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

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> On 9/3/2019 5:57 AM, Tim Rentsch wrote:
>
>> 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.)
>
> They are as "misleading" as any other statement that does not
> consist entirely of terms strictly defined by the language
> standard. [...]

Oh that's nonsense.  It's perfectly possible to give a reasonble
and not overly jargoned answer, as for example:

The C standard allows it to be okay if an implementation chooses
to act that way, and some implementations do (sometimes needing
particular compiler options).  However, the standard doesn't
require implementations to act in this way, or any way at all if
there is more than one case of "extern" being left off, so it
can't be counted on.  Unless there are special circumstances, and
you know what you're doing (and have a good reason for doing
it!), it's better to follow the usual pattern of using "extern"
in every separately compiled file where the variable is used, and
defining the variable (that is, leaving off the "extern") in
exactly one separately compiled file.  (By the way, the standard
uses "implementation" as an umbrella term to include what we
usually think of as compilers, linkers, etc.)

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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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