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Groups > comp.lang.c > #129420

Re: lvalue types

Path csiph.com!eternal-september.org!feeder.eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: lvalue types
Date Wed, 18 Apr 2018 11:01:57 -0700
Organization None to speak of
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supercat@casperkitty.com writes:
> On Wednesday, April 18, 2018 at 11:12:49 AM UTC-5, Keith Thompson wrote:
>> supercat@casperkitty.com writes:
>> [...]
>> > The ability to pass the address of an object to functions that can use
>> > the object, at least until the next time the object is addressed via other
>> > means, is no less fundamental than the ability to use the structure
>> > directly.  Both clang and gcc, however, *invent* a distinction between
>> > direct and indirect use of objects.
>> 
>> I'm probably going to regret asking this, but can you support the
>> claim in your last sentence?  (I request that you do so without
>> asking rhetorical questions.)
>
> I've posted code many times where both gcc and clang create such a
> distinction.
>
>     struct s1 { int x; };
>     struct s2 { int x; };
>     union u { struct s1 v1; struct s2 v2; } uarr[10];
>
>     int readS1x(struct s1 *p) { return p->x; }
>     void writeS2x(struct s2 *p) { p->x = 5; }
>
>     int test(union u *uarr2, int i, int j)
>     {
>       if (readS1x(&uarr[i].v1))
>           writeS2x(&uarr[j].v2);
>       return readS1x(&uarr[i].v1);
>     }
>
> Both gcc and clang will assume that there's no way an operation on a struct
> s2 can affect a struct s1, even if all operations on the storage using
> different types are separated by actions which derive pointers of those
> types from the same lvalue "uarr".

Can you substantiate your claim about what assumptions gcc and
clang make, by showing us a self-contained program whose output
clearly demonstrates those assumptions?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

lvalue types supercat@casperkitty.com - 2018-03-01 16:38 -0800
  Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 17:17 -0800
  Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-01 17:37 -0800
    Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 22:00 -0800
    Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 23:49 -0800
      Re: lvalue types asetofsymbols@gmail.com - 2018-03-05 06:56 -0800
    Re: lvalue types supercat@casperkitty.com - 2018-03-02 00:22 -0800
      Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 01:23 -0800
      Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 05:04 -0800
        Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 06:06 -0800
          Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 08:12 -0800
      Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-02 09:50 -0800
        Re: lvalue types supercat@casperkitty.com - 2018-03-02 11:33 -0800
          Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-02 11:45 -0800
            Re: lvalue types supercat@casperkitty.com - 2018-03-02 16:28 -0800
              Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-02 16:54 -0800
                Re: lvalue types supercat@casperkitty.com - 2018-03-02 17:50 -0800
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-02 19:43 -0800
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-02 22:51 -0800
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 23:07 -0800
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 01:44 -0800
                Re: lvalue types supercat@casperkitty.com - 2018-03-03 13:01 -0800
          Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-02 22:21 -0800
    Re: lvalue types supercat@casperkitty.com - 2018-03-02 09:24 -0800
    Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-03 00:58 -0800
      Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-03 01:13 -0800
      Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-03 13:01 -0800
        Re: lvalue types supercat <flatfinger@casperkitty.com> - 2018-03-03 17:43 -0800
        Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-04 02:22 -0800
          Re: lvalue types supercat@casperkitty.com - 2018-03-04 13:03 -0800
          Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-04 13:03 -0800
            Re: lvalue types bartc <bc@freeuk.com> - 2018-03-04 22:32 +0000
            Re: lvalue types supercat@casperkitty.com - 2018-03-05 09:45 -0800
            Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-19 09:54 -0700
              Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-19 11:52 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-26 23:44 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-27 09:14 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-03-27 12:21 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-28 17:01 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-29 01:54 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-30 08:32 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-30 09:25 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-03-30 10:58 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-31 00:52 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-02 15:19 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-03 10:06 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-03 11:33 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-03 12:19 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-03 12:48 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-05 10:17 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-05 10:52 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-05 12:39 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-06 06:13 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-04 16:38 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-03-27 12:09 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-28 10:48 -0700
              Re: lvalue types supercat@casperkitty.com - 2018-03-19 12:27 -0700
          Re: lvalue types jameskuyper@verizon.net - 2018-04-03 14:02 -0700
            Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-04 17:32 -0700
              Re: lvalue types jameskuyper@verizon.net - 2018-04-04 20:24 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-09 08:31 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-11 11:19 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-11 12:44 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-12 06:45 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-12 07:54 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-12 14:26 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-12 14:43 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-12 17:40 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-13 03:02 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-13 00:27 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-13 07:19 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-16 08:52 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-16 10:28 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-16 13:58 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-16 14:52 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-16 16:22 -0700
                Re: lvalue types Richard Damon <Richard@Damon-Family.org> - 2018-04-16 21:51 -0400
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-16 19:42 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-16 21:30 -0700
                Re: lvalue types Richard Damon <Richard@Damon-Family.org> - 2018-04-17 07:01 -0400
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-17 04:22 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-19 02:43 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 07:25 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-17 08:48 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 09:54 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-17 10:37 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 11:00 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-17 11:48 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 12:45 -0700
                Re: lvalue types scott@slp53.sl.home (Scott Lurndal) - 2018-04-17 19:55 +0000
                Re: lvalue types jameskuyper@verizon.net - 2018-04-17 13:13 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 14:00 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-17 19:45 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 08:27 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-18 09:12 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 10:13 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-18 11:01 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 11:51 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 12:15 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 12:30 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 13:25 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-19 10:51 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-18 12:38 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 14:39 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 21:17 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-19 02:40 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-19 03:01 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-19 06:49 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-19 07:58 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-20 08:19 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-19 12:17 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-19 12:51 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-19 13:37 -0700
                Re: lvalue types scott@slp53.sl.home (Scott Lurndal) - 2018-04-19 21:08 +0000
                Re: lvalue types supercat@casperkitty.com - 2018-04-19 14:50 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-19 14:56 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-19 16:13 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-21 10:27 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-23 11:37 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-18 16:07 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 09:20 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 11:25 -0700
                Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-04-17 14:13 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-17 15:05 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-17 19:57 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 08:43 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 09:36 -0700
                Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-17 11:37 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-17 20:23 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 09:23 -0700
                Re: lvalue types scott@slp53.sl.home (Scott Lurndal) - 2018-04-18 17:29 +0000
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 10:47 -0700
                Re: lvalue types bartc <bc@freeuk.com> - 2018-04-18 20:04 +0100
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 10:48 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 13:23 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-18 14:26 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-18 15:36 -0700
                Re: lvalue types jameskuyper@verizon.net - 2018-04-19 10:39 -0700
                Re: lvalue types supercat@casperkitty.com - 2018-04-19 12:42 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-16 21:29 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-23 11:39 -0700
                Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-06-14 02:53 -0700
              Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-04 21:30 -0700
      Re: lvalue types supercat@casperkitty.com - 2018-03-03 14:19 -0800
  Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-01 18:03 -0800
  Re: lvalue types John Bode <jfbode1029@gmail.com> - 2018-03-21 10:33 -0700
    Re: lvalue types Keith Thompson <kst-u@mib.org> - 2018-03-21 11:53 -0700
      Re: lvalue types Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-22 00:11 -0700
    Re: lvalue types supercat@casperkitty.com - 2018-03-21 11:53 -0700
    Re: lvalue types Tim Rentsch <txr@alumni.caltech.edu> - 2018-03-26 23:04 -0700

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