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From: MrSpud_1dgrmf@dx58865qyrdw30lqllb.info
Newsgroups: comp.lang.c++
Subject: Re: Is this really necessary
Date: Mon, 9 Aug 2021 08:21:10 -0000 (UTC)
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On Sun, 8 Aug 2021 15:04:52 -0000 (UTC)
Juha Nieminen <nospam@thanks.invalid> wrote:
>James Kuyper <jameskuyper@alumni.caltech.edu> wrote:
>> On 8/8/21 5:18 AM, MrSpud_85yGi2@1ahbfz.gov wrote:
>> In the C standard, 7.20.1.4p1 describes intptr_t and uintptr_t, and says
>> "These types are optional". A fully conforming implementation may have
>> pointers that are too big to be representable using any supported
>> integer type, and the same is true of pointer differences.
>
>Indeed. For example in x86 16-bit real mode (think MS-DOS) it may
>well be that a pointer is 32-bit (because it has to contain a
>segment and an offset) but ptrdiff_t may well be 16-bit, and the
>compiler may limit eg. arrays to 64 kilobytes (minus 1 byte) so
>that they will fit inside a segment.
>
>A difference between pointers will only give a valid (16-bit)
>result when they point to the same array (which would be within
>the same segment). Else you get garbage (if the two pointers are
>pointing to different segments).

I was discussing from the POV of a proper OS, not some half baked monitor
program from the 70s. However if you can't work out how to get a valid diff
from the above then clearly your maths skills need some work.