Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #103786

Re: Is 'extern' necessary?

Path csiph.com!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Is 'extern' necessary?
Date Mon, 13 Feb 2017 12:33:46 +0100
Organization A noiseless patient Spider
Lines 47
Message-ID <o7s5fd$jui$1@dont-email.me> (permalink)
References <o7ksei$n0$1@dont-email.me> <lnd1eqvtl6.fsf@kst-u.example.com> <o7l1ci$l0t$1@dont-email.me>
Mime-Version 1.0
Content-Type text/plain; charset=windows-1252
Content-Transfer-Encoding 7bit
Injection-Date Mon, 13 Feb 2017 11:31:57 -0000 (UTC)
Injection-Info mx02.eternal-september.org; posting-host="e04e5426bceb88344a663937ce34ac72"; logging-data="20434"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19NQ5PIDD3sApxgkBDR9/6wtLqWF61Bx4U="
User-Agent Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0
In-Reply-To <o7l1ci$l0t$1@dont-email.me>
Cancel-Lock sha1:QUCHX5gq+vMTCJJkeWep4AH/z6o=
Xref csiph.com comp.lang.c:103786

Show key headers only | View raw

On 10/02/17 19:41, BartC 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?
> 
> Presumably it can't mean a definition with 'extern', because if I write:
> 
>   extern int abc = 100;
> 
> then gcc complains (but splitting it into extern int abc; int abc=100;
> is OK).
> 
> In my mind, file-scope objects can be declared in a module as either
> Local, Exported or Imported. Only the first two can be initialised in
> the module (initialising of an Imported object is up to the module that
> exports it).

In assembly, that is the common case - file-scope symbols are either
local, exported, or imported.  But in C, the concept is "linkage".
"extern" means that a symbol has external linkage.  You might be
declaring it as external linkage to export it, /or/ to import it.

So "extern" does /not/ mean "this identifier is defined externally".
The way to import a symbol is to have only an "extern" declaration.
Having a definition (without "static") is sufficient to export a symbol
- but you can happily also have an "extern" declaration there.  Most
people make use of that when the put their "extern" declarations in a
header file, and #include it in the source file that defines the object
as well as ones that use it.

> 
> I understand that in C, separate declarations that may appear
> conflicting are necessary, in order to have a shared declaration of X
> that can appear, via a shared header, in both modules A and B, where
> there is also a second, perhaps initialised definition of X in one of
> those modules.
> 
> But with this extern, tentative, storage and linkage business, it's
> doing a good job of making it as fiendishly convoluted as possible.
> Having issues hidden by a compiler, or multiple definitions taken care
> of by a linker, doesn't help in trying to understand.
>

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 17:16 +0000
  Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-10 09:33 -0800
    Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 18:41 +0000
      Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-10 12:09 -0800
        Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 20:29 +0000
          Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-10 13:26 -0800
            Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 22:56 +0000
              Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-10 20:19 -0800
                Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-11 09:53 -0500
              Re: Is 'extern' necessary? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-13 05:19 -0800
      Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-10 12:09 -0800
        Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 20:50 +0000
          Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-10 13:09 -0800
          Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-10 13:33 -0800
          Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-10 23:23 +0000
            Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 23:35 +0000
              Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-11 00:34 +0000
            Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 12:41 +0100
              Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-13 13:14 +0000
                Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-13 14:05 +0000
                Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-13 07:49 -0800
          Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-10 19:33 -0800
            Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-11 09:41 -0800
              Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-11 10:19 -0800
                Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-11 13:39 -0500
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-12 10:10 -0800
      Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-10 14:59 -0800
      Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 12:33 +0100
  Re: Is 'extern' necessary? Siri Cruise <chine.bleu@yahoo.com> - 2017-02-10 09:35 -0800
    Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-10 12:12 -0800
  Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-10 10:38 -0800
  Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-10 21:10 +0000
    Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-10 13:25 -0800
  Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-10 14:53 -0800
    Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-10 15:06 -0800
    Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-10 23:24 +0000
      Re: Is 'extern' necessary? Ian Collins <ian-news@hotmail.com> - 2017-02-11 12:34 +1300
        Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 13:14 +0100
          Re: Is 'extern' necessary? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-13 13:44 +0100
            Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-13 14:13 +0000
            Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 15:16 +0100
              Re: Is 'extern' necessary? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-13 17:24 +0100
                Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-13 14:24 -0500
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 23:29 +0100
            Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-13 11:06 -0800
            Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-13 11:15 -0800
          Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-13 11:48 -0500
            Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-13 09:03 -0800
            Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-13 23:37 +0100
      Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-10 15:40 -0800
      Re: Is 'extern' necessary? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-13 05:09 -0800
        Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-13 13:46 +0000
          Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-13 09:52 -0800
            Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-13 19:19 +0000
              Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-13 11:34 -0800
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-13 20:13 +0000
              Re: Is 'extern' necessary? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2017-02-13 13:11 -0800
              Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 00:33 +0100
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-14 01:02 +0000
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 09:28 +0100
                Re: Is 'extern' necessary? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-14 03:31 -0600
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-14 11:42 +0000
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 13:29 +0100
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-14 05:16 -0800
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 14:51 +0100
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-14 07:24 -0800
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 21:35 +0100
                Re: Is 'extern' necessary? James Kuyper <jameskuyper@verizon.net> - 2017-02-14 15:53 -0500
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-15 09:31 +0100
                Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-15 11:36 -0500
                Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-15 09:12 -0800
                Re: Is 'extern' necessary? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-02-15 12:58 -0500
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-15 10:01 -0800
                Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-15 10:55 -0800
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-15 11:42 -0800
                Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-15 12:28 -0800
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-15 13:58 -0800
                Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-14 08:54 -0800
                Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-14 08:51 -0800
                Re: Is 'extern' necessary? Richard Heathfield <rjh@cpax.org.uk> - 2017-02-14 17:32 +0000
                Re: Is 'extern' necessary? Gareth Owen <gwowen@gmail.com> - 2017-02-14 17:45 +0000
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-14 21:38 +0100
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-14 23:26 +0000
                Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-14 16:33 -0800
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-15 00:47 +0000
                Re: Is 'extern' necessary? Jean-Marc Bourguet <jm@bourguet.org> - 2017-02-14 18:17 +0100
                Re: Is 'extern' necessary? Keith Thompson <kst-u@mib.org> - 2017-02-14 09:47 -0800
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-14 18:36 +0000
                Re: Is 'extern' necessary? jameskuyper@verizon.net - 2017-02-14 11:20 -0800
                Re: Is 'extern' necessary? supercat@casperkitty.com - 2017-02-14 11:42 -0800
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-14 20:22 +0000
                Re: Is 'extern' necessary? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-14 17:58 -0600
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-15 01:29 +0000
                Re: Is 'extern' necessary? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-16 05:02 -0600
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-16 11:38 +0000
                Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-16 14:37 +0000
                Re: Is 'extern' necessary? BartC <bc@freeuk.com> - 2017-02-16 15:43 +0000
                Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-16 16:08 +0000
                Re: Is 'extern' necessary? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-02-16 18:42 +0100
                Re: Is 'extern' necessary? David Brown <david.brown@hesbynett.no> - 2017-02-15 09:47 +0100
                Re: Is 'extern' necessary? Robert Wessel <robertwessel2@yahoo.com> - 2017-02-16 05:05 -0600
          Re: Is 'extern' necessary? Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-15 16:53 -0800
    Re: Is 'extern' necessary? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-10 23:55 +0000
  Re: Is 'extern' necessary? Les Cargill <lcargill99@comcast.com> - 2017-02-10 19:24 -0600
  Re: Is 'extern' necessary? void <nathan.gothier@gmail.com> - 2017-02-11 03:17 -0800

csiph-web