C語言之linux內核可變參實現printf,sprintf

昨天，我發表了一篇用可變參實現的fprintf函數,其實說實話還不完全是可變參實現的，因為用到了FILE * 這樣的指針，需要包含stdio.h這個頭文件才能實現這個函數，今天我們就來看看，如何拋棄stdio.h，全0開始實現printf , sprintf ,當然，這段代碼是我在linux內核裡面獲取的，再經過我本人修改，移植，在DevC++這個編譯環境中通過測試。我們來看看代碼:

#include 
#define NULL 0
//如果字符串中為數字，則返回數字 
static int skip_atoi(const char **s)
{
	int i = 0;

	while (isdigit(**s))
		i = i * 10 + *((*s)++) - '0';
	return i;
}

static inline int isdigit(int ch)
{
	return (ch >= '0') && (ch <= '9'); //返回從字符中提取0-9的數字
}	

#define ZEROPAD	1		/* pad with zero */
#define SIGN	2		/* unsigned/signed long */
#define PLUS	4		/* show plus */
#define SPACE	8		/* space if plus */
#define LEFT	16		/* left justified */
#define SMALL	32		/* Must be 32 == 0x20 */
#define SPECIAL	64		/* 0x */

//這個宏主要用來實現判斷是要轉化成什麼進制數 
#define __do_div(n, base) ({ \
int __res; \
__res = ((unsigned long) n) % (unsigned) base; \
n = ((unsigned long) n) / (unsigned) base; \
__res; })

static char *number(char *str, long num, int base, int size, int precision,
		    int type)
{
	/*這個字符串數組存放著0-15這16個數字，到時候要用來進制轉換*/
	static const char digits[16] = "0123456789ABCDEF"; 

	char tmp[66];
	char c, sign, locase;
	int i;

	/*locase = 0 或者 0x20 , 產生與locase相同的數字或字母，也許字母是小寫的*/
	locase = (type & SMALL);
	if (type & LEFT)
		type &= ~ZEROPAD;
	if (base < 2 || base > 36)
		return NULL;
	c = (type & ZEROPAD) ? '0' : ' ';
	sign = 0;
	if (type & SIGN) {
		if (num < 0) {
			sign = '-';
			num = -num;
			size--;
		} else if (type & PLUS) {
			sign = '+';
			size--;
		} else if (type & SPACE) {
			sign = ' ';
			size--;
		}
	}
	//檢測進制數，是要2進制還是要8進制還是16進制 
	if (type & SPECIAL) {
		if (base == 16)
			size -= 2;
		else if (base == 8)
			size--;
	}
	i = 0;
	if (num == 0)
		tmp[i++] = '0';
	else
		while (num != 0)
			tmp[i++] = (digits[__do_div(num, base)] | locase);
	if (i > precision)
		precision = i;
	size -= precision;
	if (!(type & (ZEROPAD + LEFT)))
		while (size-- > 0)
			*str++ = ' ';
	if (sign)
		*str++ = sign;
	if (type & SPECIAL) {
		if (base == 8)
			*str++ = '0';
		else if (base == 16) {
			*str++ = '0';
			*str++ = ('X' | locase);
		}
	}
	if (!(type & LEFT))
		while (size-- > 0)
			*str++ = c;
	while (i < precision--)
		*str++ = '0';
	while (i-- > 0)
		*str++ = tmp[i];
	while (size-- > 0)
		*str++ = ' ';
	return str;
}

int vsprintf(char *buf, const char *fmt, va_list args)
{
	int len;
	unsigned long num;
	int i, base;
	char *str;
	const char *s;
	int flags;		
	int field_width;	/*位寬輸出*/
	int precision;		
	int qualifier;		
	//這裡判斷，如果在字符串fmt中不存在%這個符號，那麼字符串繼續往後遍歷 
	for (str = buf; *fmt; ++fmt) {
		if (*fmt != '%') {
			*str++ = *fmt;
			continue;
		}

		//程序設置標志位 
		flags = 0;
	      repeat:
		++fmt;		/* this also skips first '%' */
		//格式控制 
		switch (*fmt) {
		case '-':
			flags |= LEFT;
			goto repeat;
		case '+':
			flags |= PLUS;
			goto repeat;
		case ' ':
			flags |= SPACE;
			goto repeat;
		case '#':
			flags |= SPECIAL;
			goto repeat;
		case '0':
			flags |= ZEROPAD;
			goto repeat;
		}

		//獲取寬度，這裡主要是要實現printf的位寬機制 
		field_width = -1;
		if (isdigit(*fmt))
			field_width = skip_atoi(&fmt);
		else if (*fmt == '*') {
			++fmt;
			field_width = va_arg(args, int);
			if (field_width < 0) {
				field_width = -field_width;
				flags |= LEFT;
			}
		}

		precision = -1;
		if (*fmt == '.') {
			++fmt;
			if (isdigit(*fmt))
				precision = skip_atoi(&fmt);
			else if (*fmt == '*') {
				++fmt;
				precision = va_arg(args, int);
			}
			if (precision < 0)
				precision = 0;
		}

		/*得到的轉換限定符*/
		qualifier = -1;
		if (*fmt == 'h' || *fmt == 'l' || *fmt == 'L') {
			qualifier = *fmt;
			++fmt;
		}

		/*默認進制為10進制*/
		base = 10;
		//以下主要是要實現printf的格式輸出 例如：%d , %c , %u ...等等 
		switch (*fmt) {
		case 'c':  //以字符形式進行輸出 
			if (!(flags & LEFT))
				while (--field_width > 0)
					*str++ = ' ';
			*str++ = (unsigned char)va_arg(args, int);
			while (--field_width > 0)
				*str++ = ' ';
			continue;

		case 's': //以字符串形式進行輸出 
			s = va_arg(args, char *);
			len = strnlen(s, precision);

			if (!(flags & LEFT))
				while (len < field_width--)
					*str++ = ' ';
			for (i = 0; i < len; ++i)
				*str++ = *s++;
			while (len < field_width--)
				*str++ = ' ';
			continue;

		case 'p': //以地址形式輸出，也就是以16進制數輸出 
			if (field_width == -1) {
				field_width = 2 * sizeof(void *);
				flags |= ZEROPAD;
			}
			str = number(str,
				     (unsigned long)va_arg(args, void *), 16,
				     field_width, precision, flags);
			continue;

		case 'n':
			if (qualifier == 'l') {
				long *ip = va_arg(args, long *);
				*ip = (str - buf);
			} else {
				int *ip = va_arg(args, int *);
				*ip = (str - buf);
			}
			continue;

		case '%':     //這裡表示字符串中存在%號這個字符 
			*str++ = '%';
			continue;

			/* integer number formats - set up the flags and "break" */
		case 'o':     //%o 表示8進制輸出 
			base = 8;
			break;

		case 'x':     //%x或者%X 表示16進制輸出 
			flags |= SMALL;
		case 'X':
			base = 16;
			break;

		case 'd':     //%d %i整形數輸出，%u無符號整形 
		case 'i':
			flags |= SIGN;
		case 'u':
			break;

		default:
			*str++ = '%';
			if (*fmt)
				*str++ = *fmt;
			else
				--fmt;
			continue;
		}
		if (qualifier == 'l') //以無符號長整型輸出 
			num = va_arg(args, unsigned long);
		else if (qualifier == 'h') {
			num = (unsigned short)va_arg(args, int);
			if (flags & SIGN)
				num = (short)num;
		} else if (flags & SIGN)
			num = va_arg(args, int);
		else
			num = va_arg(args, unsigned int);
		str = number(str, num, base, field_width, precision, flags);
	}
	*str = '\0';   //字符串遍歷到有\0的地方就停止 
	return str - buf;
}
//可變參形式實現sprintf 
int mysprintf(char *buf, const char *fmt, ...)
{
	va_list args;
	int i;
	va_start(args, fmt);
	//將獲取到的fmt格式字符串寫入到buf這個緩存裡去 
	i = vsprintf(buf, fmt, args);
	//釋放args 
	va_end(args);
	return i;
}

//可變參形式進行實現myprintf 
int myprintf(const char *fmt, ...)
{
	char printf_buf[1024];
	va_list args;
	int printed;
	va_start(args, fmt);
	printed = vsprintf(printf_buf, fmt, args);
	va_end(args);
	puts(printf_buf);
	return printed;
}

int main(void)
{
	myprintf("輸出字符串：hello world!\n")	;
	static int sum , a = 3 , b = 4; 
	sum = a + b ; 
	myprintf("sum(十進制輸出):%d\n",sum);
	myprintf("sum(16進制輸出):%p\n",sum);
	char buffer[128] = {0};
	//將字符串存到一個數組buffer裡去 
	mysprintf(buffer , "輸出字符串：hello world！\n");
	//以字符串格式輸出這個buffer的內容 
	myprintf("buffer:%s\n",buffer);
	return 0 ;
}

看完代碼就知道了，我們這個程序沒有包含stdio.h這個頭文件，一樣也就實現了printf和sprintf這兩個函數。這樣的話，以後如果要自己實現一個printf函數，這份代碼就可以作為一個文件來進行調用了，哈哈!