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Re: Indexing one past the end of array?

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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> Keith Thompson <kst-u@mib.org> writes:
>>
>>> Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> writes:
>>>
>>>> Dear comp.lang.c,
>>>>
>>>> I've been reading a thread on LWN[1], notably this quote:
>>>>
>>>>> Just some pedantry, but I believe you're allowed to point to the first
>>>>> element after the end of an array, as long as you don't dereference
>>>>> that element.  Pointer comparisons with such a pointer are supposed to
>>>>> work, too.
>>>>
>>>> Elsewhere, I've read that on embedded systems memory can be allocated
>>>> right up to the end of memory, and therefore such indexes are invalid
>>>> (but I cannot remember where).
>>>>
>>>> So, is it always legal to calculate one past the end of an array or
>>>> allocated memory?
>>>>
>>>> [1] https://lwn.net/Articles/278137/
>>>
>>> Yes, it's always legal.  The language guarantees it.
>>
>> <cut>
>>
>>> A compiler needs to do whatever it has to to make this work as
>>> specified.  Allocating an object at the very end of memory doesn't
>>> necessarily break this, as long as pointer addition and subtraction
>>> yield the expected results.
>>
>> <cut>
>>
>>> For example, assume a system with 32-bit addresses and exactly 2**32
>>> bytes of addressable memory.  Assume an array object
>>>     char buf[100];
>>> is allocated starting at address 2**32-100.  Then (buf+100) points just
>>> past the end of the array, and also just past the end of addressable
>>> memory.  As long as evaluating (buf+100)-1 yields a valid pointer to
>>> buf[99], that's ok.
>>
>> A nit:  the relational operators (<, >, <=, >=) have to work on the
>> resulting pointers as well.  For example (buf+100)-1 < (buf+100) must be
>> true, and that's not automatically the case where pointers are
>> interpreted as unsigned bit patterns.  If, however, pointers are signed
>> 2's complement values, it will all come out in the wash.
>
> It took me a while to figure out what you meant by the statement
> about signed pointers.  What I think you mean is that if the
> signed address of 'buf' is -100, then the comparisons (and
> arithmetic) all work as required.

Kind of.  It was an almost entirely unhelpful remark because I was
referring only to the 'all-bits set' to 'no bits set' wrap around case.
That becomes -1 to 0 rather than 2**32-1 to 0.  I should not have said
"it will all" because that suggests too much generality.

> But if pointers are signed
> there is still a singularity; it just occurs at a different point
> in the address space.

Yes, indeed.  And in some sense, the singularity is always in the same
place: at the (arithmetic) top of the address range.

> A 100-byte array starting at 2**31-100
> will still have the problem:  a pointer past the last element of
> the array will be less than -- instead of greater than -- a
> pointer to any element of the array.

-- 
Ben.

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Indexing one past the end of array? Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2019-10-06 04:02 +0800
  Re: Indexing one past the end of array? Keith Thompson <kst-u@mib.org> - 2019-10-05 13:13 -0700
    Re: Indexing one past the end of array? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-05 22:01 +0100
      Re: Indexing one past the end of array? Keith Thompson <kst-u@mib.org> - 2019-10-05 14:35 -0700
      Re: Indexing one past the end of array? Richard Damon <Richard@Damon-Family.org> - 2019-10-05 19:00 -0400
        Re: Indexing one past the end of array? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-06 08:05 -0700
      Re: Indexing one past the end of array? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-06 06:36 -0700
        Re: Indexing one past the end of array? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-06 16:31 +0100
    Re: Indexing one past the end of array? Richard Damon <Richard@Damon-Family.org> - 2019-10-05 17:10 -0400
      Re: Indexing one past the end of array? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-06 07:40 -0700
  Re: Indexing one past the end of array? Mateusz Viste <mateusz@wont.tell> - 2019-10-05 20:26 +0000
  Re: Indexing one past the end of array? chad <cdalten@gmail.com> - 2019-10-05 16:55 -0700
  Re: Indexing one past the end of array? "Oliver S." <Follow.Me@gmx.net> - 2019-10-06 15:50 +0200
  Re: Indexing one past the end of array? "Oliver S." <Follow.Me@gmx.net> - 2019-10-06 17:04 +0200
    Re: Indexing one past the end of array? scott@slp53.sl.home (Scott Lurndal) - 2019-10-07 13:32 +0000
      Re: Indexing one past the end of array? "Oliver S." <Follow.Me@gmx.net> - 2019-10-07 17:23 +0200

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