Dump question! Did the interviewer ask you to write DOS on 8 bit processor?
How many formats do you have to deal with in printf()?

Agree - not acceptable. Whoever asked you is a duh!
May be he wanted to hear about varargs - and how you efficiently code integers into strings...but that is it. Nothing more.

i agree.....its not possible to code the whole function during an interview, the interviewer might have been interested in knowing about stdarg.h etc...

a lame attempt using stdarg

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include<string.h>


#define STDOUT 1
#define MAX_INT_LEN 10
//function foo taken from manpages of stdarg
void Printf(char *fmt, ...)
 {
    va_list ap;
    int d;
    char c, *s;
    va_start(ap, fmt);
    while (*fmt)
        switch (*fmt++) {
            case 's': /* string */
                s = va_arg(ap, char *);
                write(STDOUT, s, strlen(s));
                break;
            case 'd': /* int */
                d = va_arg(ap, int);
                s = malloc(MAX_INT_LEN);
                sprintf(s, "%d", d); // lame attempt to convert int to string, no itoa() in linux :(
                write(STDOUT, s, strlen(s));
                free(s);
                break;
            case 'c': /* char */
                /* need a cast here since va_arg only
                   takes fully promoted types */
                c = (char) va_arg(ap, int);
                write(STDOUT, &c, 1);
                break;

}
    va_end(ap);
}

int main() {
    Printf("%s %d %c", "hello world", 123456 , 's');
}