va_start

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

int add_nums_C99(int count, ...)
{
    int result = 0;
    va_list args;
    va_start(args, count); // count can be omitted since C23
    
    for (int i = 0; i < count; ++i)
        result += va_arg(args, int);
    
    va_end(args);
    return result;
}

#if __STDC_VERSION__ > 201710L
// Same as above, valid since C23
int add_nums_C23(...)
{
    int result = 0;
    va_list args;
    va_start(args);
    
    int count = va_arg(args, int);
    for (int i = 0; i < count; ++i)
        result += va_arg(args, int);
    
    va_end(args);
    return result;
}
#endif

int main(void)
{
    printf("%d\n", add_nums_C99(4, 25, 25, 50, 50));
#if __STDC_VERSION__ > 201710L
    printf("%d\n", add_nums_C23(4, 25, 25, 50, 50));
#endif
}