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

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Question about use (and non-use) of "extern"
Date	2019-09-03 11:13 -0700
Organization	None to speak of
Message-ID	<lnblw1w8nl.fsf@kst-u.example.com> (permalink)
References	<qjgric$8u2$1@news.xmission.com> <qk49be$ac$1@dont-email.me> <86tv9ty1t4.fsf@mailhub.linuxsc.com> <qklqqd$u5t$1@dont-email.me>

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David Brown <david.brown@hesbynett.no> writes:
[...]
> If you have "int x;" in a translation unit (without any "static"
> involved), that counts as an external definition of "x" with external
> linkage.  (6.9.2p2 and 6.9.2p1).  If this identifier is used in an
> expression (other than an unevaluated expression, such as a sizeof),
> there must be exactly one definition of it (6.9p5).  These are "shall"
> rules - they are as strong as the standards get.
[...]

The C standard uses "shall" in two very different ways.

A violation of a "shall" in a constraint (all constraints are explicitly
marked as such) requires a diagnostic, just like a syntax error.
*That's* as close as the C standard gets to saying something is
illegal/invalid/disallowed (aside from #error).

A violation of a "shall" outside a constraint, like the one in 6.9p5,
cause the code to have undefined behavior, with no diagnostic required.
In some cases, such rules can leave room for common extensions.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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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