Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ?

From	Shao Miller <sha0.miller@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ?
Date	2013-01-12 04:30 -0500
Organization	A noiseless patient Spider
Message-ID	<kcraev$fv2$1@dont-email.me> (permalink)
References	(3 earlier) <lna9sflmjj.fsf@nuthaus.mib.org> <kcpj52$lv9$1@dont-email.me> <lntxqnjs1a.fsf@nuthaus.mib.org> <kcqar7$asb$1@dont-email.me> <lnlibzjf8e.fsf@nuthaus.mib.org>

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On 1/11/2013 21:40, Keith Thompson wrote:
>
> Hardly.  If I want to access every 10th element of an array, starting
> from element 0, the above is a perfectly good way to do that.  If the
> array has 10 or fewer elements, it will access only element 0.  If it
> has 23 elements, it will access elements 0, 10, and 20, and then leave i
> with the value 30.
>
> It *doesn't matter* that the value of i goes past the last valid index
> of the array, as long as I don't use that value to index the array.  The
> upper bound for i is INT_MAX.
>
> (This is a significant difference between accessing arrays with integer
> indices vs. using pointers, and one that hadn't occurred to be before.
> It probably doesn't show up very often, we don't usually skip through
> arrays like this.)
>[...]
> I'm tempted to suggest that you've misspelled "bogus".  8-)}
>

Ha ha ha, yes...  From one beard-o to another: I could very well be 
misremembering. :)}  I won't argue about the usefulness any more since 
it's subjective and because I can't seem to track down the exact warning 
or the explanatory web page that I recalled.

When Syslinux was bitten by this warning (turned error because of 
-Werror), here're some numbers, for what they're worth:

   - I'm 100% certain that the diagnostic was issued and turned out to 
be a false positive, as there was no undefined behaviour in the code.

   - I'm 95% certain that the code involved was clearly explained by the 
web page explaining the cause of the false positive.

   - I'm 90% certain that the code was indeed a bit of an odd loop 
construct, and that reworking it could've avoided the false positive.

   - I'm 100% certain that the extra attention to the loop seemed 
warranted, but certainly didn't warrant aborting the build process, as 
was happening.

   - I'm 70% certain that the web page's explanation of the false 
positive was that the translation process treated the array index and 
pointer arithmetic in the same way, so the problem with a pointer 
pointing beyond one past was being misapplied to the index.

   - I'm 60% certain that an optimization level was involved.

   - I'm 70% certain that instead of changing the code, Syslinux chose 
to change the compilation options.

   - I'm 65% certain that FORTIFY_SOURCE was involved.

Oh well, I give up.  Heh.

-- 
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.

Cheerily,"  -- Richard Harter

Thread

[MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-09 07:26 -0800
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? James Kuyper <jameskuyper@verizon.net> - 2013-01-09 11:30 -0500
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-09 11:58 -0500
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-09 12:27 -0600
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-09 11:22 -0800
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-09 14:39 -0500
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 02:40 -0800
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 08:31 -0500
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 08:49 -0500
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 06:48 -0800
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 10:33 -0500
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 08:19 -0800
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 12:46 -0500
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 14:04 -0800
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-11 22:42 +0000
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-12 01:44 +0000
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 19:30 -0500
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 20:11 -0500
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-12 01:17 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 20:46 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-12 03:14 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-12 14:17 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-12 18:02 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-12 15:13 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 16:36 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 15:39 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 19:17 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 16:57 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 20:50 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 19:08 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 00:00 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 21:21 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 17:50 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 01:23 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 21:04 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 19:13 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 21:37 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 18:28 -0800
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 18:40 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-12 04:30 -0500
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-10 14:37 +0000
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 06:51 -0800
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 07:41 -0800
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-10 16:55 +0000
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 17:41 -0500
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-11 03:03 -0800
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-11 04:14 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? James Kuyper <jameskuyper@verizon.net> - 2013-01-11 08:32 -0500

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