4.2::SYSV compatability problem

[Doug Gwyn (VLD/VMB)]

But the idea of making extern objects conform to a DEF/REF model
is a good one (forced upon the C implementor on a wide variety of
systems); it's just changing the semantics on UNIX that was not
such a hot idea.  (Although K&R described DEF/REF semantics, few
UNIX programmers paid heed, since UNIX used a COMMON model.)

The draft ANSI C spec requires DEF/REF, but in a small note in an
Appendix the COMMON model is mentioned as a "common extension".
C programmers should start conforming to DEF/REF semantics if they
are not already doing so; this works on UNIX and will continue to
work on other C systems (including perhaps future UNIXes).