Re: Who owns the variable in my header file ?

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Who owns the variable in my header file ?
Date	2012-10-03 16:21 -0700
Organization	None to speak of
Message-ID	<lnfw5vp2f2.fsf@nuthaus.mib.org> (permalink)
References	<EK-dnVoFlczbHfHNnZ2dnUVZ8hidnZ2d@bt.com> <k4iel5$633$3@blue-new.rahul.net>

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falk@rahul.net (Edward A. Falk) writes:
[...]
> Perhaps the C standard has changed since I first read it, but AFAIK,
> most of the answers so far have been wrong.
>
> If you write this in your code:
>
> 	int foo;
>
> You've *declared* foo -- that is, described what it is -- but you haven't
> *defined* it.  There's a difference.  In this case, no actual space
> for foo has been allocated yet, and it's known as a "common" symbol.
> If no module ever actually defines it, the linker will allocate space
> for it at the end.
>
> If you write
>
> 	int bar = 1;

Did you mean to change the name from "foo" to "bar"?

> Now you've defined it.  Space and an initial value for it will be
> included in your module.  If more than one module does this, there will
> be a conflict.

N1370 6.9.2p2:

    A declaration of an identifier for an object that has file scope
    without an initializer, and without a storage-class specifier or
    with the storage-class specifier static, constitutes a *tentative
    definition*. If a translation unit contains one or more tentative
    definitions for an identifier, and the translation unit contains
    no external definition for that identifier, then the behavior
    is exactly as if the translation unit contains a file scope
    declaration of that identifier, with the composite type as of
    the end of the translation unit, with an initializer equal to 0.

So

    int foo;

*can* be a definition, or at least can act as one.

> So add "int bar = 1;" to your foo.h and then compile fooget into a binary.

But then two or more translation units within your program can see that
definition, and you can get a "multiple definition" error.

Instead, add

    extern int foo;

to "foo.h", and

    int foo = 1;

to "foo.c".  Then any translation unit that includes "foo.h" can use the
object "foo", which is defined in exactly one place in your program.
(You could drop the "extern" in foo.h, making it a tentative definition,
but adding "extern" is more explicit.)

(I'm snipping some context in which you make some of these same points.)

[snip]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-03 19:13 +0100
  Re: Who owns the variable in my header file ? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-10-03 15:05 -0400
    Re: Who owns the variable in my header file ? pacman@kosh.dhis.org (Alan Curry) - 2012-10-03 19:17 +0000
      Re: Who owns the variable in my header file ? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-10-03 15:55 -0400
    Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-03 20:59 +0100
      Re: Who owns the variable in my header file ? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-10-03 16:32 -0400
        Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 08:50 +0100
          Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-04 01:30 -0700
            Re: Who owns the variable in my header file ? Angel <angel+news@spamcop.net> - 2012-10-04 10:17 +0000
            Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 12:02 +0100
              Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-04 16:20 -0700
                Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-05 08:44 +0100
                Re: Who owns the variable in my header file ? Chicken McNuggets <chicken@mcnuggets.com> - 2012-10-07 16:48 +0100
          Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-04 07:13 -0400
      Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-04 00:34 -0400
        Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 09:02 +0100
          Re: Who owns the variable in my header file ? David Brown <david@westcontrol.removethisbit.com> - 2012-10-04 10:14 +0200
          Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-04 08:30 -0400
            Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 14:18 +0100
            Re: Who owns the variable in my header file ? Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-10-06 02:30 -0700
              Re: Who owns the variable in my header file ? Les Cargill <lcargill99@comcast.com> - 2012-10-06 08:01 -0500
              Re: Who owns the variable in my header file ? Richard Damon <news.x.richarddamon@xoxy.net> - 2012-10-06 21:30 -0400
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-07 11:40 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-07 10:56 -0400
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-07 21:26 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-07 19:58 -0400
                Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-07 11:47 -0700
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-07 21:53 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-07 20:04 -0400
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-08 02:14 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-07 21:47 -0400
                Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-08 00:33 -0700
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-08 11:13 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-08 07:40 -0400
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-08 13:13 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-08 10:08 -0400
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-08 16:20 +0100
                Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-08 12:45 -0400
                Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-08 12:29 -0700
                Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-08 22:28 +0100
                Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-08 15:47 -0700
                Re: Who owns the variable in my header file ? Richard Damon <news.x.richarddamon@xoxy.net> - 2012-10-07 21:16 -0400
          Re: Who owns the variable in my header file ? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-10-04 08:35 -0600
          Re: Who owns the variable in my header file ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-04 22:31 +0100
            Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-05 08:58 +0100
        Re: Who owns the variable in my header file ? gordonb.mhaw9@burditt.org (Gordon Burditt) - 2012-10-06 20:19 -0500
          Re: Who owns the variable in my header file ? Ian Collins <ian-news@hotmail.com> - 2012-10-07 15:19 +1300
          Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-07 18:22 +0100
            Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-07 14:38 -0400
      Re: Who owns the variable in my header file ? Kaz Kylheku <kaz@kylheku.com> - 2012-10-04 05:14 +0000
  Re: Who owns the variable in my header file ? Kaz Kylheku <kaz@kylheku.com> - 2012-10-03 19:40 +0000
  Re: Who owns the variable in my header file ? Ike Naar <ike@sverige.freeshell.org> - 2012-10-03 19:52 +0000
  Re: Who owns the variable in my header file ? falk@rahul.net (Edward A. Falk) - 2012-10-03 22:36 +0000
    Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-03 16:21 -0700
      Re: Who owns the variable in my header file ? falk@rahul.net (Edward A. Falk) - 2012-10-04 19:01 +0000
      Re: Who owns the variable in my header file ? David Thompson <dave.thompson2@verizon.net> - 2012-10-11 03:09 -0400
        Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-11 13:30 -0700
    Re: Who owns the variable in my header file ? Kaz Kylheku <kaz@kylheku.com> - 2012-10-04 01:43 +0000
    Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 09:24 +0100
  Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 11:23 +0100
    Re: Who owns the variable in my header file ? David Brown <david@westcontrol.removethisbit.com> - 2012-10-04 13:26 +0200
      Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 15:52 +0100
        Re: Who owns the variable in my header file ? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-10-04 11:40 -0400
          Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-04 17:00 +0100
        Re: Who owns the variable in my header file ? hormelfree@gmail.com - 2012-10-04 17:15 -0700
          Re: Who owns the variable in my header file ? Ike Naar <ike@iceland.freeshell.org> - 2012-10-05 06:51 +0000
            Re: Who owns the variable in my header file ? David Brown <david@westcontrol.removethisbit.com> - 2012-10-05 09:35 +0200
          Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-05 09:11 +0100
            Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-05 08:04 -0400
    Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-04 08:43 -0400
    Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-04 16:08 -0700
      Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-05 09:27 +0100
        Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-05 02:16 -0700
        Re: Who owns the variable in my header file ? Angel <angel+news@spamcop.net> - 2012-10-05 11:38 +0000
          Re: Who owns the variable in my header file ? Nobody <nobody@nowhere.com> - 2012-10-05 16:13 +0100
            Re: Who owns the variable in my header file ? Angel <angel+news@spamcop.net> - 2012-10-05 16:26 +0000
  Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-08 19:02 +0100
    Re: Who owns the variable in my header file ? James Kuyper <jameskuyper@verizon.net> - 2012-10-08 14:29 -0400
      Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-08 20:15 +0100
      Re: Who owns the variable in my header file ? Keith Thompson <kst-u@mib.org> - 2012-10-08 14:04 -0700
    Re: Who owns the variable in my header file ? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-10-08 22:47 -0600
      Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-09 09:04 +0100
        Re: Who owns the variable in my header file ? "BartC" <bc@freeuk.com> - 2012-10-09 10:30 +0100
          Re: Who owns the variable in my header file ? lipska the kat <lipskathekat@yahoo.co.uk> - 2012-10-09 11:48 +0100

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