/sys$common/syshlp/helplib.hlb CC, Run-time functions, modf  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns the positive fractional part of its first argument and
  assigns the integer part, expressed as an object of type double, to
  the object whose address is specified by the second argument.

  Syntax:

       #include <math.h>

       double modf(double value, double *iptr);