Xref: utzoo comp.lang.c:38067 news.software.b:7413
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!samsung!uunet!airs!ian
From: ian@airs.UUCP (Ian Lance Taylor)
Newsgroups: comp.lang.c,news.software.b
Subject: Re: Redeclaration of Variables extern/static
Summary: It's illegal according to ANSI C
Keywords: scope, ibm, c news
Message-ID: <1243@airs.UUCP>
Date: 9 Apr 91 04:18:57 GMT
References: <^R+_H3-@uzi-9mm.fulcrum.bt.co.uk>
Reply-To: airs!ian@uunet.uu.net (Ian Lance Taylor)
Followup-To: comp.lang.c
Organization: AIRS, Waltham, MA
Lines: 37

In article <^R+_H3-@uzi-9mm.fulcrum.bt.co.uk> igb@fulcrum.bt.co.uk (Ian G Batten) writes:
>C News, amongst other things, has declarations of ``extern int foo;'' in
>either a header or towards the top of a file, and later has ``static int
>foo;''.
>
>Can anyone quote chapter and verse as to the legality of this?

The ANSI C standard 3.1.2.2:

``...

If the declaration of a file scope identifier for an object or a
function contains the storage-class specifier *static*, the identifier
has internal linkage.

If the declaration of an identifier for an object or a function
contains the storage-class specifier *extern*, the identifier has the
same linkage as any visible declaration of the identifier with file
scope.  If there is no visible declaration with file scope, the
identifier has external linkage.

...

If, within a translation unit, the same identifier appears with both
internal and external linkage, the behavior is undefined.''

So in the case you mention, the behavior is undefined.  This means
that according to the standard, the compiler can do anything it
pleases, specifically including ``terminating a translation.''  Of
course, in this particular case that's pretty tacky.  Still, in the
long run it would probably be better to change the code (I have no
idea what that would involve).
-- 
Ian Taylor              airs!ian@uunet.uu.net              uunet!airs!ian
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