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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Offsetof with array elements
Date: Sun, 16 Feb 2020 02:41:58 -0800
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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.