Groups > comp.lang.c > #20493

Re: List of undefined behaviour and other sneeky bugs

From	nick_keighley_nospam@hotmail.com
Newsgroups	comp.lang.c
Subject	Re: List of undefined behaviour and other sneeky bugs
Date	2012-05-08 00:43 -0700
Organization	http://groups.google.com
Message-ID	<28029579.3276.1336463009075.JavaMail.geo-discussion-forums@vbbgl4> (permalink)
References	<61f319f8-323e-41ec-938a-8062d532aa6a@a5g2000vbc.googlegroups.com> <a0fu3sFhkhU1@mid.uni-berlin.de> <kfn8vh3zejr.fsf@x-alumni2.alumni.caltech.edu>

Show all headers | View raw

On Tuesday, May 8, 2012 1:45:12 AM UTC+1, Tim Rentsch wrote:
> jt@toerring.de (Jens Thoms Toerring) writes:
> 
> > John Reye <jononanon@googlemail.com> wrote:
> >[snip]
> >> Or:
> >>                   struct
> >>                   {
> >>                      char a;
> >>                      union {
> >>                         int  b;
> >>                         char c;
> >>                      };
> >>                   } mystruct;
> >>                   mystruct.c = 0x12
> >
> >> Would this union work on every platform??
> >
> > Yes. But the you won't be able to use the 'b' element of
> > the union since reading a different member than has been
> > set the last time round also invokes undefined behavior.
> 
> That's wrong.  Reading a member other than the last member
> set has defined behavior.  In cases where the member read
> has a size larger than the last member set (as is the case
> here), the value read depends on unspecified values in
> some bytes.  If those unspecified values cause the object
> representation for the type in question to be a trap
> representation, then that will cause undefined behavior.
> But simply reading a member other than the last member
> set is not in and of itself undefined behavior.  And the
> usual case, ie, reading a member whose size is no larger
> than that of the member set, commonly has well-defined
> and sometimes useful behavior.

but it is implementation defined so you can't, in general, predict what value you'll get

Thread

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

csiph-web