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

 Message        <Context> "<expression>" is of an incomplete type,
                and so has no size.

 Description    An expression that has incomplete type has no size
                and therefore cannot be used as the argument to the
                sizeof operator.

 User Action    Pass an expression with a valid type to the sizeof
                operator.