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

  Computes the floating-point remainder of the first argument divided
  by the second.  If the second argument is zero, the fuction returns
  zero.

  Syntax:

       #include <math.h>

       double fmod (double x, double y);