Variable Number of Arguments - General Questions

Which standard library function is variadic?

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;
}

What will be the output of the program?

#include <stdio.h>
#include <stdarg.h>
double avg(int n, ...) {
    va_list ap;
    double s = 0;
    int i;
    va_start(ap, n);
    for (i = 0; i < n; i++) s += va_arg(ap, double);
    va_end(ap);
    return s / n;
}
int main(void) {
    printf("%.1f\n", avg(3, 1.0, 2.0, 6.0));
    return 0;
}

Which header provides va_list, va_start, va_arg and va_end?

Which macro must be called before a variadic function returns?