操作系统开发中printf函数的简单实现

虽然没有一行注释，相信研究方向是操作系统的你也能够看懂吧，是啊，它实在是太傻太天真了，我真的希望自己的代码很慌很保利！ 

// printf.c
#include "stdio.h"
 
unsigned int strlen_(const char* s);
unsigned int div_(unsigned int* i, unsigned int n);
void i2a(char* buf, unsigned int a, unsigned int b);
int vsprintf_(char* buf, const char* format_s, char* temp);
int printf_(const char* format_s, ...);
 
 
int main(void) {
	printf_(" %%% %%%% %c %s %d %% %x %b ",
	'L', "Hello, World!...", 0xff, 256, 65535);	
}
 
unsigned int strlen_(const char* s) {
	const char* t = s;
	while(*t) {
		t++;
	}
	return t - s;
}
 
unsigned int div_(unsigned int* i, unsigned int n) {
	unsigned int res = *i % n;
	*i /= n;
	return res;
}
 
void i2a(char* buf, unsigned int a, unsigned int b) {
	char* res = buf, * p;
	if(!a) {
		*res++ = '0';
		*res = '\0';
		return;
	}
	const char num[32] = "0123456789abcdef";
	char t[0x100];
	p = t;
	while(a) {
		*p++ = num[div_(&a, b)];
	}
	*p = '\0';
	while(p != t - 1) {
		*res++ = *--p;
	}
	*res = '\0';
}
 
int vsprintf_(char* buf, const char* format_s, char* temp) {
	const char* s = format_s;
	char t[0x100], * p, * res = buf;
	unsigned int i;
	while(*s) {
		if(*s == '%') {
			s++;
			switch(*s) {
			case '%':
				*buf++ = *s;
				break;
			case 'c':
				i = *((char*)(temp+=4));
				*buf++ = (char)i;
				break;
			case 's':
				p = *((char**)(temp+=4));
				while(*p) {
					*buf++ = *p++;
				}
				break;
			case 'd':
				i = *((int*)(temp+=4));
				i2a(t, i, 10);
				p = t;
				while(*p) {
					*buf++ = *p++;
				}
				break;
			case 'x':
				i = *((int*)(temp+=4));
				i2a(t, i, 16);
				*buf++ = '0';
				*buf++ = 'x';
				p = t;
				while(*p) {
					*buf++ = *p++;
				}				
				break;
			case 'b':
				i = *((int*)(temp+=4));
				i2a(t, i, 2);
				p = t;
				while(*p) {
					*buf++ = *p++;
				}
				*buf++ = 'b';
				break;
			}
			s++;
		} else {
			*buf++ = *s++;
		}
	}
	*buf = '\0';
	return strlen_(res);
}
 
int printf_(const char* format_s, ...) {
	char t[0x400];
	char* temp = (char*)&format_s;
	vsprintf_(t, format_s, temp);
	temp = NULL;
	unsigned int i = 0;
	while(t[i]) {
		printf("%c", t[i++]);
	}
	return i;
}