Path: csiph.com!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Is 'extern' necessary?
Date: Fri, 10 Feb 2017 13:33:40 -0800
Organization: None to speak of
Lines: 47
Message-ID: <lnshnlvih7.fsf@kst-u.example.com>
References: <o7ksei$n0$1@dont-email.me> <lnd1eqvtl6.fsf@kst-u.example.com> <o7l1ci$l0t$1@dont-email.me> <4920b63e-c9ac-4ddb-96f4-5c45257f6689@googlegroups.com> <o7l8uh$jip$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Injection-Info: mx02.eternal-september.org; posting-host="be0e2eec6e3a3e421f62644bd2dd2876"; logging-data="5703"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+DNeGnlOGYjY+NjuKIKtbn"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock: sha1:t1CbwIAL9B5cwWekxM0+4gFaNSY= sha1:0DkbeC9lPZsl3FtwpwEgXfmwkfs=
Xref: csiph.com comp.lang.c:103629

BartC <bc@freeuk.com> writes:
> On 10/02/2017 20:09, jameskuyper@verizon.net wrote:
>> On Friday, February 10, 2017 at 1:41:10 PM UTC-5, Bart wrote:
>>> On 10/02/2017 17:33, Keith Thompson wrote:
>>>> BartC <bc@freeuk.com> writes:
>>>>> I've been trying to find out what the point of 'extern' is.
>>>>
>>>> N1570 6.9.2, tentative definitions.
>>>
>>> It mentions 'external definition'. What does that mean?
>>
>> Go up one level. Section 6.9 is titled "External definitions", 6.9p1
>> defines what an external declaration is, and 6.9p5 defines an
>> external definition as an external declaration that is also a
>> definition, referring back to 6.7p5 for the definition of
>> "definition".
>>
>>> Presumably it can't mean a definition with 'extern', because if I write:
>>>
>>>    extern int abc = 100;
>>
>> In 6.9, "external" means "outside of any function" (6.9p4), whereas
>> in most other parts of the standard it refers to "external linkage".
>
> I remember that from a previous discussion. It comes up in K&R2 1.10 
> (p31): "An external variable... must be defined outside of any function. 
> ...The variable must also be declared in each function that wants to 
> access it...". And it goes on to suggest using an explicit 'extern' 
> statement.
>
> I think that's just ramped up the confusion by a magnitude or so!

That appears to be some unfortunate wording in K&R2.  It implies (and
the example shows) that the variables need to be declared *inside* each
function that refers to them.  In fact, the declaration merely needs to
be visible.

Note that the idea of header files has not yet been introduced at that
point.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"