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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: [union] Pointers to inherited structs are valid ? |
| Date | 2013-01-21 14:07 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <kdk3mf$td7$1@dont-email.me> (permalink) |
| References | (17 earlier) <0.0b1231aec01398fe5573.20130120235839GMT.87libn76fk.fsf@bsb.me.uk> <kdi97a$32l$1@dont-email.me> <0.3967aeb7bf6a1530832e.20130121025322GMT.87fw1v6ycd.fsf@bsb.me.uk> <kdiibb$adj$1@dont-email.me> <0.daa800410407a948d170.20130121163138GMT.874nia7b11.fsf@bsb.me.uk> |
On 1/21/2013 11:31, Ben Bacarisse wrote: > Shao Miller <sha0.miller@gmail.com> writes: >> >> Not explicitly, no. I thought it would be obvious that it was the >> context of a Windows debugger notifying the programmer, since Geoff >> had just discussed that. > > I don't recall Geoff saying that anything was unexpected. That's your > phrase that you admit you have not defined. I can't fathom what you > think is unexpected behaviour or what point that would made. Should I > be able to work it out? > No, I'm explaining the nature of my mistake. "I thought it would be obvious." <- Wrong. > Geoff's post looks simple and correct: the standard permits trap > representations that produce undefined behaviour so an implementation is > permitted to use special values to trigger interesting effects (either > in a debugger or else where). > I agree that his post does look that way and that C does allow for that. There's a subtle bit here, though: If we're discussing a program, once that program invokes undefined behaviour, anything goes. However, if I'm not mistaken, with Windows NT, if you have a pointer value which does not point to an object, the program will continue to operate as per the C semantics until such a time (if ever) as that pointer might be used for indirect access. So it is imprecise to say that such a value has a trap representation, because the behaviour is still well-defined. Otherwise, the last sentence of N1570's 6.5.3.2p4 is redundant: "If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.102)" But anyway, a pointer value pointing to no object can still be "trapped" by Windows during indirection and can still provide useful information to a Windows debugger. It just does not precisely match C's trap representation, for behavioural differences. > > No I read in context and try to respond in context as well. I don't > think I missed the context. > Well then that's my mistake. >> In that quote, I thought it was obvious that this >> was in the context of using a Microsoft debugger, where the behaviour >> would agree with the semantics of an "unspecified value"; not warning >> the programmer during a read of the value (which it _could_). > > It could be an unspecified value (you are explicitly using C terms here) > if it is a valid pointer value. It may well be. Is it? It may equally > well not be. It could be either a trap representation or an unspecified > value but you seem to suggest that it is one not the other. You seem to > suggest that one possibility is more likely than the other for reasons > that are spurious. The best evidence that it might not be a trap > representation is that it's a valid pointer, but you give no evidence for > that -- quite the contrary in fact. > I'd say that it is not a trap representation. Take Windows NT's 'IRP' structure. It has a sub-member called 'Tail.Overlay.DriverContext', which is an array of 4 'void *'. This is one of _very_few_ places where a driver can associate information with an IRP, and is extremely valuable for that reason. The implementation defines the results of casting an appropriately-sized integer value to a 'void *', so such an integer can be "passed" via this mechanism. We _certainly_ would not wish to believe that this results in undefined behaviour, so we certainly would not wish to believe that such a result is a trap representation. >> >> B: "Here are Microsoft's documented [non-C] trap values..." > > If this was Geoff, he correctly called then "trap representation" > (rather than values) and he referred to "they" meaning the C committee. > He was talking at C trap representations not "[non-C] trap values". > This was in regards to the lower part of his post where he specifically says "Microsoft documents their compiler trap values". >> Me: "Nice summary! Say, isn't there another one, 0x0000000C, as a >> [non-C] trap for [non-C] null pointers? I don't remember..." >> >> C: "Oh no, that wouldn't be a [C] null pointer." >> >> Me: "Yes, well I was talking about B's [non-C] trap values. I see a >> Microsoft debugger catch these things and call them [non-C] null >> pointers." > > I think you missed Geoff's point. > I doubt it. -- - Shao Miller -- "Thank you for the kind words; those are the kind of words I like to hear. Cheerily," -- Richard Harter
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[union] Pointers to inherited structs are valid ? Maciej Labanowicz <m.labanowicz@gmail.com> - 2013-01-01 03:45 -0800
Re: [union] Pointers to inherited structs are valid ? Barry Schwarz <schwarzb@dqel.com> - 2013-01-01 17:59 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:18 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-01 21:06 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:45 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-02 19:56 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 08:28 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-03 15:31 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 20:55 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-03 21:00 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:56 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-04 12:59 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 20:00 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 00:47 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-06 23:06 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 04:03 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 08:25 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-13 10:34 -0800
Re: Off-Topic: Irritating Posting Patterns Shao Miller <sha0.miller@gmail.com> - 2013-01-13 16:00 -0500
Re: Off-Topic: Irritating Posting Patterns Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:27 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-07 07:41 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-12 14:52 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 15:19 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-14 12:04 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 18:52 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 00:01 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 05:43 -0500
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 09:01 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 18:32 -0500
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-16 17:32 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-16 20:57 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-17 07:55 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-17 16:47 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 08:41 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 15:53 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-18 17:38 -0800
Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-19 02:31 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:15 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-19 20:03 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 02:19 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 07:57 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 15:25 -0500
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-20 23:58 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:30 -0500
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 02:53 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 00:05 -0500
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 16:31 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 14:07 -0500
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 21:05 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:13 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 16:11 -0800
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 04:59 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 01:05 -0500
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 14:53 +0000
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:38 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 20:53 -0800
Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-22 00:58 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:40 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:32 -0800
Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-23 23:21 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 04:53 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-25 10:49 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:25 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 23:41 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 13:32 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 17:46 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 07:43 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:07 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:15 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 14:31 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 21:32 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:17 -0500
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-23 19:49 -0500
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-23 18:55 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-24 11:01 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-24 14:11 -0500
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-22 17:51 -0800
Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-20 16:48 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 21:33 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 09:23 -0800
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 17:07 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-20 22:20 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-20 22:02 -0800
Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 07:00 +0000
Re: [union] Pointers to inherited structs are valid ? James Kuyper <jameskuyper@verizon.net> - 2013-01-21 08:17 -0500
Re: [union] Pointers to inherited structs are valid ? Dr Nick <nospam-4@temporary-address.org.uk> - 2013-01-21 14:13 +0000
Re: [union] Pointers to inherited structs are valid ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-21 14:12 +0000
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 08:20 -0800
Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:41 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:37 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 12:17 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 16:33 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-21 15:50 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-21 19:45 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-22 00:18 -0500
Re: [union] Pointers to inherited structs are valid ? Keith Thompson <kst-u@mib.org> - 2013-01-22 08:01 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-18 23:02 -0500
Re: [union] Pointers to inherited structs are valid ? Philip Lantz <prl@canterey.us> - 2013-01-19 01:28 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-19 12:08 -0500
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 09:27 -0800
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 12:46 -0500
Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-21 20:48 +0000
Re: [union] Pointers to inherited structs are valid ? Geoff <geoff@invalid.invalid> - 2013-01-21 18:52 -0800
Re: [union] Pointers to inherited structs are valid ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 04:07 +0000
Re: [union] Pointers to inherited structs are valid ? Shao Miller <sha0.miller@gmail.com> - 2013-01-21 23:15 -0500
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-16 10:59 -0800
Re: [union] Pointers to inherited structs are valid ? Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-02 10:34 -0800
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