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| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: To include or not to include |
| Date | 2016-09-26 20:07 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <87k2dycweu.fsf@bsb.me.uk> (permalink) |
| References | (8 earlier) <ns8akc$o3j$1@dont-email.me> <ns8i3i$fks$1@dont-email.me> <ns8m5r$sqi$1@dont-email.me> <ns9m0h$fij$1@dont-email.me> <nsbju0$4vl$1@dont-email.me> |
BartC <bc@freeuk.com> writes:
> On 26/09/2016 00:18, James Harris wrote:
>> On 25/09/2016 15:15, BartC wrote:
>
>>> So it seems superfluous. Unless, for some bizarre reason, you want to
>>> actually declare a file-scope name inside a function.
>>
>> IIRC this goes back to support for single pass. Consider this code:
>>
>> int x = 1;
>> int f(void) {
>> return x;
>> }
>>
>> When the compiler gets to see x in the function it knows its type.
>
> Presumably single passes are still used because C still has trouble
> with references to functions and variables that occur later on. (Or
> more likely, it has to maintain that behaviour even though forward
> declaration requirements could easily be relaxed.)
>
>>> And it doesn't really explain however why 'extern' needs to be used for
>>> names of file-scope objects and functions that are shared between
>>> modules.
>>
>> From memory I think this is a subtle difference between declarations
>> (i.e. setting type information) and definitions (reserving storage).
>> Consider at file scope
>>
>> int x;
>>
>> Even though that does not assign a value, IIRC the compiler might treat
>> it as a definition (rules as to when are fiddly). But compare that with
>>
>> extern int x;
>>
>> The compiler will always see that as a declaration (again, with very
>> fiddly rules) and not a definition.
>
> All C compilers I've tried have no problem with:
>
> int x;
> int x;
> int x;
> int x;
> int x;
All conforming C compilers must have no problem with this.
> within the same module, provided only one is initialised.
I'd say "at file scope in the same translation unit, provided no more
than one is initialised". These are all what are call tentative
definitions. The syntax is there because of the long, deep history of
C. I doubt anyone would design it this way if starting from scratch.
> Even when some of these are in other modules: the linker sees multiple
> instances and creates only one.
If some appear in other translation units (C does not have modules), the
result is undefined (as I said before).
> EXCEPT when I use Tiny C, then it will
> report multiple definitions of x. That can be fixed by using extern in
> all either modules except one or even when extern is used everywhere.
Using extern everywhere fixes nothing -- it only "works" if the object
is not used since it prevents there being any definition of it.
> I don't know if that's a problem with Tiny C, or if it's all the other
> compilers that are being lax.
With gcc you are, implicitly, asking for this behaviour because gcc's
default behaviour is to place such objects into a common block. You can
stop this by using -fno-common, in which case the linker will complain
about the multiple definitions. All of these behaviours are conforming.
<snip>
--
Ben.
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To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-23 10:45 +0100
Re: To include or not to include David Brown <david.brown@hesbynett.no> - 2016-09-23 12:43 +0200
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-23 12:17 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-23 13:32 +0100
Re: To include or not to include David Brown <david.brown@hesbynett.no> - 2016-09-23 14:57 +0200
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-24 20:34 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-24 23:55 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 01:43 +0100
Re: To include or not to include Ian Collins <ian-news@hotmail.com> - 2016-09-25 14:38 +1300
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-25 11:42 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 11:58 +0100
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-25 14:06 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 15:15 +0100
Re: To include or not to include Reinhardt Behm <rbehm@hushmail.com> - 2016-09-25 23:33 +0800
Re: To include or not to include Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-25 10:04 -0600
Re: To include or not to include Reinhardt Behm <rbehm@hushmail.com> - 2016-09-26 00:21 +0800
Re: To include or not to include "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-25 13:17 -0400
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-26 00:18 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-26 17:55 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-26 20:07 +0100
Re: To include or not to include supercat@casperkitty.com - 2016-09-26 12:13 -0700
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-27 01:25 +0100
Re: To include or not to include supercat@casperkitty.com - 2016-09-26 18:05 -0700
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-27 14:27 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-27 14:49 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-27 15:15 +0100
Re: To include or not to include Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-27 07:42 -0700
Re: To include or not to include Keith Thompson <kst-u@mib.org> - 2016-09-27 09:30 -0700
Re: To include or not to include Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-27 10:06 -0700
Re: To include or not to include supercat@casperkitty.com - 2016-09-27 08:12 -0700
Re: To include or not to include Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 10:46 -0700
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-27 17:55 +0100
Re: To include or not to include supercat@casperkitty.com - 2016-09-27 10:19 -0700
Re: To include or not to include Keith Thompson <kst-u@mib.org> - 2016-09-27 11:42 -0700
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-27 19:57 +0100
Re: To include or not to include Keith Thompson <kst-u@mib.org> - 2016-09-27 12:38 -0700
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-27 21:00 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-28 01:50 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-28 10:55 +0100
Re: To include or not to include Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-27 13:33 -0700
Re: To include or not to include Ike Naar <ike@iceland.freeshell.org> - 2016-09-28 19:13 +0000
Re: To include or not to include supercat@casperkitty.com - 2016-09-27 12:38 -0700
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-27 20:57 +0100
Re: To include or not to include supercat@casperkitty.com - 2016-09-27 07:25 -0700
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-28 00:28 +0100
Re: To include or not to include supercat@casperkitty.com - 2016-09-30 07:53 -0700
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-30 16:06 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-27 13:53 +0100
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-30 19:46 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-30 20:27 +0100
Re: To include or not to include Reinhardt Behm <rbehm@hushmail.com> - 2016-09-25 23:27 +0800
Re: To include or not to include Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-25 11:38 -0400
Re: To include or not to include Reinhardt Behm <rbehm@hushmail.com> - 2016-09-25 23:40 +0800
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 15:06 +0100
Re: To include or not to include Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-25 11:49 -0400
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 18:05 +0100
Re: To include or not to include Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-25 16:42 -0400
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 21:53 +0100
Re: To include or not to include Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-25 17:57 -0400
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-25 21:20 +0100
Re: To include or not to include BartC <bc@freeuk.com> - 2016-09-25 21:49 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-26 00:40 +0100
Re: To include or not to include David Brown <david.brown@hesbynett.no> - 2016-09-23 14:56 +0200
Re: To include or not to include Richard Heathfield <rjh@cpax.org.uk> - 2016-09-23 13:31 +0100
Re: To include or not to include James Harris <james.harris.1@gmail.com> - 2016-09-23 13:52 +0100
Re: To include or not to include Richard Heathfield <rjh@cpax.org.uk> - 2016-09-23 14:35 +0100
Re: To include or not to include Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-23 18:55 +0100
Re: To include or not to include Richard Heathfield <rjh@cpax.org.uk> - 2016-09-23 20:33 +0100
Re: To include or not to include Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-23 08:22 -0600
Re: To include or not to include Richard Damon <Richard@Damon-Family.org> - 2016-09-23 11:37 -0400
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