modf() - extract signed integral and fractional values from floating-point number
#include <math.h>
double modf (double value, double *iptr)
The modf(3) 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.
The modf(3) function returns the signed fractional part of value.
frexp(3)
ldexp(3)