C语言变长参数解释（stdarg.h）

typedef char *va_list;

/* 参数压栈是双字对齐，从左到右，从低地址到高地址的 */

/* Amount of space required in an argument list for an arg of type TYPE.
   TYPE may alternatively be an expression whose type is used. */
#define __va_rounded_size(TYPE) \
   (((sizeof (TYPE) + sizeof (int) - 1) / sizeof (int)) * sizeof (int))
 
/* AP指向可变参数 */
#define va_start(AP, LASTARG) \
  (AP = ((char *) &(LASTARG) + __va_rounded_size (LASTARG)))

void va_end (va_list AP)
{
     AP = 0;
}
#define va_end(AP)

/* AP 指向下一个参数, 然后给出当前参数的地址 */
#define va_arg(AP, TYPE) \
 (AP += __va_rounded_size (TYPE), \
  *((TYPE *) (AP - __va_rounded_size (TYPE))))