Re: on understanding & and pointer arithmetic

From	Meredith Montgomery <mmontgomery@levado.to>
Newsgroups	comp.lang.c
Subject	Re: on understanding & and pointer arithmetic
Date	2021-12-18 12:02 -0300
Organization	Aioe.org NNTP Server
Message-ID	<86o85e6jo0.fsf@levado.to> (permalink)
References	<86a6h0eiy9.fsf@levado.to> <spfllt$vkn$1@dont-email.me> <spg7sd$10ue$1@gioia.aioe.org> <87fsqsur88.fsf@bsb.me.uk>

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

> Manfred <noname@add.invalid> writes:
>
>> On 12/16/2021 4:23 PM, David Brown wrote:
>>> On 16/12/2021 15:13, Meredith Montgomery wrote:
>> [...]
>>> 
>>>>    printf("a: %s\n", &a + 3);
>>> "a" here is the full array, so "&a" is a pointer to an array of 13 char.
>>>   Adding 3 gives a new pointer to an array of 13 char, at the address 39
>>> bytes higher than address of "a".  Basically, you are pretending that
>>> there is an array of 4 elements, each of which is itself an array of 13
>>> chars - with "a" being the first of those 4 elements.  Now you are
>>> asking to print the 4th element here, which is just whatever happens to
>>> be in the memory at that address.
>>> 
>>
>> It might be worth mentioning that accessing memory at that address (&a
>> + 3) technically yields undefined behavior, because it is an attempt
>> to access a location outside the array (a).
>> 
>
> That's a good point to make.  

Indeed.  Thanks.  (I was aware of that.  In fact, I thought that my OP
would produce a lot of --- that's undefined behavior and, so, C has
nothing to do with it.  Lol.  But, thankfully, you guys went straight
into helping me clear up my troubles.)

> It reminds me of a construct that I rather like:
>
>   char buffer[some-complex-size];
>   char *end_of_buffer = (&buffer)[1];
>
> (&buffer)[1] means *(&buffer + 1).  The constructed pointer &buffer + 1
> is valid, and the * does not attempt to dereference it as *(&buffer + 1)
> is an array-valued expression.  It is, instead, converted to a pointer
> to the first element of this non-existent array object -- a pointer one
> past the end of 'buffer'.

Wow, that's a cool application.  Thanks for sharing.  

I needed to redefine my definition of ``end''.  I tend to think the end
of an array is its last byte, but agreed --- that's not quite its end
yet.  It seems hard to define the exact end of something.  

The end of my property should be on my property or should it be outside
of it?  If *it* is outside, then because because *it* is relative to
*my* property, this *it* must be mine, so it belongs to my property.  So
it's not outside of it. :-)

I would have done

  char *end_of_buffer = (&buffer)[1] - 1;

Thread

on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:13 -0300
  Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 14:23 +0000
    Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:57 -0300
      Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 17:42 +0000
        Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 18:14 +0000
          Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:14 +0000
        Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:42 -0300
          Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:44 -0800
            Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:06 -0300
          Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:34 +0000
            Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 14:14 -0800
              Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:44 +0000
            Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:09 -0300
  Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 16:23 +0100
    Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:25 -0300
      Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 21:08 +0100
        Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:54 -0800
          Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 10:53 +0100
            Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:39 -0300
              Re: on understanding & and pointer arithmetic scott@slp53.sl.home (Scott Lurndal) - 2021-12-18 16:31 +0000
              Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-18 18:56 +0100
                Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-25 21:29 -0300
              Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-18 18:29 +0000
        Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:32 -0300
    Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-16 21:34 +0100
      Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 22:20 +0000
        Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:02 -0300
          Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:34 -0300
          Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-18 18:36 +0100
          Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-18 21:17 +0000
      Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:59 -0500
  Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:38 -0500
    Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 23:50 +0000
      Re: on understanding & and pointer arithmetic "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-12-17 07:34 -0800
        Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-17 17:48 +0000
    Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:56 +0000
      Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 19:06 -0500
        Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-17 00:10 +0000
        Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:32 -0300
    Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:30 -0300
  Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 11:08 +0100
    Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-17 16:24 +0100
      Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 18:17 +0100
    Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:11 -0300

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