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Groups > comp.lang.c > #64595

Re: "core C"

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: "core C"
Date 2015-07-01 10:56 -0700
Organization None to speak of
Message-ID <lntwtn2245.fsf@kst-u.example.com> (permalink)
References (8 earlier) <mmugvp$mpc$1@dont-email.me> <8de0b72b-0f2c-45cc-92c7-0c3c498134e3@googlegroups.com> <mn0gap$cf3$1@dont-email.me> <lny4iz2a0i.fsf@kst-u.example.com> <mn12ql$2mi$1@dont-email.me>

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Bartc <bc@freeuk.com> writes:
> On 01/07/2015 16:05, Keith Thompson wrote:
>> Bartc <bc@freeuk.com> writes:
>>> I'm curious: how can a pointer and offset together be the same size as a
>>> pointer?
>>>
>>> Or do all pointers have a spare 32-bit field on the off-chance they
>>> might have to store an offset?
>>
>> An example I've mentioned here many times: Cray vector machines are
>> word-oriented, where a word is 64 bits.  A machine-level address/pointer
>> is a 64-bit quantity containing the address of a word.  Cray's C
>> compiler has CHAR_BIT==8 (for Unix/POSIX compatibility), and implements
>> a byte pointer as a word pointer with the byte offset stored in the
>> otherwise unused high-order 3 bits.
>>
>> Any system with 64-bit pointers that doesn't actually support 16
>> exabytes of memory is likely to be able to do something similar.
>
> We're talking about offsets into an array which could be as big as 
> available memory, and that's for a byte array. For a bit-array, the 
> offset could be 8 times as big!
>
> So it's not just a few extra bits.

Ok, I missed some of the context.

If the base pointer points to the word containing the data, then the
offset can be no bigger than 6 bits (assuming 64-bit pointers and 1-bit
elements).  If the base pointer points to the beginning of an array
object, and the offset can refer to any element of the array, then the
offset has to be a size_t or equivalent.

[...]

> Otherwise it gets tight. My point was that the natural way to do this 
> was with a discrete pointer/offset pair that is larger (perhaps double 
> the size) than a regular pointer. (With my preferred method of using bit 
> pointers, then there's a better chance of storing the 0..7 bit offset in 
> a spare corner of the pointer.)

It can vary a bit if the maximum size of an object is substantially
smaller than the total size of the memory space; then size_t can be
smaller than void*.  Of course monolithic address spaces are all the
rage these days.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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"core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 08:38 +0200
  Re: "core C" jt@toerring.de (Jens Thoms Toerring) - 2015-06-30 07:54 +0000
    Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 02:55 -0700
    Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 17:31 +0200
      Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 18:15 +0200
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 12:19 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:27 -0700
    Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:48 -0700
      Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:07 +0100
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 05:15 -0700
    Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:02 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 05:13 -0700
        Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 13:58 +0100
          Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:11 -0700
            Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 15:11 +0100
              Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:50 -0700
                Re: "core C" Rosario19 <Ros@invalid.invalid> - 2015-06-30 17:24 +0200
                Re: "core C" Bartc <bc@freeuk.com> - 2015-06-30 17:44 +0100
                Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 10:04 -0700
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-01 11:46 +0100
                Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-01 04:40 -0700
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 08:05 -0700
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-01 17:01 +0100
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 10:56 -0700
                Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 12:59 -0500
                Re: "core C" Bartc <bc@freeuk.com> - 2015-07-04 20:25 +0100
                Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 20:56 -0500
        Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:17 +0100
    Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:03 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:16 -0700
    Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-06-30 15:46 +0200
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:54 -0700
      Re: "core C" Steve Thompson <stevet810@gmail.com> - 2015-07-03 12:25 +0000
    Re: "core C" jt@toerring.de (Jens Thoms Toerring) - 2015-07-01 15:04 +0000
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-01 08:12 -0700
    Re: "core C" BGB <cr88192@hotmail.com> - 2015-07-04 10:36 -0500
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:41 -0700
    Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 14:19 +0100
      Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 06:25 -0700
        Re: "core C" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-30 15:48 +0100
          Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 07:52 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 04:42 -0700
  Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 08:28 -0700
    Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-06-30 22:28 +0200
      Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 08:45 +1200
        Re: "core C" "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-06-30 13:56 -0700
          Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-07-01 10:27 +0200
        Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 14:56 -0700
          Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 10:03 +1200
            Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 15:31 -0700
              Re: "core C" Ian Collins <ian-news@hotmail.com> - 2015-07-01 10:52 +1200
                Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-30 16:58 -0700
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-06-30 17:30 -0700
                Re: "core C" David Brown <david.brown@hesbynett.no> - 2015-07-01 08:54 +0200
                Re: "core C" Keith Thompson <kst-u@mib.org> - 2015-07-01 07:54 -0700
                Re: "core C" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-01 09:42 -0700

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