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Re: List of undefined behaviour and other sneeky bugs

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On 05/07/2012 07:32 AM, Old Wolf wrote:
> On May 4, 8:35 am, John Reye <jonona...@googlemail.com> wrote:
>> A trap? How can that possibly be justified? Why does the C language
>> have unions then? What is the intended use of unions?
> 
> Unions are for saving memory, when you know you
> need to store a value of some sort, but the value
> could be one of various types. If there were no
> unions you'd either have to have a struct (wasting
> memory), or use a byte buffer and write some ugly
> cast and alignment stuff.
> 
> They're not supposed to be a hack to do type conversions,
> although that hasn't stopped people trying that over the years :)

This is not hacking this is foreseen as such by the standard:

 -  Unions have been the only way to guarantee that an object is
correctly aligned for a collection of types. (Since C11 there are also
_Alignas and friends.)
 -  Other than casting pointers, unions avoid aliasing problems.

Jens

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List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 10:04 -0700
  Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 17:38 +0000
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 10:50 -0700
      Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 11:09 -0700
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:18 -0700
          Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 11:24 -0700
            Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:35 -0700
              Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:56 -0700
              Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 12:16 -0700
                Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 12:20 -0700
                Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 12:39 -0700
          Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:51 -0700
            Re: List of undefined behaviour and other sneeky bugs "Joachim Schmitz" <nospam.jojo@schmitz-digital.de> - 2012-05-08 09:10 +0200
              Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 05:31 -0700
      Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 18:11 +0000
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 11:29 -0700
          Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 19:19 +0000
            Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 13:31 -0700
              Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 14:18 -0700
                Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 15:20 -0700
            Re: List of undefined behaviour and other sneeky bugs Nobody <nobody@nowhere.com> - 2012-05-04 01:47 +0100
      Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:47 -0700
    Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 17:45 -0700
      Re: List of undefined behaviour and other sneeky bugs nick_keighley_nospam@hotmail.com - 2012-05-08 00:43 -0700
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 06:17 -0700
  Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 12:46 -0700
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 13:35 -0700
      Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 14:05 -0700
        Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 14:11 -0700
          Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-03 15:02 -0700
            Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:10 -0700
              Re: List of undefined behaviour and other sneeky bugs nick_keighley_nospam@hotmail.com - 2012-05-08 00:49 -0700
                Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-08 06:37 -0400
                Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 07:49 -0700
                Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 07:25 -0700
              Re: List of undefined behaviour and other sneeky bugs Ben Pfaff <blp@cs.stanford.edu> - 2012-05-08 09:46 -0700
          Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-03 18:24 -0400
            Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:24 -0700
      Re: List of undefined behaviour and other sneeky bugs jt@toerring.de (Jens Thoms Toerring) - 2012-05-03 21:17 +0000
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:28 -0700
      Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 15:54 -0700
      Re: List of undefined behaviour and other sneeky bugs Old Wolf <oldwolf@inspire.net.nz> - 2012-05-06 22:32 -0700
        Re: List of undefined behaviour and other sneeky bugs Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-07 08:26 +0200
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:31 -0700
    Re: List of undefined behaviour and other sneeky bugs Jens Gustedt <jens.gustedt@loria.fr> - 2012-05-03 23:18 +0200
      Re: List of undefined behaviour and other sneeky bugs Keith Thompson <kst-u@mib.org> - 2012-05-03 15:58 -0700
  Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-04 00:25 -0400
    Re: List of undefined behaviour and other sneeky bugs John Reye <jononanon@googlemail.com> - 2012-05-03 22:28 -0700
      Re: List of undefined behaviour and other sneeky bugs James Kuyper <jameskuyper@verizon.net> - 2012-05-04 07:33 -0400
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 19:20 -0700
      Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:50 -0700
    Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 18:42 -0700
      Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-07 23:45 -0400
        Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-07 23:01 -0700
          Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-08 06:28 -0400
            Re: List of undefined behaviour and other sneeky bugs pete <pfiland@mindspring.com> - 2012-05-08 06:37 -0400
              Re: List of undefined behaviour and other sneeky bugs Tim Rentsch <txr@alumni.caltech.edu> - 2012-05-08 06:19 -0700

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