Вершины в порядке обхода по часовой стрелке - C (СИ)

#include <stdlib.h>
#include <stdio.h>
 
typedef struct Xy
{
  int x, y;
} Xy;
 
static const Xy *xy_origin;
 
int cmp_polar_angle(const void *p1, const void *p2)
{
  const Xy *xy1 = p1, *xy2 = p2;
 
  Xy 
    origin_xy1 = { xy1->x - xy_origin->x, xy1->y - xy_origin->y },
    origin_xy2 = { xy2->x - xy_origin->x, xy2->y - xy_origin->y };
 
  long long area = 
    (long long) origin_xy1.x * origin_xy2.y - 
    (long long) origin_xy1.y * origin_xy2.x;
 
  return (area > 0) - (area < 0);
}
 
int main()
{
  Xy poly[] = { { 0, 0 }, { 2, 5 },  { -3, -1 }, { 0, 9 }, { -5, 3 } };
  const unsigned n = sizeof poly / sizeof *poly;
 
  /* Find min-y point */
 
  for (unsigned i = 1; i < n; ++i)
    if (poly[i].y < poly[0].y)
    {
      Xy temp = poly[0];
      poly[0] = poly[i];
      poly[i] = temp;
    }
 
  /* Sort the vertices */
 
  xy_origin = &poly[0];
  qsort(poly + 1, n - 1, sizeof *poly, cmp_polar_angle);
 
  /* Done */
 
  for (unsigned i = 0; i < n; ++i)
    printf("{ %d, %d } ", poly[i].x, poly[i].y);
  printf("\n");
}