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Re: List of undefined behaviour and other sneeky bugs

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John Reye <jononanon@googlemail.com> wrote:
> On May 3, 9:46 pm, Keith Thompson <ks...@mib.org> wrote:
> > As for whether it "works", that depends on what you want it to do.  It
> > stores the value 0x12 in mystruct.u.c, which is also the first byte of
> > mystruct.u.b.  If you access the first byte of mystruct.u.b,
> > *(char*)&mystruct.u.b, you'll get 0x12.  If you access mystruct.u.b as
> > an int, you'll get garbage, possibly a trap representation.

> Will I really get garbage? I think not. The lowest-addressed byte is
> 0x12, and the other bytes of mystruct.u.b are what they were before,
> right?

As you have seen there are different architectures, having
different ways of representing e.g. an int - it can start
wtith the most significant bute at the lowest address or
with the least significant byte (or some other byte - the
C standard must be written in a way that it can be used
on all possible architectures). Thus when you change just
the byte at the lowest adress the value you get when you
read out the whole int can be different on different archi-
tectures.

What is meant when the C standard says that something is un-
defined is that it can't promise a certain outcome on all
possible architectures. Nothing more. Thus, if you want to
write a truely portable program you must avoid all kinds of
undefined behaviour. But if you write platform dependent
code and the platform defines what the C standard doesn't
it's completely fine to just use it, you just have to be
aware of that. Actually, without undefined behaviour you
wouldn't be able to write platform-dependent code, so it
would be impossible to e.g. write an operating system or
a device driver.

> A trap? How can that possibly be justified?

By the existence of machines with properties that may
seem unusual to you but for which probably good reasons
existed. The C standard can only mandate behaviour that
has a chance to be implemented on as many architectures
as possible - otherwise it would be impossible to write
a C compiler for them. So it's restricted to a smallest
common divder. And that's exactly one of the primary rea-
sons why C has been that successful and is still going
strong after 40 years - it's possible to have a C com-
piler for a supercomputer and for whatever system runs
a wrist-watch, a phone etc., and a program written in
standard C will run the same way on each of them.

> Why does the C language
> have unions then? What is the intended use of unions?

So that you can store values of different types in the
same location. But, of course, it hardly makes sense to
expect that after one has written a value of type A in-
to that location to be able to read something of a dif-
ferent type from it. Or do you often put a handkerchief
into a drawer and, when you open it again, it's a rabbit?

                            Regards, Jens
-- 
  \   Jens Thoms Toerring  ___      jt@toerring.de
   \__________________________      http://toerring.de

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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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