可变参数函数的实现原理

比较下面的程序：
#include "stdafx.h"
#include <conio.h>
#include <stdarg.h>
int Print ( char *str, ...);
int main()//主函数
{
int i;
i = Print("Integer:%d string:%s float:%f %t", 1, "this is test.", 20.2);
printf("\n%d%t", i);
_getche();
return 0;
}
int Print ( char *str, ...)
{
char *ap;
char *p;
int argd;
char *args;
double argf;
int count;
ap = (char *)&str+sizeof(str);
//va_start(ap, str);
p = str;
count = 0;
while(*p)
{
if(*p=='%')
{
p++;
switch(*p)
{
case 'd':
argd = *(int *)((ap += sizeof(int)) - sizeof(int));
count += printf("%d", argd);
break;
case 's':
args = *(char **)((ap += sizeof(char *)) - sizeof(char *));
count += printf("%s", args);
break;
case 'f':
argf = *(double *)((ap += sizeof(double)) - sizeof(double));
count += printf("%f", argf);
break;
default:
_putch(*p);
count++;
break;
}
}
else
{
_putch(*p);
count++;
}
p++;
}
ap=0;
return count;
}