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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: [union] Pointers to inherited structs are valid ? |
| Date | 2013-01-22 14:15 -0800 |
| Organization | None to speak of |
| Message-ID | <lnfw1s50ft.fsf@nuthaus.mib.org> (permalink) |
| References | (16 earlier) <kdk3mf$td7$1@dont-email.me> <lnpq0y5iil.fsf@nuthaus.mib.org> <kdkgfv$hpd$1@dont-email.me> <lnd2wx5iko.fsf@nuthaus.mib.org> <kdmv2g$g34$1@dont-email.me> |
Shao Miller <sha0.miller@gmail.com> writes:
> On 1/22/2013 10:43, Keith Thompson wrote:
>> Shao Miller <sha0.miller@gmail.com> writes:
>>> On 1/21/2013 16:32, Keith Thompson wrote:
>>>> Shao Miller <sha0.miller@gmail.com> writes:
>>>>> 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)"
>>>>
>>>> No. Accessing an object with a trap representation has undefined
>>>> behavior.
>>>
>>> I don't understand this as an answer for "it is imprecise to say that
>>> such a value has a trap representation, because the behaviour is still
>>> well-defined." I don't understand it as an answer for "Otherwise, the
>>> last sentence of N1570's 6.5.3.2p4 is redundant." Did you think that I
>>> didn't think that accessing a trap representation was undefined behaviour?
>>
>> I *think* the phrase "such a value" refers to a pointer value such as
>> (void*)0x0000000C, in a Microsoft Windows 32-bit environment. If you're
>> talking about something else, the following probably won't make much
>> sense.
>>
>> Let's try a concrete example. Given this code snippet:
>>
>> int *ptr;
>> uintptr_t x = 0x0000000C;
>> memcpy(&ptr, &x, sizeof ptr);
>> int deref = *ptr;
>>
>> I assert that evaluating `*ptr` has undefined behavior.
>
> I'd say that it is implementation-defined behaviour; the implementation
> knows
I disagree, and I'll elaborate below.
> All I've been trying to point out is that, for Windows NT, 'ptr' does
> not contain a trap representation, so any use _other_ than indirection
> (via '*', '->', or function-call '()', for example) is well-defined.
> For indirection, it depends on if 'ptr' points to an 'int' object, as
> always.
Assume that there is no int object at address 0x0000000C. Do you
believe that the behavior of an access to ptr is well-defined *by
the C standard*?
A trap representation is a representation such that accessing an object
holding it has undefined behavior. Undefined behavior is behavior that
is not defined by the C standard, regardless of whether the
implementation chooses to define it.
[...]
> I don't think there needs to be an inference that were was a dereference
> to a null pointer, but other than that, I think you're right. I really
> wish I'd said "NULL_CLASS_PTR_DEREFERENCE" in the beginning, so it
> wouldn't have been an issue of discussion. The trap values that Geoff
> had discussed were not C trap representations, so I was likewise being
> loose with "null pointers." I am filled with regret about that. As
> we've already discussed, such a pointer value does not compare equal to
> a C null pointer, so the debugger could hardly claim that it's a C null
> pointer.
And in fact it doesn't claim either that 0x000000C a C null pointer, or
that it's any kind of null pointer.
> In order to understand what I mean by "loose" and why I might've done
> so, for what it's worth, there are some people who don't use the C
> definition for "null pointer" in non-pedantic discussion:
>
> Scott Noone (well known at OSR Online):
> http://social.msdn.microsoft.com/Forums/en-AU/windbg/thread/669a4137-f328-495f-a002-120fb6841542
>
> Chad Bramwell:
> http://www.altdevblogaday.com/2011/09/26/how-did-i-crash-in-that-func/
I disagree with your interpretation of both linked discussions. In both
cases, there was a null pointer dereference in C (or C++) code, which
resulted in an attempted dereference of a non-null but invalid pointer
(similar to 0x0000000C) in the generated machine code.
Machine code *implements* C semantics; it needn't precisely mirror them.
[...]
> Considering a Windows pointer with representation 0xC, I meant with
> regards to what Ben had asked about and what I had typed later in the
> same post: "The C Standard _plus_ the implementation say: It can be
> stored, read, passed, discarded, converted, compared, its size
> determined, etc. Pretty well anything that doesn't involve using the
> pointer for indirect access."
Sure, but the C standard doesn't permit any of those things. It's well
known that an implementation can define behavior that isn't defined by
the C standard. Such behavior is still "undefined behavior" as defined
by 3.4.3.
[...]
>> Can you provide a small self-contained program that's relevant to
>> the point you're making?
>>
>> If you're saying that there's some construct whose behavior you
>> say is implementation-defined, and I say is undefined, then please
>> point out the construct in question in your program.
>
> int main(void) {
> void * vp1 = (void *) 0xC;
> void * vp2 = vp1;
> return 0;
> }
>
> This is implementation-defined, not undefined. If the implementation
> does not define the result of the cast to be a trap representation, or
> better yet, defines that pointers are implemented as unsigned integers
> with all value bits, then there is no undefined behaviour. This is the
> case for Windows NT, as far as I'm aware.
No, it's undefined. It may additionally be defined by the
implementation, but it's not implementation-defined.
The phrase "implementation-defined behavior", as defined by the C
standard, refers *only* to behavior that is explicitly referred to
by the standard as "implementation-defined". It doesn't just mean
"behavior that is defined by the implementation".
In your program, the behavior of the initialization:
void * vp2 = vp1;
is not defined by the C standard. For example, it could cause the
program to crash in a conforming implementation. It is therefore,
by definition, undefined behavior. If an implementation chooses
to define its behavior, that doesn't change any of the above --
and the implementation is not obligated to document its choice.
> If someone wants to suggest that 'vp1' and 'vp2' have trap
> representations, I'd like to know why. If we can agree that their
> representations can provide hints to a debugger, then I'll be glad to
> read that.
They are pointer values such that accessing them has undefined
behavior. They are neither null pointers, nor pointers to any
object, nor pointers just past the end of any object. It's not 100%
clear to me, from the standard's definition of "trap representation",
that they *must* be trap representations, but I believe that
they are.
Speculation: Perhaps what bothers you is the idea that "undefined
behavior" and "trap representation" imply "This is evil, don't
touch!!!". They don't. It's perfectly valid for an implementation
to define the behavior of something that has "undefined behavior" in
the standard, or to use a C trap representation for its own purposes.
Certainly their representation can provide hints to a debugger; we've
seen that demonstrated. That doesn't cause them not to be trap
representations.
--
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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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
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