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To: Poul-Henning Kamp <phk@critter.freebsd.dk>
Cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: Global variables defined several times. 
In-reply-to: Your message of "Mon, 03 Jul 2000 22:43:10 +0200."
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Date: Mon, 03 Jul 2000 22:22:30 +0100
From: David Malone <dwmalone@maths.tcd.ie>
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> In message <200007032140.aa37554@salmon.maths.tcd.ie>, David Malone writes:
> >I've just noticed that usr.bin/ftp/ftp_var.h defines a large
> >selection of global variables, and then this header file is included
> >in multiple C source files.
> >
> >I thought this should lead to one copy of the global varible per
> >source file, and then a warning or error at link time due to symbols
> >being defined multiple times. This doesn't seem to be the case with
> >the toolchain - you seem to get one copy of each variable for the
> >final linked unit.

> This is called "common" variables.  They're documented with that
> behaviour in the old and new testament.

In the K&R1 I have to hand, the last line of page 76 and top of
page 77 say:

	If the lines
		int sp;
		double val[MAXVAL];
	apprar outside any function, they \emph{define} the expernal
	variables sp and val...

Further down page 77:

	There must be only one \emph{definition} of an external variable
	among all the files that make up the source program; other files
	may contain extern declarations to access it.

I can't find my second edition at the moment. This behavior is
commented on in the C FAQ as something the ANSI standard describes
as a common extension. (http://www.eskimo.com/~scs/C-faq/q1.7.html)
It also seems to suggest it is mostly a Unix thing.

	David.


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