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Re: on understanding & and pointer arithmetic

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David Brown <david.brown@hesbynett.no> writes:

> On 16/12/2021 21:54, Keith Thompson wrote:
>> David Brown <david.brown@hesbynett.no> writes:
>>> On 16/12/2021 20:25, Meredith Montgomery wrote:
>> [...]
>>>> Yes, so that corrects me at least once.  I didn't think &a would be of a
>>>> different type.  So the type of &a is array of 13 char.  I get that.
>>>
>>> /No/.  "a" is of type "array of 13 char".  "&a" is of type "pointer to
>>> array of 13 char".  These are different.
>>>
>>>> The value of &a is the same /value/ as &a[0].  I also get that.  I'm
>>>> good here.
>>>
>>> Correct.  As Ben said (and Ben is very good at explaining things
>>> accurately), a pointer has a type and has an address as it's value.  So
>>> the values of pointers here are the same, but the pointers themselves
>>> are different because they have different types.
>> 
>> I don't think I'd say that &a and &a[0] have the same *value*, though it
>> depends on just what you mean by "value".
>
> Yes, indeed it does.  As always in threads like these, there is a
> balance to be found between trying to have approximate explanations and
> terms that are easier for the relative beginner OP, and trying to be as
> accurate as possible with precise terms to help the OP get the details
> right from the start (and of course there are always other people
> reading the thread, who might benefit from more insight).
>
> Objects in C always have a type, and they contain some sort of data -
> which can reasonably be called its "value".  For pointers, that will be
> the address they contain - the value the OP is getting when he casts
> these to an unsigned long and prints them out.
>
> I know the real picture can be more complicated - different pointer
> types can have different sizes, pointers can contain more than just an
> address or memory location, two pointers of different types might
> contain the same raw value but refer to different address spaces, they
> might contain different raw values but refer to the same location and
> may or may not compare equal, there can be trap representations, etc.
> (That list is not complete.)  So what I am writing is "lies to
> children", rather than trying to be complete and precise (partly because
> others here, such as yourself, are significantly better at that kind of
> answer).  If you are not familiar with the specific phrase "lies to
> children", ask Santa for "The Science of the Discworld" :-)

For the record, I enjoyed the simplicity of this (sub)thread.  I was
trying to understand why my expectation was wrong.  So, I was already in
some trouble; bringing in more details at that point would not have
helped more than not bringing them in.

It's also nice to see that the situation is even deeper than I would
have thought, though.  For instance, right now my intuition is that the
size of a pointer on a 64-bit machine is always 8 bytes.  Can anyone
show an easy example of when this isn't true?

[...]

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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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