Re: gcc and 'include'

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From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: gcc and 'include'
Date	Mon, 30 Mar 2026 05:33:06 -0700
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Bart <bc@freeuk.com> writes:

> On 29/03/2026 00:53, Waldek Hebisch wrote:
>
>> Bart <bc@freeuk.com> wrote:
>>
>>> That you declare 'abc' as something to be imported, yet in the next line
>>> it is initialised as though it was exported.
>>
>> Well, C is not able to directy express notion of export and import.
>> Also, C does not have modules/interfaces etc.  Instead C object
>> may be global (which is specified by 'extern' or by lack of storage
>> class at file scope) or local.  C rules allow you to emulate
>> modules using header files.  Namely, in header file you put
>> 'extern' declaration for exports of given module.  Any user of
>> of the module must include the corresponding header file.
>> To ensure consistency also implementation of the module should
>> include the header file.  So everywhere elso you have just
>> 'extern' declaration, but in implementation there is 'extern'
>> declaration first followed by the definition.  If such
>> sequence was disallowed, then it would be harder to check
>> consistency of implementation and corresponding header.
>>
>> So in reasonable code 'extern' really means "part of interface"
>> without specifying if it is import or export.
>
> But, the rules are lax, and implementations exploit that by all
> behaving a little differently.

The rules are not lax;  they are carefully and precisely defined.
To see that one needs to know what the rules are, which in turn
depends on reading and understanding the C standard.

> TBF, the diverse implementations probably came first, and the
> standard had to accommodate existing practice.

Diverse implementations certainly came first, including especially
the original description of C in "The C Programming Language", by
Kernighan and Ritchie.  The rules for global symbols in K&R C are
markedly different than those in the current C standard.  There is
a long explanation of different options considered for standard C,
given in the C Rationale document.

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gcc and 'include' Bart <bc@freeuk.com> - 2026-03-26 22:36 +0000
  Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-26 16:12 -0700
    Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-27 10:55 +0000
      Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-27 13:49 +0100
      Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-27 10:51 -0700
      Re: gcc and 'include' Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-03-27 21:27 +0000
        Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-27 22:05 +0000
          Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-27 17:03 -0700
          Re: gcc and 'include' Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-03-28 05:10 +0100
          Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-28 20:37 +0300
            Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-28 18:33 +0000
              Re: gcc and 'include' antispam@fricas.org (Waldek Hebisch) - 2026-03-29 00:53 +0000
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-29 22:37 +0100
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 05:33 -0700
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-30 14:42 +0100
                Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-30 16:53 +0300
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-30 18:11 +0100
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 08:27 -0700
                Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-30 11:54 -0700
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-30 21:54 +0100
                Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-30 18:07 -0700
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-31 11:39 +0100
                Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-03-31 13:56 -0700
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-04-06 20:56 -0700
                Re: gcc and 'include' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-04-06 23:12 -0700
                Re: gcc and 'include' James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-30 21:06 -0400
              Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-29 11:24 +0200
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-29 12:44 +0100
                Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-31 15:57 +0200
            Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 07:20 -0700
          Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 05:07 -0700
  Re: gcc and 'include' Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-03-27 00:25 +0000
    Re: gcc and 'include' Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-03-30 07:13 +0000
      Re: gcc and 'include' Andrey Tarasevich <noone@noone.net> - 2026-03-30 07:54 -0700
        Re: gcc and 'include' Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-03-31 01:46 +0000
          Re: gcc and 'include' antispam@fricas.org (Waldek Hebisch) - 2026-03-31 05:28 +0000
  Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-27 04:10 +0300
    Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-26 19:08 -0700
      Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-27 16:47 +0300
        Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-27 16:43 +0100
        Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-27 09:03 -0700
          Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-29 11:46 +0300
            Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 08:19 -0700
              Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-30 20:08 +0300
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-31 00:26 -0700
                Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-31 11:27 +0300
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-04-07 09:45 -0700
        Re: gcc and 'include' Andrey Tarasevich <noone@noone.net> - 2026-03-28 10:25 -0700
          Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-29 10:37 +0300
            Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-29 11:30 +0200
            Re: gcc and 'include' Andrey Tarasevich <noone@noone.net> - 2026-03-29 07:22 -0700
            Re: gcc and 'include' James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-29 13:56 -0400
              Re: gcc and 'include' Michael S <already5chosen@yahoo.com> - 2026-03-29 21:39 +0300
                Re: gcc and 'include' James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-29 20:08 -0400
                Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-30 01:58 +0100
                Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 07:59 -0700
  Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-26 19:06 -0700
  Re: gcc and 'include' Bart <bc@freeuk.com> - 2026-03-27 16:20 +0000
    Re: gcc and 'include' David Brown <david.brown@hesbynett.no> - 2026-03-27 18:07 +0100
    Re: gcc and 'include' James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-03-28 18:48 -0400
  Re: gcc and 'include' Andrey Tarasevich <noone@noone.net> - 2026-03-27 22:38 -0700
    Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-28 00:30 -0700
      Re: gcc and 'include' Andrey Tarasevich <noone@noone.net> - 2026-03-29 16:15 -0700
        Re: gcc and 'include' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-30 00:41 -0700

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