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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: How does strict aliasing work with parallel arrays in a union or const-initialized data |
| Date | 2015-10-30 13:35 -0700 |
| Organization | None to speak of |
| Message-ID | <ln611o9jt2.fsf@kst-u.example.com> (permalink) |
| References | (4 earlier) <b49266e4-9532-4179-9d4c-fd2c02d7236b@googlegroups.com> <lnlhambmrj.fsf@kst-u.example.com> <kfnoafibkpk.fsf@x-alumni2.alumni.caltech.edu> <lnh9labhuv.fsf@kst-u.example.com> <kfnwpu486q9.fsf@x-alumni2.alumni.caltech.edu> |
Tim Rentsch <txr@alumni.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>> Keith Thompson <kst-u@mib.org> writes:
>>>> supercat@casperkitty.com writes:
>>>>> On Wednesday, October 28, 2015 at 2:15:29 PM UTC-5, Tim Rentsch wrote:
>>>>>> supercat writes:
>>>>>>> [snip]
>>>>> If I have one translation unit which contains the definitions:
>>>>>
>>>>> struct FOO { int x,y; char b[]; };
>>>>> struct GOO { int x,y; char b[4]; };
>>>>>
>>>>> and another that contains those two definitions plus
>>>>>
>>>>> union FOOGOO { FOO f; GOO g; };
>>
>> [...]
>>
>>>> No. The standard explicitly says that the layouts must be identical in
>>>> the second unit. It makes no such guarantee for the first.
>>>
>>> I believe it does, indirectly. The struct FOO in TU1 and the
>>> struct FOO in TU2 are compatible types, so they must use the same
>>> layout. Similarly for struct GOO's in TU1 and TU2. Therefore
>>> the two struct types in TU1 must use the same layout, because the
>>> corresponding compatible types might have to in another (unknown)
>>> TU that has those corresponding types in a union.
>>
>> Right. I was thinking of the more general case of two distinct
>> struct types with a common initial subsequence. I missed the fact
>> that struct FOO and struct GOO are compatible.
>
> Why do you say that struct FOO and struct GOO are compatible?
> They don't look compatible to me, even inter-TU. Or did you (I
> hesitate to even ask the question) mean something different from
> compatible in the sense the Standard uses it?
I didn't entirely think it through, but ...
The x and y members have the same type, and the b members have
compatible types; see N1570 6.7.6.2 for compatibility of array types.
Normally two structure types are compatible only if they're the
same type, but 6.2.7p1 says that two structure types are compatible
if, among other requirements, their corresponding members have
compatible type *and* the structure types are declared in separate
translation units.
But structs FOO and GOO are declared in the same translation unit,
so as far as I can tell they're not compatible.
This isn't a part of the standard I've spent a lot of time thinking
about, so it's possible I'm missing something.
--
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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How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-21 16:47 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-24 21:20 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-25 11:44 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 12:15 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-28 14:25 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 16:23 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 17:08 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 18:09 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-28 20:02 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Bartc <bc@freeuk.com> - 2015-10-29 10:55 +0000
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-29 06:50 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-29 08:52 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 10:21 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-29 08:50 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-29 12:25 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 13:02 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-30 13:35 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 19:57 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-31 12:27 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-30 17:02 -0400
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-30 14:16 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 08:13 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-31 08:42 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-10-31 10:40 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-02 01:45 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-02 06:57 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-02 08:12 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-02 08:54 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-04 07:24 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-04 09:22 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-04 19:48 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-05 09:44 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-07 10:17 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data raltbos@xs4all.nl (Richard Bos) - 2015-11-10 15:37 +0000
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data supercat@casperkitty.com - 2015-11-10 09:01 -0800
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 16:48 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Keith Thompson <kst-u@mib.org> - 2015-10-28 16:11 -0700
Re: How does strict aliasing work with parallel arrays in a union or const-initialized data Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 16:37 -0700
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