Inverse square root function

invsqrt.c

/*
 * http://www.beyond3d.com/content/articles/8/
 * http://www.beyond3d.com/content/articles/15/
 */
#include <stdio.h>
#include <math.h>

float inv_sqrt(float x)
{
  float xh = 0.5f*x;
  //unsigned i=*(unsigned *)&x;
  int i = *(int *) & x;
  i = 0x5f3759df - (i>>1);
  x = *(float *) & i;
  x = x*(1.5f - xh*x*x);
  return x;
}

int main(void)
{
  float x, xs1, xs_f;
  double xs_d;

  printf("Enter x: ");
  scanf("%f", &x);
  xs1 = inv_sqrt(x);
  xs_d = 1.0/sqrt(x);
  xs_f = (float)(xs_d);
  printf("x           = %.14f\n"
         "inv_sqrt(x) = %.14f\n"
         "c.f.        = %.14f(float)\n"
         "            = %.14f(double)\n"
         , x, xs1, xs_f, xs_d);

  return 0;
}