double scalbn(double x, int exp);
scalbn multiplies a floating-point number by an integral power of DBL_RADIX.
As floating-point arithmetic conforms to IEC 60559, DBL_RADIX is 2 and scalbn is (in this implementation) identical to ldexp.
scalbn returns x * DBL_RADIX^exp.
If the result overflows, errno is set to ERANGE and scalbn returns HUGE_VAL.
If x is ∞ or NaN, scalbn returns x.
If the result overflows, scalbn returns ∞.