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