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Groups > comp.lang.c > #103643
| From | BartC <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Is 'extern' necessary? |
| Date | 2017-02-10 23:24 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <o7li0r$m52$1@dont-email.me> (permalink) |
| References | <o7ksei$n0$1@dont-email.me> <o7lg60$df0$1@dont-email.me> |
On 10/02/2017 22:53, Andrey Tarasevich wrote:
> On 2/10/2017 9:16 AM, BartC wrote:
>> I've been trying to find out what the point of 'extern' is.
>>
>> Usually, if you declare:
>>
>> int abc;
>>
>> at filespec in modules A and B, then you end up with one copy of 'abc'
>> shared across both modules. It doesn't matter which of A or B is abc's
>> 'home' module.
>
> No, you will end up with a constraint violation. C language prohibits
> this, although it lists it as a "popular extension" in a non-normative
> appendix of the language standard.
>
> (It is often incorrectly claimed that "tentative definition" mechanism
> in C allows this. In reality, it doesn't. However, this is allegedly a
> side-effect of the approach compilers use to implement tentative
> definitions in C. Nevertheless, it is illegal.)
>
>> If one of them uses 'extern', then it's the same.
>
> Then it is correct (as opposed to your previous example).
The 'extern' method is what I use exclusively when my tools
auto-generate C code, as it's the method that more closely matches how I
think about it:
static int A [= expr]; // Defined here, not exported
int B [= expr]; // Defined here, exported
extern int C; // Defined elsewhere, imported
Yet I rarely see 'extern' used in typical C code, and I need to
understand how it's used in other people's code because I want to try
and process C source.
(My generated code doesn't use headers, so if 3 modules want to use 'B'
from my example, and 'B' is defined in module X, then:
X contains: int B; // export
Y contains: extern int B; // import
Z contains: extern int B; // import
Using headers like normal C code, it might be more like this:
X sees: extern int B; // from #include "X.h"
X contains: int B; //
Y sees: extern int B; // from #include "X.h"
Z sees: extern int B; // from #include "X.h"
This combination I can understand. However no one else seems to write C
like I do!)
> Additionally, it can be used to refer to a hidden name of a global variable
>
> int i;
>
> void foo()
> {
> int i; // Hides global `i`
>
> {
> extern int i; // Refers to global `i` and brings it to
> // this scope
> ...
> }
> }
That's an interesting use. But I suspect very little used.
> Pre-C89 implementations exhibit at least four different models, listed
> here in order of increasing restrictiveness:
>
> *Common* Every object declaration with external linkage, regardless of
> whether the keyword 'extern' appears in the declaration, creates a
> definition of storage. When all of the modules are combined together,
> each definition with the same name is located at the same address in
> memory. (The name is derived from common storage in Fortran.) This model
> was the intent of the original designer of C, Dennis Ritchie.
Yes. I liked Fortran...
> *Relaxed Ref/Def* The appearance of the keyword 'extern' in a
> *Strict Ref/Def* This is the same as the relaxed ref/def model, save
> *Initialization* This model requires an explicit initialization to
> define storage. All other declarations are references.
....
This is heavy stuff. But is it really necessary; do linkers really work
like this?
Sure all that's needed is a Local, Imported, Exported model?
--
Bartc
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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
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