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From: rgenter@BBN-LABS-B.ARPA (Rick Genter)
Newsgroups: net.lang.c
Subject: Re: Another bug in C compilers
Message-ID: <2703@brl-smoke.ARPA>
Date: Thu, 31-Jul-86 08:41:33 EDT
Article-I.D.: brl-smok.2703
Posted: Thu Jul 31 08:41:33 1986
Date-Received: Fri, 1-Aug-86 03:23:36 EDT
Sender: news@brl-smoke.ARPA
Lines: 41


     Accepting the declaration:

	struct	ABC;

is not a bug, it's a feature.  This is termed a *vacuous declaration*, and
permits the definition of cyclic recursive structures in an inner scope where
one of the structures renames a structure from on outer scope.  For example,

	struct	ABC	{
		int	foo;
		double	mumble;
	};

	main ()
	{
		struct	ABC;
		struct	DEF	{
			char	bar[ 128 ];
			int	bletch;
			struct	ABC	*foop;
		} a;

		struct	ABC	{
			double	giant[ 5000 ];
			struct	DEF	*barp;
		} b;

		<the rest of main>
	}

If you didn't have the vacuous declaration, the compiler would think that
"foop" pointed at a structure consisting of "int foo" and "double mumble".
Referencing foop->giant would get you "illegal structure pointer combination".
--------
Rick Genter 				BBN Laboratories Inc.
(617) 497-3848				10 Moulton St.  6/512
rgenter@labs-b.bbn.COM  (Internet new)	Cambridge, MA   02238
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