this is a gcc file, you can download it and learn its usetility, for further detail please look at t

				f (double *ty)				{				  *ty = -1.0;				}								main ()				{				  double foo[6];				  double tx = 0.0, ty, d;								  f (&ty);								  if (ty < 0)				    ty = -ty;				  d = (tx > ty) ? tx : ty;				  if (ty != d)				    abort ();				  exit (0);				}