double modf(double value, double *iptr);

The modf() function breaks the argument value into integral and fractional parts, each of which has the same sign as the argument. It stores the integral part as a double in the object pointed to by iptr.

double frexp(double value, int *exp);

The frexp() function breaks a floating-point number into a normalized fraction and an integral power of 2. It stores the integer in the int object pointed to by exp.

Your question does not make it clear as to the output format. You ask for a hexadecimal representation of something. But what?