VMS Help CC, Messages, QUESTCOMPARE1  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        <Context> the unsigned expression "<expr>" is being
                compared with an equality operator to a constant
                whose value is negative.  This might not be what you
                intended.

 Description    An unsigned value and a signed constant whose value
                is negative are being compared for equality.
                Logically, these value would never be equal.  But in
                C the negative constant value is converted to an
                unsigned value before the comparison, and may well
                compare equal.

 User Action    Cast (or otherwise rewrite) one of the operands of
                the compare to match the signedness of the other
                operand.