Named parameters and default argument values in C

named_params.c

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

struct rectangle {
    double x1, x2, y1, y2;
};

double area(struct rectangle r)
{
    return (r.x2 - r.x1) * (r.y2 - r.y1);
}

#define area(...) area((struct rectangle) \
    { .x1 = 1.0, .x2 = 4.0, .y1 = 1.0, .y2 = 5.0, __VA_ARGS__})

int main(void)
{
    printf("%g\n", area());
    printf("%g\n", area(.x1 = 0.0));
    printf("%g\n", area(.x2 = 2.5, .y2 = 10.5));
    
    return 0;
}