How to loop through a va_list if the number of arguments is unknown?

Question

How do I loop through a va_list if the number of additional arguments is unknown?

#include 
#include 

int add(int x, int y, ...         


        

Answer 1

The two answers above that suggest adding a first argument to indicate the number of arguments miss the point: the number of arguments is unknown.

Using a sentinel value (or series of sentinel values) at the end is the only way to solve the problem.