modf − extract signed integral and fractional values from floating-point number

#include <math.h> double modf(double x, double *iptr);

The modf() function breaks the argument x into an integral part and a fractional part, each of which has the same sign as x. The integral part is stored in iptr.

The modf() function returns the fractional part of x.

SVID 3, POSIX, BSD 4.3, ISO 9899

frexp(3), ldexp(3)