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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Fortran Guy Learning C - Structures... |
| Date | 2015-02-09 08:22 -0800 |
| Organization | None to speak of |
| Message-ID | <ln61bbhx6i.fsf@nuthaus.mib.org> (permalink) |
| References | (6 earlier) <7fc0036a-112b-45e1-94c5-f54536464ef3@googlegroups.com> <7uufdad245u251a7r74ledo8au0alvbqhb@4ax.com> <507b368f-7ae8-410a-b7e6-e42f389fb2e3@googlegroups.com> <mba784$t2g$1@dont-email.me> <mbad9n$k0c$1@speranza.aioe.org> |
glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
>> On 09/02/15 11:24, Malcolm McLean wrote:
[...]
>>> int x[10] = {1, 2,3};
>>> if(x[3] == 0)
>>> printf("This compiler zero initialises arrays\n"):
[...]
>> The comparison of x[3] with 0 invokes undefined behaviour because the
>> value of x[3] is indeterminate. This has nothing to do with the fact
>> that x[3] is an array member and everything to do with the fact that its
>> value is indeterminate.
>
> On the usual processors now, it is unlikely that any unusual effects
> occur, other than that you don't know the value.
>
> I don't know of any current systems, but there used to be ones where
> it was possible to set the parity bits on memory such that a parity
> error would occur on access.
>
> Systems with ECC memory usually require a special process to initialize
> the ECC bits. When writing only part of an ECC block, such as a byte
> on word ECC, the memory system has to read and validate the existing
> value, change part, and rewrite. That fails if the ECC bits aren't
> valid. But the system should do all that before running user programs.
>
> Anyway, there are good reasons for the standard to say that
> it is UB, but it is rare on current systems.
As discussed, the remaining elements of x are actually initialized to 0.
But if they were uninitialized, an optimizing compiler could perform
code transformations based on the assumption that the program's behavior
is defined. Reading an uninitialized object can misbehave in ways that
go beyond just reading the arbitrary contents of that memory location.
--
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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Fortran Guy Learning C - Structures... jeffkrob16@gmail.com - 2015-02-03 09:33 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-03 17:59 +0000
Re: Fortran Guy Learning C - Structures... James Kuyper <jameskuyper@verizon.net> - 2015-02-03 13:01 -0500
Re: Fortran Guy Learning C - Structures... Barry Schwarz <schwarzb@dqel.com> - 2015-02-03 10:49 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <invalid@see.sig.invalid> - 2015-02-03 18:54 +0000
Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-03 11:50 -0800
Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-03 12:07 -0800
Re: Fortran Guy Learning C - Structures... Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-02-03 15:14 -0500
Re: Fortran Guy Learning C - Structures... Barry Schwarz <schwarzb@dqel.com> - 2015-02-03 15:06 -0800
Re: Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-03 22:19 -0800
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-03 21:40 +0000
Re: Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-03 22:56 -0800
Re: Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-03 23:02 -0800
Re: Fortran Guy Learning C - Structures... asetofsymbols@gmail.com - 2015-02-04 00:01 -0800
Re: Fortran Guy Learning C - Structures... jeffkrob16@gmail.com - 2015-02-04 10:53 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-04 11:21 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-04 19:32 +0000
Re: Fortran Guy Learning C - Structures... Barry Schwarz <schwarzb@dqel.com> - 2015-02-04 11:59 -0800
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-04 15:30 -0800
Re: Fortran Guy Learning C - Structures... jeffkrob16@gmail.com - 2015-02-04 19:41 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-05 04:06 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-05 10:36 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-05 10:59 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-05 11:03 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-05 12:30 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-05 12:46 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-05 12:49 +0000
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-05 15:08 -0800
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-05 08:48 -0800
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-05 15:16 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-05 23:35 +0000
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-07 12:11 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-07 12:22 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-07 22:03 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-07 14:47 -0800
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-07 17:15 -0800
Re: Fortran Guy Learning C - Structures... Ian Collins <ian-news@hotmail.com> - 2015-02-08 17:22 +1300
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-08 02:47 -0800
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 11:20 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-08 14:36 -0800
Re: Fortran Guy Learning C - Structures... Ian Collins <ian-news@hotmail.com> - 2015-02-09 11:51 +1300
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 15:02 -0800
Re: Fortran Guy Learning C - Structures... Geoff <geoff@invalid.invalid> - 2015-02-08 16:55 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-09 03:24 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 11:56 +0000
Re: Fortran Guy Learning C - Structures... David Brown <david.brown@hesbynett.no> - 2015-02-09 14:25 +0100
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 14:00 +0000
Re: Fortran Guy Learning C - Structures... David Brown <david.brown@hesbynett.no> - 2015-02-09 15:26 +0100
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 14:39 +0000
Re: Fortran Guy Learning C - Structures... David Brown <david.brown@hesbynett.no> - 2015-02-09 16:59 +0100
Re: Fortran Guy Learning C - Structures... raltbos@xs4all.nl (Richard Bos) - 2015-02-09 16:01 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-09 13:39 +0000
Re: Fortran Guy Learning C - Structures... James Kuyper <jameskuyper@verizon.net> - 2015-02-09 09:46 -0500
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 14:50 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-09 08:22 -0800
Re: Fortran Guy Learning C - Structures... gazelle@shell.xmission.com (Kenny McCormack) - 2015-02-09 13:24 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-09 16:03 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-09 11:01 -0800
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-09 21:54 +0000
Re: Fortran Guy Learning C - Structures... Öö Tiib <ootiib@hot.ee> - 2015-02-09 19:22 -0800
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-10 11:34 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-10 14:15 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-10 00:38 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-10 09:21 +0000
Re: Fortran Guy Learning C - Structures... Ian Collins <ian-news@hotmail.com> - 2015-02-10 22:24 +1300
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-10 09:25 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-10 09:03 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-10 17:12 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-10 09:56 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-10 18:12 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-10 11:24 -0800
Re: Fortran Guy Learning C - Structures... gazelle@shell.xmission.com (Kenny McCormack) - 2015-02-10 20:18 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-10 09:16 -0800
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-10 22:12 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-11 02:58 +0000
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-10 19:00 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-11 07:56 +0000
Re: Fortran Guy Learning C - Structures... David Thompson <dave.thompson2@verizon.net> - 2015-02-22 18:08 -0500
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-10 16:01 -0800
Re: Fortran Guy Learning C - Structures... Ian Collins <ian-news@hotmail.com> - 2015-02-10 11:26 +1300
Re: Fortran Guy Learning C - Structures... David Brown <david.brown@hesbynett.no> - 2015-02-10 10:29 +0100
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-09 08:32 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-09 11:09 -0800
Re: Fortran Guy Learning C - Structures... gazelle@shell.xmission.com (Kenny McCormack) - 2015-02-09 19:39 +0000
Re: Fortran Guy Learning C - Structures... Martin Shobe <martin.shobe@yahoo.com> - 2015-02-09 14:04 -0600
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-09 12:24 -0800
Re: Fortran Guy Learning C - Structures... David Brown <david.brown@hesbynett.no> - 2015-02-10 10:34 +0100
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 11:42 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-07 22:50 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-07 17:07 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 11:29 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-08 12:14 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 13:44 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-08 22:39 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 11:12 -0800
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 20:01 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 20:06 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 21:04 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 21:11 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-08 23:48 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 13:30 -0800
Re: Fortran Guy Learning C - Structures... Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-02-08 16:47 -0500
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 14:53 -0800
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-09 02:31 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-09 10:35 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-09 13:57 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-09 07:59 -0800
Re: Fortran Guy Learning C - Structures... Öö Tiib <ootiib@hot.ee> - 2015-02-09 18:15 -0800
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-10 05:54 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-09 16:17 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 22:04 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 14:57 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 23:45 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-09 02:53 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 09:29 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-09 10:10 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 11:24 +0000
Re: Fortran Guy Learning C - Structures... Ike Naar <ike@iceland.freeshell.org> - 2015-02-09 17:36 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-09 17:56 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-09 10:51 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-09 11:10 +0000
Re: Fortran Guy Learning C - Structures... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-02-09 11:25 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-09 11:43 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-10 02:13 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-10 03:26 +0000
Re: Fortran Guy Learning C - Structures... raltbos@xs4all.nl (Richard Bos) - 2015-02-09 21:18 +0000
Re: Fortran Guy Learning C - Structures... Bartc <bc@freeuk.com> - 2015-02-09 23:11 +0000
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 12:10 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-08 11:18 -0800
Re: Fortran Guy Learning C - Structures... Richard Heathfield <rjh@cpax.org.uk> - 2015-02-08 19:27 +0000
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-09 15:00 -0800
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-09 16:00 -0800
Re: Fortran Guy Learning C - Structures... Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-02-05 02:49 -0800
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-05 11:16 +0000
Re: Fortran Guy Learning C - Structures... Keith Thompson <kst-u@mib.org> - 2015-02-05 09:00 -0800
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-06 00:29 +0000
Re: Fortran Guy Learning C - Structures... Robert Wessel <robertwessel2@yahoo.com> - 2015-02-05 19:20 -0600
Re: Fortran Guy Learning C - Structures... "Charles Richmond" <numerist@aquaporin4.com> - 2015-02-06 16:15 -0600
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-07 00:57 +0000
Re: Fortran Guy Learning C - Structures... Kaz Kylheku <kaz@kylheku.com> - 2015-02-07 01:40 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-07 08:22 +0000
Re: Fortran Guy Learning C - Structures... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-07 12:06 +0000
Re: Fortran Guy Learning C - Structures... John Bode <jfbode1029@gmail.com> - 2015-02-06 15:03 -0800
Re: Fortran Guy Learning C - Structures... hormelfree@gmail.com - 2015-02-05 15:12 -0800
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