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

 Message        The use of __typeof__ is a language extension.

 Description    Support for __typeof__ is a language extension
                provided for compatibilty with some other C
                compilers.  Although some other C compilers will
                accept this syntax, many compilers will reject it.

 User Action    Be aware of this difference if you plan to port this
                source to another compiler.