Re: What is the difference between (r+ and w+)

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: What is the difference between (r+ and w+)
Date	2017-06-20 11:53 +0100
Organization	A noiseless patient Spider
Message-ID	<87shivaq1s.fsf@bsb.me.uk> (permalink)
References	(7 earlier) <oi63gm$odt$1@dont-email.me> <87zid43n05.fsf@bsb.me.uk> <kfnfuewmbo4.fsf@x-alumni2.alumni.caltech.edu> <87zid4c4iy.fsf@bsb.me.uk> <kfn60fr8chr.fsf@x-alumni2.alumni.caltech.edu>

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Tim Rentsch <txr@alumni.caltech.edu> writes:
<snip>
> Now about inline.  For functions with internal linkage, 'inline'
> is just a performance hint, more or less the same way that
> 'register' is.
>
> For functions with external linkage, 'inline' carries some
> additional constraints (eg, function body has no modifiable
> static variables) to avoid semantic difficulties.  Aside from
> that, such functions behave just the way they would without
> 'inline', _except_ for one important case, which is most
> relevant here.
>
> If every declaration of a function has 'inline' but does NOT
> have 'extern', then the TU must define the function (again
> with 'inline' but not 'extern'), and that gives an /inline
> definition/.  An inline definition is special in that it
> does /define/ the function, with external linkage, but /does
> not provide an external definition/.  Another TU can provide
> an external definition but this TU does not.
>
> The point of this definitional sleight-of-hand is to have a
> function that behaves like it has a single definition (in another
> TU) but still can provide speedup by means of inline expansion at
> call sites.  Taking the address of such a function yields the
> address of the (single) external definition in another TU, but
> calling the function might call the global one or might result in
> an inline expansion.  We get to have our cake and eat it too.
>
> Getting back to functions like printf(), using an inline
> definition can work only if every declaration has 'inline' but
> does not have 'extern'.  That means redundant declarations in
> regular user code pretty much need to be excluded (because how
> would they know whether to use 'inline' or not?).  So the
> question I raised earlier, about whether redundant declarations
> are allowed, has implications regarding whether headers can
> give inline definitions for public functions such as printf().
>
> You may consider this a thank you to your pedantry for getting my
> interest up enough to dig into 'inline', etc.  I hope you feel my
> comments here have returned the favor. :)

Thank you, that was very clear and informative.  I had supposed that the
rules (which I had read at some point) were intended to achieve this but
I would not have been able to say how.  The confusing part is mainly due
to the terminology which the authors were stuck with, particularly the
difference between a name with external linkage and an external
definition.

That's an easier distinction to get hold of without inline because
inline allows a new and unusual-sounding combination: the definition of
a function whose name has external linkage but which is not, as is,
"linkable" -- there is no external definition to link to (though one may
be provided somewhere else).

-- 
Ben.

Thread

What is the difference between (r+ and w+) Christopher <invalid@invalid.invalid> - 2017-06-17 23:59 -0100
  Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-17 17:51 -0700
    Re: What is the difference between (r+ and w+) Christopher <invalid@invalid.invalid> - 2017-06-18 00:00 -0100
      Re: What is the difference between (r+ and w+) Jerry Stuckle <jstucklex@attglobal.net> - 2017-06-17 21:32 -0400
        Re: What is the difference between (r+ and w+) Christopher <invalid@invalid.invalid> - 2017-06-18 00:50 -0100
          Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 03:40 +0100
            Re: What is the difference between (r+ and w+) jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-18 11:48 +0200
              Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 13:13 +0100
                Re: What is the difference between (r+ and w+) Gareth Owen <gwowen@gmail.com> - 2017-06-18 16:20 +0100
            Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-18 14:49 +0100
              Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 15:46 +0100
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 00:16 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-19 05:37 +0100
                Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-19 09:25 -0700
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 17:35 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-19 18:19 +0100
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 20:51 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-19 21:16 +0100
                Re: What is the difference between (r+ and w+) Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 04:58 -0700
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-19 17:43 +0100
                Re: What is the difference between (r+ and w+) Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 22:16 -0700
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-20 11:53 +0100
                Re: What is the difference between (r+ and w+) Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-20 22:45 -0700
              Re: What is the difference between (r+ and w+) Good Guy <hello.world@example.com> - 2017-06-18 18:17 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 18:20 +0100
                Re: What is the difference between (r+ and w+) bartc <bc@freeuk.com> - 2017-06-18 18:27 +0100
                Re: What is the difference between (r+ and w+) jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-20 11:11 +0200
                Re: What is the difference between (r+ and w+) bartc <bc@freeuk.com> - 2017-06-20 10:27 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-20 10:54 +0100
                Re: What is the difference between (r+ and w+) jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-20 12:25 +0200
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-20 12:34 +0100
                Re: What is the difference between (r+ and w+) jameskuyper@verizon.net - 2017-06-20 05:37 -0700
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-20 14:11 +0100
                Re: What is the difference between (r+ and w+) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-20 14:35 +0100
                Re: What is the difference between (r+ and w+) raltbos@xs4all.nl (Richard Bos) - 2017-06-18 17:30 +0000
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 21:21 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 21:23 +0100
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 21:47 +0100
                Re: What is the difference between (r+ and w+) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-06-18 17:01 -0400
                Re: What is the difference between (r+ and w+) "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-19 10:41 -0400
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 21:50 +0100
                Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-18 13:52 -0700
                Re: What is the difference between (r+ and w+) jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-20 11:16 +0200
                Re: What is the difference between (r+ and w+) raltbos@xs4all.nl (Richard Bos) - 2017-06-26 15:54 +0000
                Re: What is the difference between (r+ and w+) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-06-18 17:13 -0400
                Re: What is the difference between (r+ and w+) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-06-18 17:17 -0400
                Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-18 15:14 -0700
                Re: What is the difference between (r+ and w+) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-06-18 18:24 -0400
                Re: What is the difference between (r+ and w+) Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-19 04:11 -0700
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 23:01 +0100
          Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-17 19:55 -0700
            Re: What is the difference between (r+ and w+) Christopher <invalid@invalid.invalid> - 2017-06-18 02:10 -0100
              Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 04:15 +0100
              Re: What is the difference between (r+ and w+) raltbos@xs4all.nl (Richard Bos) - 2017-06-18 08:31 +0000
                Re: What is the difference between (r+ and w+) Good Guy <hello.world@example.com> - 2017-06-18 18:28 +0100
                Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-18 11:25 -0700
                Re: What is the difference between (r+ and w+) raltbos@xs4all.nl (Richard Bos) - 2017-06-18 19:13 +0000
                Re: What is the difference between (r+ and w+) David Kleinecke <dkleinecke@gmail.com> - 2017-06-18 14:30 -0700
                Re: What is the difference between (r+ and w+) Richard Heathfield <rjh@cpax.org.uk> - 2017-06-18 23:04 +0100
              Re: What is the difference between (r+ and w+) GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-06-18 10:43 +0200
                Re: What is the difference between (r+ and w+) Jerry Stuckle <jstucklex@attglobal.net> - 2017-06-18 10:33 -0400
                Re: What is the difference between (r+ and w+) GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-06-19 01:27 +0200
                Re: What is the difference between (r+ and w+) Jerry Stuckle <jstucklex@attglobal.net> - 2017-06-18 21:16 -0400
      Re: What is the difference between (r+ and w+) Ian Collins <ian-news@hotmail.com> - 2017-06-18 13:45 +1200
      Re: What is the difference between (r+ and w+) Keith Thompson <kst-u@mib.org> - 2017-06-17 19:05 -0700

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