Re: Aliasing through a union pointer

From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c
Subject	Re: Aliasing through a union pointer
Date	2015-11-25 12:57 +1300
Message-ID	<dbkbrsFjpqnU25@mid.individual.net> (permalink)
References	(4 earlier) <slrnn5749h.5q5.grahn+nntp@frailea.sa.invalid> <n318h6$6lq$1@dont-email.me> <slrnn58t2f.5q5.grahn+nntp@frailea.sa.invalid> <n32md3$v2d$1@dont-email.me> <n32oaa$789$1@dont-email.me>

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Eric Sosman wrote:
> On 11/24/2015 4:58 PM, David Brown wrote:
>> [... "packed" structs and similar underhanded artifices ...]
>> 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.
>
>       It's been my experience that these questions of packing and so on
> nearly always arise in connection with an externally-imposed format of
> some kind: You want to use a struct to represent an IP header or a
> disk partition table entry or something of that sort.  If that's the
> situation, I've almost always found that trying to sculpt the struct
> into the desired shape is inferior to just letting the struct take its
> own preferred form and using a mediating buffer to translate to and
> from the imposed format.  Use separate "native" and "serialized" forms,
> and you're quite likely to be better off than by trying to force both
> forms into one object.
>
>       Not only does the separation of forms avoid all the alignment
> issues discussed elsethread, but it also offers a convenient hook for
> dealing with other matters.  Does the external form require a specific
> endianness that may or may not match your machine's?  Does it use a
> weird representation for negative numbers, or for floating-point
> quantities?  Are text strings supposed to be a count word followed
> by a payload in some strange five-bit encoding?  The import() and
> export() functions can handle all such idiosyncracies cleanly and
> quietly, so the rest of the program can deal with perfectly normal,
> native-format structs with native-format numbers, zero-terminated
> native-encoded strings, and so on.  The translation functions are
> also good places to compute and verify things like checksums, which
> are usually specified in terms of the external format.
>
>       Somewhere, somebody is shrieking "Efficiency!  All that copying?
> Why should I un-pickle an entire object just to test the one field
> that tells me whether I care about the others?  Eeeek!"  This shrieker
> is overly fearful and insufficiently imaginative; there are plenty of
> ways to attack such issues if they should prove important.  (Also,
> for some reason the shrieker seldom yowls about the speed penalties
> of using the misaligned elements in his packed structs.  Go figure.)
>
>       There are many situations where it is tempting to map an imposed
> format with a struct, but resisting temptation is nearly always a
> virtue, and a virtue with rewards.

I agree with you here.  I leaned that lesson many years ago when testing 
embedded code on SPARC systems.  The cost of a misaligned access was way 
more than the cost of unpacking the messages (and there were a lot of 
them!).

The "Why should I un-pickle an entire object just to test the one 
field?" problem can be solved by lazy extraction: only extract that one 
field and keep the raw buffer around to be processed later.

-- 
Ian Collins

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

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