#include <math.h>
double scalbn(double x, int exp);
scalbn multiplies a floating-point number by an integral power of FLT_RADIX.
As floating-point aritmetic conforms to IEC 60559, FLT_RADIX is 2 and scalbn is (in this implementation) identical to ldexp.
scalbn returns x * FLT_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 ?.
scalbn conforms to ISO/IEC 9899:1999 (C99).