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Re: Offsetof with array elements

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: Offsetof with array elements
Date 2020-02-16 02:41 -0800
Organization A noiseless patient Spider
Message-ID <86wo8mg6fd.fsf@linuxsc.com> (permalink)
References <r1udrq$hoe$1@dont-email.me> <r1v150$nb7$1@news.albasani.net> <87mu9ogkcq.fsf@bsb.me.uk>

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Bonita Montero <Bonita.Montero@gmail.com> writes:
>
>> How about this:
>>
>> #include <stddef.h>
>> #define array_offsetof(type, member, index) (size_t)(offsetof(type,
>> member) + sizeof(((type *)NULL)->##member[0]) * index)
>
> The ## is wrong.  Maybe it was typo?  There are no valid tokens that
> start -> (other than -> itself) so pasting any other token to -> will
> always be an error.
>
> It has undefined behaviour (dereferencing a null pointer) but may well
> work.  [...]

Why do you say the pointer dereference has undefined behavior?
The -> operator is part of an operand to 'sizeof', and hence is
not evaluated.

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Thread

Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-11 15:31 +0100
  Re: Offsetof with array elements Guillaume <message@bottle.org> - 2020-02-11 16:08 +0100
    Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-11 08:32 -0800
      Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-11 18:46 +0100
        Re: Offsetof with array elements Manfred <noname@add.invalid> - 2020-02-11 19:24 +0100
          Re: Offsetof with array elements Manfred <noname@add.invalid> - 2020-02-11 19:36 +0100
          Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-11 22:50 -0500
            Re: Offsetof with array elements Manfred <noname@add.invalid> - 2020-02-12 13:53 +0100
            Re: Offsetof with array elements Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-18 01:43 -0800
    Re: Offsetof with array elements Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-18 01:39 -0800
  Re: Offsetof with array elements Bart <bc@freeuk.com> - 2020-02-11 16:56 +0000
    Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-11 18:49 +0100
  Re: Offsetof with array elements Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-11 21:00 +0100
    Re: Offsetof with array elements Bart <bc@freeuk.com> - 2020-02-11 20:13 +0000
      Re: Offsetof with array elements Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-11 21:24 +0100
        Re: Offsetof with array elements Bart <bc@freeuk.com> - 2020-02-11 20:33 +0000
          Re: Offsetof with array elements Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-11 21:39 +0100
            Re: Offsetof with array elements Bart <bc@freeuk.com> - 2020-02-11 20:48 +0000
    Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-12 10:56 +0100
      Re: Offsetof with array elements Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-12 14:07 +0100
        Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-12 15:24 +0100
    Re: Offsetof with array elements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-12 10:39 +0000
      Re: Offsetof with array elements Bonita Montero <Bonita.Montero@gmail.com> - 2020-02-12 14:08 +0100
        Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-12 05:29 -0800
          Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-12 11:45 -0800
        Re: Offsetof with array elements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-12 17:50 +0000
      Re: Offsetof with array elements Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-02-16 02:41 -0800
        Re: Offsetof with array elements Ben Bacarisse <ben.usenet@bsb.me.uk> - 2020-02-16 11:18 +0000
    Re: Offsetof with array elements already5chosen@yahoo.com - 2020-02-13 02:42 -0800
      Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-14 00:20 -0500
        Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-14 08:24 +0100
          Re: Offsetof with array elements already5chosen@yahoo.com - 2020-02-14 03:59 -0800
            Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-14 15:25 +0100
              Re: Offsetof with array elements already5chosen@yahoo.com - 2020-02-16 03:15 -0800
                Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-16 14:09 +0100
                Re: Offsetof with array elements already5chosen@yahoo.com - 2020-02-16 06:36 -0800
          Re: Offsetof with array elements James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-02-15 08:18 -0500
            Re: Offsetof with array elements David Brown <david.brown@hesbynett.no> - 2020-02-15 15:49 +0100
  Re: Offsetof with array elements fir <profesor.fir@gmail.com> - 2020-02-12 04:40 -0800
  Re: Offsetof with array elements "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-02-16 16:03 -0800

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