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Re: Indexing one past the end of array?

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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.

Good nit, thanks.

A compiler *could* generate code that checks for that special case, and
optimize away the extra code when it can prove that the pointer didn't
wrap around.  But I'd be surprised if any actual compiler did that.

>> (This is just an example.  The standard doesn't
>> assume this kind of mapping between addresses and integer values.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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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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