/sys$common/syshlp/helplib.hlb CC, Language topics, Storage Class Modifiers, readonly  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Readonly variables are assigned the PSECT attribute NOWRT and are
  stored in the PSECT $CODE, which is a nonwritable data area.  Other
  programs can access the PSECT directly, but none of the information
  can be overwritten.  You can use the readonly modifier with the
  storage-class keywords [extern], static, globaldef, and
  globaldef{"name"}.

  You can use both the readonly and noshare modifiers with the
  [extern] and the globaldef{"name"} specifiers.  If you use both
  modifiers with either the static or the globaldef specifiers, the
  compiler ignores noshare and accepts readonly.