VMS Help DCE_RPC, idl, ARGUMENTS, -Dname  *Conan The Librarian (sorry for the slow response - running on an old VAX)

   -Dname[=definition]

   Defines a symbol name and an optional value to be passed to the C
   preprocessor.  You can use this method of defining a symbol instead
   of using #define in the source code.  You can use more than one
   -Dname argument on the command line.  This argument has no effect if
   you use the -no_cpp argument.