struct quad_params {
  double p, q, r;  /* coefficients of quadratic:
                      p*x*x+q*x+r */
};

double quadratic(void *params, double x)
{
  struct quad_params *qp;
  qp = (struct quad_params *)params;
  return x*(qp->p*x+qp->q) + qp->r;  /* Horner's rule */
}