# include & ltmath.h & gt means to include this math.h file.

(# include & ltmath.h & gt； # include "math.h" contains both. The details are still a little different)

double frexp(double value，int * exp)； This is a method to split a value into a fractional part f and an exponential part exp (base 2), and return the fractional part f, that is, f * 2 exp. Where the value of f is in the range of 0.5~ 1.0 or 0.

double ldexp(double x，int exp)； This function is just the opposite of the frexp function above, and the return value is x * 2 exp.

double modf(double value，double * iptr)； Split a value and return its fractional part, with iptr pointing to the integer part.

Extended data:

When the result of a function cannot be represented by a floating-point number. If the result is too large to be represented, the function will set errno to ERANGE to indicate the range error, and return a specific large value named by the macro hug _ val or its negation (-hug _ val).

If the size of the result is too small, a value of zero will be returned. In this case, error can be set to ERANGE or not.