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

 Message        <Context> accepting the expression "<expr>" as a
                constant is a language extension.

 Description    In many cases Compaq C accepts the subtraction of two
                addresses within the same array or struct/union as a
                constant.  The C standard does not consider such an
                expression to be a constant.  Therefore, this program
                does not conform to the standard and may be rejected
                by other compilers.

 User Action    Change the expression to be a constant.