(v,l)
#define va_end(v)               __builtin_va_end(v)
#define va_arg(v,l)             __builtin_va_arg(v,l)

int mr_printf(char *fmt, ...)
{
	va_list ap;
	char putc_buf[20]; //输出缓冲区，减少运算加速输出
	 unsigned int u_val;
	 int val, bits, flag;
	 double f_val;
	 char *str;
	 int res = 0;
	
  	va_start(ap,fmt);
	while(*fmt != '\0')
	{
		if(*fmt == '%')
		{
			++ fmt;
			"处理函数"
		}
		else
		{
			mr_putc(*fmt);
	        ++ res;
	        ++ fmt;
		}
	}
	
    va_end(ap);
    return res;
}

switch (*fmt)
{
}

case 'd':
	
    val = va_arg(ap,int);
    if(val < 0)						//判断正负
    {
      val = - val;
      mr_putc('-');
      ++ res;
    }
    
    bits = 0;
    while(val)
    {
      putc_buf[bits] = '0' + val % 10;		//获取整型位数的同时，将每一位按低位到高位存入缓冲区
      val /= 10;
      ++ bits;
    }
    res += bits;
    
    while (bits)
    {
      -- bits;
      mr_putc(putc_buf[bits]);			//将每一位从高到低从缓冲区中输出
    }
    ++ fmt;
    continue;

case 'u':
	
	u_val = va_arg(ap,unsigned int);
	
	bits = 0;
	while(u_val)
	{
	  putc_buf[bits] = '0' + u_val % 10;
	  u_val /= 10;
	  ++ bits;
	}
	res += bits;
	
	while (bits)
	{
	  -- bits;
	  mr_putc(putc_buf[bits]);
	}
	++ fmt;
	continue;

 
case 'x':
	 
	 u_val = va_arg(ap,unsigned int);
	 
	 bits = 0;
	 while(u_val)
	 {
	   putc_buf[bits] = '0' + u_val % 16;
	   if(putc_buf[bits] > '9')
	     putc_buf[bits] = 'A' + (putc_buf[bits] - '9' - 1);
	   u_val /= 16;
	   ++ bits;
	 }
	 res += bits;
	 
	 while (bits)
	 {
	   -- bits;
	   mr_putc(putc_buf[bits]);
	 }
	 ++ fmt;
	 continue;

case 'o':
	 
	 u_val = va_arg(ap,unsigned int);
	 
	 bits = 0;
	 while(u_val)
	 {
	   putc_buf[bits] = '0' + u_val % 8;
	   u_val /= 8;
	   ++ bits;
	 }
	 res += bits;
	 
	 while (bits)
	 {
	   -- bits;
	   mr_putc(putc_buf[bits]);
	 }
	 ++ fmt;
	 continue;

case 's':
	str = va_arg(ap,char *);
	while (*str != '\0')
	{
	  mr_putc(*str);
	  ++ str;
	  ++ res;
	}
	++ fmt;
	continue;

case 'c':
	mr_putc(va_arg(ap,int));
	++ res;
	++ fmt;
	continue;

if(*fmt == '.')
{
  ++ fmt;
  flag = (int)(*fmt - '0');
  ++ fmt;
}
else
{
  flag = 187; // N(46) + U(53) + L(44) + L(44) = NULL(187)
}

case 'f':
	
	f_val = va_arg(ap,double);
	if(f_val < 0)
	{
	  f_val = - f_val;						//判断正负
	  mr_putc('-');
	  ++ res;
	}
	
	val = (int)f_val;					// 分离整数和小数，整数将按上面处理整数的部分输出，小数部分单独处理
	f_val -= (double)val;
	
	bits = 0;
	if(val == 0)
	{
	  mr_putc('0');
	  ++ res;
	}
	while (val)
	{
	  putc_buf[bits] = '0' + val % 10;
	  val /= 10;
	  ++ bits;
	}
	res += bits;
	
	while (bits)
	{
	  --bits;
	  mr_putc(putc_buf[bits]);
	}
	
	if(flag != 0)
	{
	  mr_putc('.');
	  ++ res;
	}
	if(flag > 6)													//判断需要输出几位小数
	  flag = 6;
	val = (int)((f_val * 1000000.0f) + 0.5f);		//仅做一次浮点运算，同时对误差进行处理忽略最低几位小数引入的误差
	
	bits = 0;
	while (bits < 6)
	{
	  putc_buf[bits] = '0' + val % 10;					//使用输出整数的方法将小数输出出去
	  val /= 10;
	  ++ bits;
	}
	res += flag;
	
	while (flag)
	{
	  --flag;
	  -- bits;
	  mr_putc(putc_buf[bits]);
	}
	++ fmt;
	continue;