应该要控制精度的。
double fmode(double x, double y)
{
    return x-floor(x/y)*y;
}
int dblcmp(double d){
    if(fabs(d)<EPS)
        return 0;
    return d>0?1:-1;
}
double fgcd(double a, double b)
{
   double t;
   if(dblcmp(a-b) == 1) 
   {
       t=a;
       a=b;
       b=t;
   }
   if(dblcmp(a) == 0) return b;
   return fgcd(fmod(b,a),a);
}