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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: So You Think You Can Const?
Date: Sun, 12 Jan 2025 16:25:23 -0800
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Julio Di Egidio <julio@diegidio.name> writes:

> On 11/01/2025 12:14, David Brown wrote:
>
>> On 10/01/2025 19:56, Keith Thompson wrote:
>
> <snip>
>
>> The idea was to place the emphasis on "free" changing the pointer,
>> rather than the data pointed to.
>
> I feel I am still altogether missing the point.
>
> Is my understanding correct that when freeing a pointer: 1) the
> pointer value, i.e. the address it holds, does not change;  OTOH, 2)
> the pointed-to object does change, in the sense that it is marked
> unusable (and, supposedly, made available to re-allocation)?

Not exactly.  The bits in the pointer don't change.  The _meaning_
of those bits, which is to say what address the bits correspond to,
or even if there is a corresponding address, _does_ change, in the
sense that any attempt (in a C program) to look at the bits as an
address is -- as far as the C standard is concerned -- completely
unpredictable.

> Moreover, while the pointer value has not changed, it is in fact
> changed in the sense that it has become invalid, namely the pointer
> cannot be used (validly dereferenced) anymore.  Not just that, but
> *every* pointer to the same object, i.e. holding the same address, has
> become invalid.

It isn't just that those pointers cannot be dereferenced.  Even
trying to load their values might blow up the program, or even do
anything else one might imagine.