cc bug in definition of global variables?

[Marc Pucci]

> > The following seems to be a bug in the SYSV.2 C-compiler (loader really):
> > You can DEFINE the same global variable in two files and the loader does
> > not even issue a warning about it.  
 ...

> This is not a bug.  Although Ritchie preferred the COMMON model
> for extern data, which is what most UNIX PCCs implement, there
> are some systems supporting C that have unacceptable restrictions
...
> There is a story that some early release of USG UNIX (5.0?) had
> been changed to enforce DEF/REF semantics, and so much code broke
> that they had to back out the change (by providing "mcc", or "cc -m",
> or some such temporary scheme) to still the outcries.  I heard that
> some AT&T internal sites refused to install the changed SGS until
> extern data linkage was put back the way it used to be.  If there
> is an interesting story here, perhaps someone involved will tell us.

 - - - -

Doug Gwyn is right about the internal differences in some flavors of
USG C compilers (actually SGS's).  Forgive me if I start to ramble on.
Most of this took place 4 or 5 years ago, so some details are sketchy.

Back when I did the original UNIX port for the 3B20S,
the SGS systems had just appeared.  I believe they
were based on the 3B20 D (for duplex) development systems from BTL
Indian Hill.  These were cross generation systems that ran on pdp-11's
running either RT or UNIX and produced code to be later down-loaded into
the 3B20-D through various nifty hardware.  These adhered to the spec
that made muliple externs illegal.

The timing now (~1980-82?) is also about that of the UNIX shop's real growth
period - a transition when it grew from a few groups to a few departments.
Support would be provided for several processor types: 3B20, 3B5, VAX, PDP-11
and so adopting a common SGS structure for the language maintainers
was a reasonable thing to do.

We were the first internal customers for the new SGS and had to port
UNIX for the 3B20S.  However, even though the C spec claimed no
multiple externs, the UNIX system code made extensive use of this.
Just look at sys/systm.h.  It contained things like "char runrun;"
which had to be changed to "extern char runrun;" with a single instance
of "char runrun;" appearing in some other file.   Hence, header files were
changed and variables added to related source files.

At the time, we were too excited about doing the UNIX port to complain
about the externs.  (Another problem that did get fixed was that the
loader would complain and exit when it hit its first conflict.  Couple
this with the fact that it was a slow cross-compiler with many temporary
files to support software demand paging of data structures and you
can imagine what a joy it was hitting conflict after conflict - ah but
we got over-time in those days).

Anyway, the SGS's were released in internal UNIX release 4.something or
other.  People were upset about the change concerning multiple externs,
but it was decided to go with the new SGS.  This decision was subsequently
reversed and multiple externs were added to the SGS.  During this time
you could find cc, mcc (or was it Mcc), and occ on various internal BTL
machines.  Some had installed the new and kept the old as occ; some
had reinstalled the old and moved the new to mcc, and some had an Mcc
that was one flavor or the other (my memory is very sketchy here - but
confusion certainly had set in.  I think that on at least one system
there was a cc (pre-SGS), an mcc (SGS - no multiple externs), and an
Mcc (SGS with multiple externs)).

I think by internal release 4.3, the multiple externs were put in
to stay.

Marc Pucci
Bell Communications Research, Morristown, NJ