一些实用的小函数记录

int _finite( 
   double x 
);

_finite returns a nonzero value if its argument x is not infinite; that is, if –INF < x < +INF. It returns 0 if the argument is infinite or a NAN.

int _isnan( 
   double x  
);
_isnan returns a nonzero value if the argument x is a NAN; otherwise it returns 0.