Variable Number of Arguments - General Questions

What will be the output of the program?

#include <stdio.h>
#include <stdarg.h>
int count(const char *first, ...) {
    va_list ap;
    int n = 0;
    const char *s = first;
    va_start(ap, first);
    while (s) { n++; s = va_arg(ap, const char *); }
    va_end(ap);
    return n;
}
int main(void) {
    printf("%d\n", count("a", "b", "c", (char *)NULL));
    return 0;
}

Which standard library function is variadic?

Which macro copies a va_list so it can be traversed twice?

In a variadic call, a float argument is passed as:

What will be the output of the program?

#include <stdio.h>
#include <stdarg.h>
int maxof(int n, ...) {
    va_list ap;
    int i, m, v;
    va_start(ap, n);
    m = va_arg(ap, int);
    for (i = 1; i < n; i++) { v = va_arg(ap, int); if (v > m) m = v; }
    va_end(ap);
    return m;
}
int main(void) {
    printf("%d\n", maxof(4, 3, 9, 2, 7));
    return 0;
}