C/C++ :: Unlimited Number Of Arguments - Float To String Conversion

So, I'm supposed to do : Create a function with unlimited number of arguments, which forms a dynamic string based on the following form (%d, %s, %f, %lf, %c), with the following prototype:

char*create(char*form, ...);

The function is supposed to have the following output:

create("Peter is %d years old and is in %s-%c class.",7,"second",'A');
-> Peter is 7 years old and is in 7-A class.
create("His GPA is %lf.",4.96);
-> His GPA is 4.96.
create("His favourite subject is math!");
-> His favourite subject is math!

I've managed to do the following :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
char *create(char *form, ...) {
char *res =(char*)calloc(1,1),*pos_int,*pos_float,*pos_str,pos_char,*pos_long;

[Code] ....

The part with %d and %s string was not that hard, but now I'm supposed to convert %f and %lf to string, I've tried using sprintf but I've had no luck so far, another problem is the fact that I've gotta use lists to complete the task. I've been trying to convert float to string for the past 2 hours, but I'm drawing a blank now.