Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!iuvax!bobmon
From: bobmon@iuvax.cs.indiana.edu (RAMontante)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Possible Bug in Turbo Linker (and question about Turbo Debug)
Message-ID: <14962@iuvax.cs.indiana.edu>
Date: 13 Nov 88 17:33:55 GMT
References: <1254@helios.ee.lbl.gov>
Reply-To: bobmon@iuvax.UUCP (RAMontante)
Organization: malkaryotic
Lines: 48

(A better answer is probably to be had in comp.lang.c...)

forrest@ux1.lbl.gov (Jon Forrest) writes:

+I've been doing some playing around with Turbo C 2.0 and I have
+found something that I didn't expect. This is the fact that
+if you declare the same external variable in more than one file,
+the linker treats them as separate variables instead of treating
+them as references to the same variable. You can see this be
+creating 2 files, each with "int x" as the first line. Compile
+them and link them. You'll see an error message from the linker
+saying "Error: x defined in module YYY is duplicated in module ZZZ"
+(where YYY and ZZZ are the names of the files). Furthermore, if you
+run OBJXREF /RC you see that "_x" exists in both object files
+and uses space in both.	[...]
+
+The reason I think this is wrong, in additon to being different
+than the behavior on Unix and VMS, is that K&R (1st edition)
+on page 206, section 11.1 says "... all references to the same
+external identifier refer to the same object ...".	[...]

Looking at K&R sections 10 - 11.2, I conclude that Jon's definition of

	int x;

in each file attempts to create an 'x' whose name is available to other
files.  Repeating the definition in other files does not create
additional references to the same external identifier; rather that
creates new externally-available variables with a single, conflicting
name.  In order for a file to _use_ an 'x' that was created/stored in
another file, it must define

        extern int x;

which says that the x *comes from* somewhere external to the file (i.e.,
another file).  Consider this K&R quote, from section 11.2:

   "Thus in a multi-file program, an external data definition
   _without_ the  extern  specifier must appear in _exactly_one_ of the
   files.  Any other files which wish to give an external definition
   for the identifier must include the  extern  in the definition."
   [Emphases mine]

So Turbo is in agreement with K&R here, as well as the ANSI draft (I presume).
-- 
--  bob,mon			(bobmon@iuvax.cs.indiana.edu)
-- "I thought it must be one of the prime numbers of the Zeeman Series.
    *I* haven't changed."