Side issue
"%f" does not well express the state of a floating point (FP) object
When a FP is large, printing 100s of digits is not informative. More compact options exist.
Worse, when a FP is much smaller than 1, "%f" only retains a few or zero digits - losing perhaps all precession.
As FP are encoded in exponential notation, use an exponential format
printf("%a", fp); //Hexadecimal significant with binary exponent
// or
printf("%.*e", DBL_DECIMAL_DIG - 1, fp); // Decimal exponential
// or
printf("%.*g", DBL_DECIMAL_DIG, fp); // Decimal exponential when needed, else fix point.