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NNTP-Posting-Date: Thu, 30 Jun 2011 16:37:55 -0500
Message-ID: <4E0CECB1.3D9D@mindspring.com>
Date: Thu, 30 Jun 2011 17:37:53 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: Use of EXTERN
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Tom Young wrote:
> 
> Hi all.  I've read through all the
> K&R2 chapters and I'm now reading the appendices.
> Specifically, I'm reading section A11 - Scope & Linkage p227. 
> I have a question about the "extern" property.
> 
> Section A11.2 Linkage p228 says:
> 
> "All declarations for the same object or function identifier
> with external linkage refer to the same thing,
> and the object or function is shared by
> the entire program."
> 
> and
> 
> "... the first external declaration for an identifier
> gives the identifier
> internal linkage if the static specifier is used,
> external linkage otherwise."
> 
> My questions are,
> is there any point to declaration an external object "extern"?
> Does it have any effect(s)?
> I'm almost positive I've seen code
> with external declarations that included the keyword "extern",
> but given
> the quoted statements above, that seems redundant.  Am I missing
> something?

I have a program called "e_driver".

It consists of six files:
 http://www.mindspring.com/~pfilandr/C/e_driver/e_driver.c
 http://www.mindspring.com/~pfilandr/C/e_driver/e_driver.h
 http://www.mindspring.com/~pfilandr/C/e_driver/e_sort.c
 http://www.mindspring.com/~pfilandr/C/e_driver/e_sort.h
 http://www.mindspring.com/~pfilandr/C/e_driver/e_distributions.c
 http://www.mindspring.com/~pfilandr/C/e_driver/e_distributions.h

e_driver.c has this external declaration:
 long unsigned cmp_count;

e_driver.h has this external declaration:
 extern long unsigned cmp_count;

e_driver.c and e_sort.h and e_distributions.h
all have this line:
 #include "e_driver.h"

e_sort.c has this line:
 #include "e_sort.h
and e_distributions.c has this line:
 #include "e_distributions.h"

All three of the *.c files in the e_driver program
use the same external object (cmp_count),
which is declared in e_driver.c

-- 
pete