Re: Aliasing through a union pointer

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Aliasing through a union pointer
Date	2015-11-24 22:58 +0100
Organization	A noiseless patient Spider
Message-ID	<n32md3$v2d$1@dont-email.me> (permalink)
References	(2 earlier) <n2voss$m44$1@dont-email.me> <n2vtq1$avd$1@dont-email.me> <slrnn5749h.5q5.grahn+nntp@frailea.sa.invalid> <n318h6$6lq$1@dont-email.me> <slrnn58t2f.5q5.grahn+nntp@frailea.sa.invalid>

Show all headers | View raw

On 24/11/15 15:29, Jorgen Grahn wrote:
> On Tue, 2015-11-24, David Brown wrote:
>> On 23/11/15 23:20, Jorgen Grahn wrote:
>>> On Mon, 2015-11-23, David Brown wrote:
>>> ...
>>>> I'd advise against __attribute__((packed)) (or equivalents on other
>>>> compilers) - either it makes no difference because there is no padding,
>>>> or you will end up with structures with fields that don't match their
>>>> alignment.  On some targets, that means run-time errors - on others, it
>>>> means seriously degraded code speed.  So it is rarely a good idea.
>>>
>>> I think I agree, but are there really cases where you say
>>> __attribute__((packed)) and then suffer a crash because of it?  The
>>> compiler shouldn't generate a struct, and code to access it which will
>>> crash if it's ever run.
>>>
>>
>> The attribute tells gcc that it should generate the struct layout
>> exactly as /you/ tell it - /not/ as the target system normally requires
>> with respect to alignment.
> ...
>> So if it is critical, you can use "packed" if you make sure it is safe
>> to do so on /your/ target, with /your/ compiler, using /your/ access
>> methods - and treat it as highly implementation-dependent.
>
> Makes sense.  I just wish the gcc documentation had expressed it that
> way, so I could point people to it when they abuse it.
>
>> Or just use explicit padding with a static assertion on the final size
>> (or particular offsets, if you prefer) to make the code clear, safe, and
>> portable (or at least giving clear error messages when you hit
>> portability issues).
>
> I prefer reading uint8_t by uint8_t from a buffer in most cases where
> I could have used a struct "overlay".  But yes, your way works too in
> many important cases (and importantly will not fail at runtime).
>

memcpy is good for doing such copies - it has the advantage that if it 
could be done faster using 16-bit or bigger moves, the compiler can do 
that (if it is reasonably smart), with no worries about aliasing :-)

Also explicit packing "dummies" have the advantage that there is no 
chance of undefined, implementation-defined, unspecified or generally 
unexpected or undesired behaviour when they are accessed.  (When storing 
something in a struct, padding bytes get unspecified values.)  This is 
particularly important if you want to compare structs, or do something 
like calculate a checksum over them.

And as noted before, be generous with your static assertions.  You find 
out problems at compile time, with no run-time costs.

Thread

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

csiph-web