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Groups > comp.lang.c > #5428

Re: offsetof() Macro Attempt

From Shao Miller <sha0.miller@gmail.com>
Newsgroups comp.lang.c
Subject Re: offsetof() Macro Attempt
Date 2011-06-04 09:32 -0500
Organization A noiseless patient Spider
Message-ID <isdfme$kvf$1@dont-email.me> (permalink)
References <isb79p$oao$1@dont-email.me> <4DE92C83.824@mindspring.com> <iscl9q$dgc$1@dont-email.me> <4DEA0DB0.5BE6@mindspring.com>

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On 6/4/2011 5:49 AM, pete wrote:
> Shao Miller wrote:
>>
>> On 6/3/2011 1:48 PM, pete wrote:
>>> Shao Miller wrote:
>>>
>>>> So let's please go on to:
>>>>
>>>>      char ** helper;
>>>>
>>>>      enum foo {
>>>>          bar = *helper + 1 - *helper
>>>>        };
>>>>
>>>>      int main(void) {
>>>>          return 0;
>>>>        }
>>>
>>> That one has the same problem as
>>>
>>>       int main(void)
>>>       {
>>>           int x;
>>>
>>>           x - x;
>>>           return 0;
>>>       }
>>>
>>
>> In your example, 'x' is not initialized, and I agree that such is a
>> concern.  In the example just above your example, 'helper' is at file
>> scope[6.9.2p2].  What is the problem, exactly?
>
> I read it wrong. Sorry about that.
>

No problem.

> But, if (helper) is equal to NULL,
> which I believe is the case,
> then (*helper) is undefined.
>

I agree that the initially stored value for 'helper' would be comparable 
with 'NULL'.  I also agree that if '*helper' was evaluated while 
'helper' has a null pointer value, then there'd be undefined behaviour.

> But even if (*helper) was defined,
> then (*helper + 1 - *helper) is still not a constant expression
> and the compiler that I'm using,
> won't compile the above program.
>

May I ask which compiler, please?  Since I am concerned about 
portability, that'd be great to know.  Thanks!

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Thread

offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-03 13:57 -0400
  Re: offsetof() Macro Attempt Billy Mays <noway@nohow.com> - 2011-06-03 13:59 -0400
    Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-03 14:33 -0400
      Re: offsetof() Macro Attempt Noob <root@127.0.0.1> - 2011-06-08 15:03 +0200
  Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-03 14:48 -0400
    Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 02:02 -0500
      Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-04 06:49 -0400
        Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 09:32 -0500
          Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-04 12:55 -0400
            Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 13:24 -0500
              Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-04 14:56 -0400
                Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-04 15:38 -0400
                Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 18:25 -0500
                Re: offsetof() Macro Attempt pete <pfiland@mindspring.com> - 2011-06-05 07:10 -0400
  Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-03 13:39 -0700
    Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-03 19:58 -0700
      Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-04 01:57 -0700
        Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-04 05:22 -0700
          Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-04 10:16 -0700
            Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-04 23:03 -0700
              Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-05 02:24 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-06 08:08 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-06 13:26 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-08 12:34 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-08 15:07 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-08 15:19 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-14 14:53 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-14 14:52 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-14 16:24 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-15 06:44 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-15 10:40 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 13:48 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-19 14:57 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-23 00:52 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-23 12:34 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-27 10:03 -0700
                Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-07-01 10:03 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-07-03 14:33 -0700
                Re: offsetof() Macro Attempt Michael Press <rubrum@pacbell.net> - 2011-06-18 21:17 -0700
                Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-27 10:13 -0700
    Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 14:05 -0500
      Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-05 02:44 -0700
        Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-06 17:58 -0500
          Re: offsetof() Macro Attempt Harald van Dijk <truedfx@gmail.com> - 2011-06-06 15:37 -0700
            Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-06 18:58 -0500
  Re: offsetof() Macro Attempt Andrey Tarasevich <andreytarasevich@hotmail.com> - 2011-06-03 17:15 -0700
    Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-03 20:20 -0700
    Re: offsetof() Macro Attempt Shao Miller <sha0.miller@gmail.com> - 2011-06-04 12:50 -0500
  Re: offsetof() Macro Attempt Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-03 21:43 -0400
    Re: offsetof() Macro Attempt Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-03 21:54 -0400
      Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-03 20:36 -0700
  Re: offsetof() Macro Attempt Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-03 19:46 -0700

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