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Re: Indexing one past the end of array?

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Richard Damon <Richard@Damon-Family.org> writes:

> On 10/5/19 4:13 PM, Keith Thompson wrote:
>
>> [..addressing past the end of an array..]
>>
>> 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.  (This is just an example.  The standard doesn't
>> assume this kind of mapping between addresses and integer values.)
>
> One possible issue you (or maybe it is the compier) can run into is that
> buf + 100 must be > buf + 99.  In you case above, buf + 100 would be
> 2**32, which if addresses are treated as 32 bit numbers, wraps to zero
> and thus the relationship doesn't hold.

The Rationale mentions this issue and points out that there need
be only one never-allocated byte at the end of memory (or of
course a few more than one if alignment causes the second-to-last
byte not to be usable).

> an issue with segmented memory/pointers, with a segment
> size of 2**16.  In such an environment, you can't have both 2**16 byte
> arrays and 16 bit pointers and full conforming, and practically, often
> the 2**16 byte object is just defined to be not supported in a fully
> conforming manner (I don't think it is actually required to be)

In C90 hosted implementations were required to support objects of
at least 32767 bytes.

In C99 onwards (assuming it hasn't changed) hosted implementations
are required to support objects of at least 65535 bytes (and the
minimum value for SIZE_MAX is 65535).  So segments of 2**16 bytes
can still be conforming.

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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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