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Re: Defined and undefined C pointer manipulation

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"James Harris" <james.harris.1@gmail.com> writes:
> "Keith Thompson" <kst-u@mib.org> wrote in message 
> news:lnzjj6pmui.fsf@nuthaus.mib.org...
[...]
>> I don't believe there's any advantage to using an alignment stricter
>> than 1 64-bit word.  I'd hesitate to impose such a requirement.
>
> This would only be for allocating blocks of memory. It can make sense to 
> allocate those blocks on two-word boundaries (or coarser such as four-word 
> boundaries) and won't do any harm.

Probably true.

For a byte pointer, setting the low-order bit to zero deosn't mean much;
it can still be misaligned.  But if you're limiting yourself to word
pointers, then yes, you can do that.

>> You could put the flag in the 4th bit from the top, which I believe is
>> always 0 for a valid pointer.  (The addressing space is much less than
>> 2**60 words.)  Use a macro to define which bit you're using for the
>> flag.  You'd have to configure it manually, but that's not unreasonable
>> for this kind of low-level bit-twiddling.
>
> If I were to align memory in any program it would always have to be the 
> lowest bits that got zeroed. If that wouldn't cause coarser alignement on 
> some unusual machines that would be surprising. The point of this thread is 
> to avoid writing code specifically for any given target machine as far as 
> possible.

You're still making assumptions about pointer representation that go
beyond what the C standard guarantees.

Parameterizing which bit you use for a flag could *in principle* let
your code work correctly on some implementations where it might fail if
you always use the low-order bit, at the expense of having to change a
one-line macro definition for some targets.  I don't know enough about
your goals and requirements to tell you whether that's worthwhile.
Using a bit outside the pointer could let you write 100% portable C
code, but at the expense of allocating extra memory.  The tradeoffs are
up to you.

-- 
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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Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-26 18:35 +0100
  Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-26 12:04 -0700
    Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 19:11 +0100
      Re: Defined and undefined C pointer manipulation Barry Schwarz <schwarzb@dqel.com> - 2014-04-27 11:46 -0700
        Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 12:46 -0700
        Re: Defined and undefined C pointer manipulation Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-28 02:53 +0100
      Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 12:57 -0700
        Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 21:56 +0100
          Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 14:25 -0700
            Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:51 +0000
  Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-27 06:21 -0700
    Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-27 16:51 +0100
      Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-27 10:36 -0700
      Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 11:40 +1200
  Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 19:23 +0100
    Re: Defined and undefined C pointer manipulation Richard Damon <Richard@Damon-Family.org> - 2014-04-27 14:56 -0400
      Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 20:52 +0100
        Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 13:16 -0700
    Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-27 20:55 +0100
      Re: Defined and undefined C pointer manipulation "James Harris" <james.harris.1@gmail.com> - 2014-04-27 21:20 +0100
        Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:39 +0000
        Re: Defined and undefined C pointer manipulation glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-27 22:43 +0000
        Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-28 00:13 +0100
    Re: Defined and undefined C pointer manipulation Keith Thompson <kst-u@mib.org> - 2014-04-27 13:01 -0700
    Re: Defined and undefined C pointer manipulation James Kuyper <jameskuyper@verizon.net> - 2014-04-27 22:07 -0400
    Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 00:47 -0700
      Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 20:01 +1200
        Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 02:11 -0700
          Re: Defined and undefined C pointer manipulation Ian Collins <ian-news@hotmail.com> - 2014-04-28 22:38 +1200
            Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 04:50 -0700
      Re: Defined and undefined C pointer manipulation "BartC" <bc@freeuk.com> - 2014-04-28 09:22 +0100
        Re: Defined and undefined C pointer manipulation Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-28 05:31 -0700

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