va_start로 고정의 매개변수가 없는 함수에서의 가변 매개변수를 어떻게 사용하나요?

EXAMPLES
       The function foo takes a string of format characters and prints out the
       argument associated with each format character based on the type.
              #include <stdio.h>
              #include <stdarg.h>
 
              void foo(char *fmt, ...) {
                   va_list ap;
                   int d;
                   char c, *p, *s;
 
                   va_start(ap, fmt);
                   while (*fmt)
                        switch(*fmt++) {
                        case ’s’:           /* string */
                             s = va_arg(ap, char *);
                             printf("string %s\n", s);
                             break;
                        case ’d’:           /* int */
                             d = va_arg(ap, int);
                             printf("int %d\n", d);
                             break;
                        case ’c’:           /* char */
                             /* need a cast here since va_arg only
                                takes fully promoted types */
                             c = (char) va_arg(ap, int);
                             printf("char %c\n", c);
                             break;
                        }
                   va_end(ap);
              }