That presumes the existence of a "mapping". Think in small enough
devices that don't have an MMU.

On Thu, Jul 31, 2025 at 7:21 AM zxuiji via Std-Proposals
<std-proposals_at_[hidden]> wrote:
>
> Then the answer to the potential null+-1 being valid is simple, mandate in the next standard that the 0+-PAGE_SIZE be premapped as sealed no rwx pages. That resolves the problem completely. They don't need to have anything mapped to them, just that they be mapped as invalid.
>
> On Thu, 31 Jul 2025 at 15:17, Thiago Macieira via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>
>> On Thursday, 31 July 2025 04:16:23 Pacific Daylight Time zxuiji via Std-
>> Proposals wrote:
>> > Uh, what is wrong with NULL? Nobody expects a pointer to be below
>> > sysconf(SC_PAGE_SIZE) so regardless of if the pointer is incremented or not
>> > one would recognise it as an invalid pointer while debugging.
>>
>> The standard doesn't *guarantee* that. The only officially-mandated invalid
>> address is that of nullptr itself (which as others have said, doesn't need to
>> be a bitwise zero). Strictly speaking, the address of (char*)nullptr + 1 could
>> be valid and could be the result of some memory allocation.
>>
>> In practice, it doesn't happen. In practice, the smallest granularity that
>> virtual memory managers can control is a full page, so they make the full page
>> containing nullptr invalid. And in practice nullptr is the lowest address in
>> that page.
>>
>> Interestingly, the addresses before nullptr are by necessity invalid, not
>> because one could decrement nullptr, but because you're not allowed to
>> calculate one-past-the-end of a valid object and reach nullptr. Considering
>> this depends on object size, everything prior to nullptr within "reasonable
>> object size" is also invalid. In practice, that's also one page.
>>
>> Though in practice it's actually more than one page, for "defence in depth".
>>
>> --
>> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
>> Principal Engineer - Intel Platform & System Engineering
>>
>>
>>
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