What does ... means in c code

Question

I'm reading the code of wiredtiger. I see a function definition as

WT_CURSOR::set_key(WT_CURSOR * cursor, ...)

what does the '...' means here? how can the compiler compile such code?

Thanks

Answer 1

It means that the function accepts a variable number of arguments after the named arguments (possibly zero). The function would use a va_list and the associated functions (va_start, va_arg, and va_end) to process the arguments.

An example:

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

// n: number of doubles
// ... list of doubles
double average(int n, ...)
{
    double accum = 0.0;
    int i;
    va_list vl;
    va_start(vl, n);

    for(i = 0; i < n; i++)
    {
        accum += va_arg(vl, double);
    }
    va_end(vl);
    return accum/(double)n;
}

int main()
{
    double avg = average(5, 1.0, 1.0, 6.5, 3.3, -5.8);
    printf("%f\n", avg);
    return 0;
}

Output: 1.200000