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| From | Ian Pilcher <arequipeno@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Aliasing through a union pointer |
| Date | 2015-11-23 13:22 -0600 |
| Organization | A noiseless patient Spider |
| Message-ID | <n2voss$m44$1@dont-email.me> (permalink) |
| References | <n2vdrr$6c5$1@dont-email.me> <n2vfjp$e1t$1@dont-email.me> |
On 11/23/2015 10:43 AM, Eric Sosman wrote: > The first difficulty that occurs to me is one that's already > occurred to you: You're screwed if the struct has any padding, > either interstitial or at the end. That seems fairly unlikely, > but is permitted by the language. As a "Just In Case" measure, > you might try asserting that > > offsetof(struct header, cksum) + sizeof(uint64_t) > == sizeof(struct header) > > ... which would detect the presence of padding. Ah nice. I'm planning using gcc's __attribute__((packed)), so this shouldn't be an issue, but I like the idea of the assert, or better yet static_assert. > It seems to me that once you've ensured there's no padding -- > which you ought to do in any case -- the union doesn't buy you much: > Just cast `hdr' to a `uint64_t*' and blaze away. This also has the > virtue of avoiding the (highly unlikely) alignment problem, because > the struct alignment is at least good enough for a uint64_t. For some reason, I thought that I wasn't allowed to do that. I have vague memories of gcc warnings about type punning and strict aliasing. I definitely don't get any such warnings when doing that, however, so it looks like a case of over-thinking. (Root cause being that I'm still not confident that I grasp the aliasing rules.) On 11/23/2015 10:49 AM, Eric Sosman wrote: > An even Bigger Actual Problem is the dope slap I'm about > to administer to myself for reading too hastily. "Sorry about > that, Chief." At least you caught it on your own. Always more dignified that way. Thanks for the help! -- ======================================================================== Ian Pilcher arequipeno@gmail.com -------- "I grew up before Mark Zuckerberg invented friendship" -------- ========================================================================
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Aliasing through a union pointer Ian Pilcher <arequipeno@gmail.com> - 2015-11-23 10:14 -0600
Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 11:43 -0500
Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 11:49 -0500
Re: Aliasing through a union pointer Ian Pilcher <arequipeno@gmail.com> - 2015-11-23 13:22 -0600
Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-23 14:38 -0500
Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 10:38 -0800
Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-25 14:17 -0500
Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 13:16 -0800
Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-23 21:46 +0100
Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 22:20 +0000
Re: Aliasing through a union pointer Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 16:52 -0600
Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 23:10 +0000
Re: Aliasing through a union pointer Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 19:05 -0600
Re: Aliasing through a union pointer Nick Bowler <nbowler@draconx.ca> - 2015-11-23 22:51 +0000
Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-23 15:49 -0800
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-23 16:01 -0800
Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 09:55 +0100
Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-24 14:29 +0000
Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 22:58 +0100
Re: Aliasing through a union pointer Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-24 17:30 -0500
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 15:27 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-25 12:57 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 16:31 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-25 18:28 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 06:54 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 08:59 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 13:01 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 12:58 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 16:44 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 14:12 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 21:34 -0800
Re: Aliasing through a union pointer Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-25 21:55 -0800
Re: Aliasing through a union pointer Ian Collins <ian-news@hotmail.com> - 2015-11-26 20:51 +1300
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-26 11:39 -0800
Re: Aliasing through a union pointer Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-26 00:06 +0000
Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-24 11:51 -0800
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-24 12:20 -0800
Re: Aliasing through a union pointer Keith Thompson <kst-u@mib.org> - 2015-11-24 21:52 -0800
Re: Aliasing through a union pointer supercat@casperkitty.com - 2015-11-25 08:30 -0800
Re: Aliasing through a union pointer David Brown <david.brown@hesbynett.no> - 2015-11-24 23:04 +0100
Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 10:10 -0800
Re: Aliasing through a union pointer Richard Damon <Richard@Damon-Family.org> - 2015-11-25 11:02 -0800
Re: Aliasing through a union pointer Philip Lantz <prl@canterey.us> - 2015-11-26 13:06 -0800
Re: Aliasing through a union pointer Richard Damon <Richard@Damon-Family.org> - 2015-11-26 13:09 -0800
Re: Aliasing through a union pointer Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 13:08 -0800
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