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Re: Function casting - UB?

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Am 28.06.2012 09:27, schrieb Johannes Bauer:
> On 28.06.2012 09:23, Johannes Bauer wrote:
>> On 27.06.2012 19:37, Tim Rentsch wrote:
>>> The other example you give,
>>> assigning to one member of a union and reading from another, is
>>> actually defined behavior, _not_ undefined behavior.
>>
>> I also was under the impression that writing to member x and reading
>> member y of a union is UB. Wikipedia says "This is not, however, a safe
>> use of unions in general.", which is pretty vague (i.e. it's not clear
>> which cases are safe and which are not).
>>
>> Could you elaborate on why you think this is well-defined?
> 
> Ah, I just read James' response further down. Interesting. Really
> thought this was undefined. Is this a recent change?

I think this was not considered as a change in contents but given as
more precision on the intent. n1256.pdf has modification marks in this
region so I suppose that these came with TC3. They state

 > When a value is stored in a member of an object of union type, the
 > bytes of the object representation that do not correspond to that
 > member but do correspond to other members take unspecified values.

which in terms of the standard means that it is only UB if these
unspecific values are "forbidden" values for that type, in particular
trap representations.

This means that for most of modern architectures manipulating integer
values (except of _Bool) through unions is completely ok. Floating
point values, _Bool, and pointer types must be treated with more care.

Jens

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Thread

Function casting - UB? nroberts <roberts.noah@gmail.com> - 2012-06-27 08:50 -0700
  Re: Function casting - UB? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-27 12:49 -0400
    Re: Function casting - UB? Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-06-27 17:54 +0000
      Re: Function casting - UB? "io_x" <a@b.c.invalid> - 2012-06-28 18:58 +0200
  Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-27 10:37 -0700
    Re: Function casting - UB? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-28 09:23 +0200
      Re: Function casting - UB? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-06-28 09:27 +0200
        Re: Function casting - UB? Jens Gustedt <jens.gustedt@loria.fr> - 2012-06-28 12:03 +0200
          Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-29 11:10 -0700
      Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-29 11:03 -0700
  Re: Function casting - UB? Les Cargill <lcargill99@comcast.com> - 2012-06-27 12:58 -0500
  Re: Function casting - UB? Les Cargill <lcargill99@comcast.com> - 2012-06-27 12:59 -0500
    Re: Function casting - UB? James Kuyper <jameskuyper@verizon.net> - 2012-06-27 14:45 -0400
      Re: Function casting - UB? "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-06-27 14:55 -0700
        Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-27 17:44 -0700
          Re: Function casting - UB? Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-07 16:46 -0700
            Re: Function casting - UB? Barry Schwarz <schwarzb@dqel.com> - 2012-07-08 09:19 -0700
              Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-08 11:13 -0700
                Re: Function casting - UB? Barry Schwarz <schwarzb@dqel.com> - 2012-07-08 15:24 -0700
                Re: Function casting - UB? Joshua Maurice <joshuamaurice@gmail.com> - 2012-07-08 18:12 -0700
                Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-10 10:40 -0700
            Re: Function casting - UB? Tim Rentsch <txr@alumni.caltech.edu> - 2012-07-08 11:06 -0700

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