VMS Help CC, Messages, EMPTYSTRUCT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        Allowing struct/union type with no members is a
                language extension.

 Description    The C standard requires that a struct/union type have
                at least one member.  The Compaq C compiler will
                accept this for compatibility with older compilers.
                The struct/union type will be treated as if it were
                declared { :  0; }

 User Action    Provide at least one member for the struct/union.