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Groups > comp.lang.c++ > #44773
| Subject | Re: This will work or not? |
|---|---|
| Newsgroups | comp.lang.c++ |
| References | (10 earlier) <a86af030-51bd-42d4-bc60-ae4be6f6ab0f@googlegroups.com> <nqbhfb$12q1$1@gioia.aioe.org> <nqbk65$bmh$1@dont-email.me> <RUdyz.14073$yi7.3443@fx42.iad> <nqbslm$7qn$1@dont-email.me> |
| From | Richard Damon <Richard@Damon-Family.org> |
| Message-ID | <2_pyz.47258$M27.22464@fx33.iad> (permalink) |
| Organization | Forte - www.forteinc.com |
| Date | 2016-09-02 21:48 -0400 |
On 9/2/16 8:51 AM, BartC wrote:
> On 02/09/2016 13:04, Richard Damon wrote:
>>
>> No it does not. a must match as_array[0] but the compiler is allowed to
>> insert padding between a and b. (Yes, for most machines it is unlikely
>> to, but it can). One motivation would be if you had a machine that was
>> naturally faster accessing 64 bit aligned objects than 32, but could
>> access 32 bit objects (perhaps with multiple instructions). An example
>> would be a 64 bit word based machine that natively only could access at
>> at 64 bit level, but to look like a more conventional machine the
>> compiler simulated smaller accesses (to avoid having CHAR_BIT == 64 and
>> sizeof(long long) == 1 which would give many programs problems, not
>> unlike the 16 bit access machine of yesteryear). On such a machine, b
>> would be at as_array[2], not [1] as expected, and [1] would map to a
>> padding word.
>>
>
> Easy to fix:
>
> #pragma pack(1) // or better, push then later pop
>
> union {
> struct {
> uint32_t a;
> uint32_t b;
> etc.
>
Since the pragma doesn't begin with STDC it is implementation defined,
so does nothing as far as what the standard says must happen. Once you
are willing to depend on implementation defined behavior,you don't need
the pragma, as the implementation can document their padding rules and
define that padding won't be added in this case (the standard doesn't
require this to be documented, but it can be).
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This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-08-31 14:59 +0200
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Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 12:37 +0100
Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:39 +0200
Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-02 17:40 +0200
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Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 08:04 -0400
Re: This will work or not? BartC <bc@freeuk.com> - 2016-09-02 13:51 +0100
Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 21:48 -0400
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Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 08:50 -0400
Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:59 +0200
Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 12:49 -0400
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Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 16:32 -0400
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Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 21:48 -0400
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Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:18 -0400
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Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 18:56 -0400
Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 00:27 +0100
Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:53 -0400
Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 18:18 +0100
Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 15:27 -0400
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Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-04 02:36 -0700
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Re: This will work or not? Vir Campestris <vir.campestris@invalid.invalid> - 2016-09-05 21:25 +0100
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