Re: Question about use (and non-use) of "extern"

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Question about use (and non-use) of "extern"
Date	2019-09-03 05:57 -0700
Organization	A noiseless patient Spider
Message-ID	<86tv9ty1t4.fsf@mailhub.linuxsc.com> (permalink)
References	<qjgric$8u2$1@news.xmission.com> <qk49be$ac$1@dont-email.me>

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> On 8/20/2019 6:11 AM, Kenny McCormack wrote:
>
>> Suppose you have multiple source files and you want to share
>> access to a global (see footnote) variable.  [how to do this?]
>>
>> 2) If you "extern" it nowhere, it works fine.
>
> ...which is non-stnadard, but happens to be a popular extension
> among compilers with Unix roots.

>> My question is:  Is the later case OK?  Is it OK to just declare
>> it normally in every module and not bother with "extern" ?
>
> It is illegal from the formal point of view.  The language standard
> does not allow that.

These statements are misleading.  An implementation may legally
support this pattern, with no diagnostics, and still conform to
the Standard.  Nothing in the Standard forbids that.  Neither does
the Standard require it, but that doesn't make it illegal, just
not advisable.  (More specifically, the behavior is undefined.)

> [...] The practice of allowing redundant [uninitialized] variable
> definitions comes from Unix compilers.  [...]

More significantly, it was the usual way of doing business in the
era of K&R C, and the C community at large expected it would work
even if not everyone followed it.

Thread

Question about use (and non-use) of "extern" gazelle@shell.xmission.com (Kenny McCormack) - 2019-08-20 13:11 +0000
  Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-20 16:09 +0200
  Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-20 16:41 +0100
    Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-20 16:47 +0100
    Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-20 19:27 +0200
      Re: Question about use (and non-use) of "extern" Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2019-08-20 13:54 -0400
        Re: Question about use (and non-use) of "extern" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-08-21 02:09 +0100
        Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-21 10:54 +0200
      Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-21 00:41 +0100
        Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-21 10:58 +0200
  Re: Question about use (and non-use) of "extern" richard@cogsci.ed.ac.uk (Richard Tobin) - 2019-08-20 16:10 +0000
    Re: Question about use (and non-use) of "extern" Siri Cruise <chine.bleu@yahoo.com> - 2019-08-20 16:23 -0700
  Re: Question about use (and non-use) of "extern" already5chosen@yahoo.com - 2019-08-21 06:50 -0700
    Re: Question about use (and non-use) of "extern" gazelle@shell.xmission.com (Kenny McCormack) - 2019-08-22 20:20 +0000
      Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-23 00:25 +0200
      Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-23 01:35 +0100
        Re: Question about use (and non-use) of "extern" gazelle@shell.xmission.com (Kenny McCormack) - 2019-08-23 03:40 +0000
          Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-23 11:09 +0100
        Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-08-23 08:16 +0200
      Re: Question about use (and non-use) of "extern" scott@slp53.sl.home (Scott Lurndal) - 2019-08-23 15:02 +0000
        Re: Question about use (and non-use) of "extern" Bart <bc@freeuk.com> - 2019-08-23 17:06 +0100
          Re: Question about use (and non-use) of "extern" gazelle@shell.xmission.com (Kenny McCormack) - 2019-08-23 18:40 +0000
          Re: Question about use (and non-use) of "extern" scott@slp53.sl.home (Scott Lurndal) - 2019-08-23 20:38 +0000
            Re: Question about use (and non-use) of "extern" Richard Damon <Richard@Damon-Family.org> - 2019-08-23 17:55 -0400
              Richard Damon weighs in... (Was: Question about use (and non-use) of "extern") gazelle@shell.xmission.com (Kenny McCormack) - 2019-08-23 23:50 +0000
  Re: Question about use (and non-use) of "extern" Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-08-27 15:03 -0700
    Re: Question about use (and non-use) of "extern" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-03 05:57 -0700
      Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-09-03 15:45 +0200
        Re: Question about use (and non-use) of "extern" Keith Thompson <kst-u@mib.org> - 2019-09-03 11:13 -0700
          Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-09-04 12:11 +0200
            Re: Question about use (and non-use) of "extern" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-04 08:18 -0400
          Re: Question about use (and non-use) of "extern" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-06 02:25 -0700
            Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-09-06 11:45 +0200
          Re: Question about use (and non-use) of "extern" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-09-06 21:15 +0100
        Re: Question about use (and non-use) of "extern" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-03 21:10 -0400
          Re: Question about use (and non-use) of "extern" David Brown <david.brown@hesbynett.no> - 2019-09-04 12:21 +0200
      Re: Question about use (and non-use) of "extern" Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-09-04 12:04 -0700
        Re: Question about use (and non-use) of "extern" James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-09-04 15:29 -0400
        Re: Question about use (and non-use) of "extern" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-06 02:46 -0700
  Re: Question about use (and non-use) of "extern" Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 10:01 +0200
    Re: Question about use (and non-use) of "extern" Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-09-05 10:43 -0700
      Re: Question about use (and non-use) of "extern" Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-06 20:24 +0200

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