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Re: Reference counting

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On 6/13/2011 2:47 PM, ImpalerCore wrote:
> On Jun 13, 3:26 pm, Shao Miller<sha0.mil...@gmail.com>  wrote:
>> On 6/13/2011 9:34 AM, ImpalerCore wrote:
>>
>>
>>
>>> On Jun 11, 6:23 pm, Shao Miller<sha0.mil...@gmail.com>    wrote:
>>>> On 6/11/2011 2:36 PM, ImpalerCore wrote:
>>
>>>>> If I understand correctly, malloc should always return an aligned
>>>>> pointer, so by knowing ALIGN_MAX, the pointer at offset ALIGN_MAX
>>>>> should also be suitably aligned for any other object.  I think this
>>>>> might work, but I hesitate as the nuances of alignment issues continue
>>>>> to elude my understanding.
>>
>>>> Or you can put your accounting object at/near the end of the allocated
>>>> memory, as mentioned else-thread.  Would anyone else mind mentioning
>>>> this or does nobody really do that?  I thought it was fairly common
>>>> practice for some I/O buffers.  Filtering... *sigh*
>>
>>> I just find the interface with your footer method to be more verbose
>>> without any tangible benefit.  You add these four functions, yet you
>>> haven't tried to use them to come up with the reference counted
>>> allocation, copy, and release functions.
>>
>>> size_t footer_at(size_t size);
>>> size_t size_with_footer(size_t size);
>>> footer_t * get_footer(void * mem, size_t size);
>>> void * malloc_with_footer(size_t size);
>>
>>> My opinion is that the interface is cluttered, and doesn't even do
>>> what was asked, to reference count.  When you come to the release
>>> step, are you going to require everyone to pass in the size of the
>>> allocated object?
>>
>>> void rc_release( void* p, size_t
>>> I_need_to_know_the_object_size_to_find_the_footer );
>>
>>> Raw pointers don't carry the size of the allocated memory with them.
>>> I don't have access to what malloc uses internally.  If you have a
>>> malloced character string, are you going to require the user to pass
>>> in the original allocated size of each allocated string so you can
>>> find the footer when you go to decrement its reference count?  Placing
>>> the bookkeeping before the object pointer allows one to know where the
>>> bookkeeping object is without knowing the size or contents of the
>>> allocated object.
>>
>>> Write your version of rc_malloc, rc_copy, and rc_release in terms of
>>> your footer interface and make a pros vs cons list.  I just see more
>>> cons to this technique without any perceived pros.
>>
>> Thanks.  I posted more code and thoughts about half a day before your
>> response.  Perhaps your news-reader has filtered it? :)
>>
>> Is keeping the accounting info before the object and working out some
>> way to get the alignment right really much more common than keeping it
>> after the object?  (Obviously this is only a poll of anyone here who
>> responds.)
>
> It depends, if you're a debugging allocator, you'll likely want to put
> markers at each end to detect buffer overruns and the like.  The
> dmalloc and SQLite allocators are examples that place information at
> both sides.
>
> The dmalloc allocator simply guesses the alignment by defining
> '#define ALLOCATION_ALIGNMENT N' in a conf.h file that just guesses
> the alignment from 'ac_cv_sizeof_long'.  I haven't investigated how
> SQLite goes about it.
>
> For myself, I've never implemented a reference counted pointer scheme,
> so what you saw is just conjecture about how I'd attempt it.

Well, two people in another C-devoted forum suggested another concern 
with keeping the accounting information at/near the end of the allocated 
storage: Someone sloppy can overwrite it.  By accounting near the 
beginning, that might be less likely.

I'm just not sure how one gets the alignment at the beginning to be 
guaranteed as correct without making an extra burden to callers, for C 
implementations conforming to < C1X.

Given an adaptation of Mr. Chris M. Thomasson's macro:

   #define ALIGNOF(type) (offsetof(struct{char c; type t;}, t))

(which, by the way, I just noticed doesn't appear to be guaranteed as 
giving the same result every time due to a lack of tag, though that 
might be highly unlikely)

We could have a 'malloc' wrapper:

   void * malloc_aligned(size_t size, size_t alignment);

But then callers would need to:

   some * p = malloc_aligned(
       sizeof *p + optional_variable_length,
       ALIGNOF(some)
     );

where we'd forfeit the convenience of having 'some' appear just once.

Any pad-to-nice-alignment-at-the-beginning strategy for < C1X is just 
guessing, isn't it?  And the more conservative we are, the greater the 
risk of wasted space?

If one only requires a 'ptrdiff_t' or 'size_t' at the beginning, this 
might be minimal and could specify where additional accounting info at 
the end is, but that does seem complicated.

But even with the overhead of tracking the size of the allocated data 
and of requiring that paired with the pointer in order to find the 
accounting info, are there scenarios where a caller _shouldn't_ know the 
apparent size of the allocated data?  Isn't that asking for trouble? 
Sentinels are great for unknown lengths, but a call to 'malloc' requires 
a known length...  Shouldn't this be tracked somewhere as a good 
programming practice in order to avoid out-of-bounds accesses?

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Thread

Reference counting daknok <radekslupik@gmail.com> - 2011-06-05 12:52 +0200
  Re: Reference counting Angel <angel+news@spamcop.net> - 2011-06-05 11:17 +0000
    Re: Reference counting Gene <gene.ressler@gmail.com> - 2011-06-06 20:07 -0700
      Multiple Inheritance (Was Re: Reference counting) Shao Miller <sha0.miller@gmail.com> - 2011-06-11 03:57 -0500
        Re: Multiple Inheritance (Was Re: Reference counting) Dr Nick <3-nospam@temporary-address.org.uk> - 2011-06-12 07:22 +0100
  Re: Reference counting China Blue Angels <chine.bleu@yahoo.com> - 2011-06-05 05:07 -0700
  Re: Reference counting "BartC" <bc@freeuk.com> - 2011-06-05 13:14 +0100
    Re: Reference counting daknok <daknok@example.com> - 2011-06-05 14:22 +0200
      Re: Reference counting "BartC" <bc@freeuk.com> - 2011-06-05 14:45 +0100
        Re: Reference counting daknok <daknok@example.com> - 2011-06-05 17:01 +0200
          Re: Reference counting Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-05 18:06 -0400
          Re: Reference counting Dr Nick <3-nospam@temporary-address.org.uk> - 2011-06-12 07:25 +0100
  Re: Reference counting daknok <daknok@example.com> - 2011-06-05 15:29 +0200
  Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-05 12:17 -0500
  Re: Reference counting cri@tiac.net (Richard Harter) - 2011-06-05 17:11 +0000
  Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-10 11:30 -0700
    Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-10 16:04 -0400
    Re: Reference counting James Kuyper <jameskuyper@verizon.net> - 2011-06-10 22:56 -0400
      Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-11 12:36 -0700
        Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-11 17:23 -0500
          Re: Reference counting luser- -droog <mijoryx@yahoo.com> - 2011-06-12 15:15 -0700
            Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-12 21:13 -0500
              Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-13 12:37 -0700
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-13 12:54 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-13 16:40 -0500
          Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-13 07:34 -0700
            Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-13 14:26 -0500
              Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-13 12:47 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-13 17:33 -0500
                Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-14 12:14 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-15 02:16 -0500
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-15 15:34 -0700
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-15 15:42 -0700
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 13:45 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 17:02 -0500
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 15:18 -0700
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 16:01 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 18:44 -0500
                Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-16 17:11 -0700
                Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 18:39 -0500
        Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-11 16:51 -0700
          Re: Reference counting "Chris M. Thomasson" <cristom@charter.net> - 2011-06-11 18:28 -0700
            Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-12 16:10 -0500
            Re: Reference counting ImpalerCore <jadill33@gmail.com> - 2011-06-13 07:36 -0700
      Re: Reference counting Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-14 15:00 -0700
  Re: Reference counting luser- -droog <mijoryx@yahoo.com> - 2011-06-13 21:46 -0700
    Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-14 03:19 -0500
      Re: Reference counting Angel <angel+news@spamcop.net> - 2011-06-14 07:31 +0000
        Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-14 05:02 -0500
        Re: Reference counting Keith Thompson <kst-u@mib.org> - 2011-06-14 09:03 -0700
          Re: Reference counting luser- -droog <mijoryx@yahoo.com> - 2011-06-14 10:23 -0700
            Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-14 13:46 -0500
          Re: Reference counting Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-14 15:05 -0700
            Re: Reference counting Keith Thompson <kst-u@mib.org> - 2011-06-14 16:44 -0700
              Re: Reference counting Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-15 06:54 -0700
                Re: Reference counting Keith Thompson <kst-u@mib.org> - 2011-06-15 08:46 -0700
                Re: Reference counting pete <pfiland@mindspring.com> - 2011-06-15 22:19 -0400
                Re: Reference counting James Kuyper <jameskuyper@verizon.net> - 2011-06-15 23:03 -0400
                Re: Reference counting Tim Rentsch <txr@alumni.caltech.edu> - 2011-06-19 14:48 -0700
        Re: Reference counting Seebs <usenet-nospam@seebs.net> - 2011-06-14 17:16 +0000
    Re: Reference counting luser- -droog <mijoryx@yahoo.com> - 2011-06-14 20:53 -0700
      Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 18:49 -0500
  Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-14 08:30 +0200
    Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-14 09:15 +0200
      Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-14 04:21 -0500
      Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-16 10:35 +0200
        Re: Reference counting Ian Collins <ian-news@hotmail.com> - 2011-06-16 20:36 +1200
        Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-16 11:21 +0200
          Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-19 17:57 +0200
          Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-19 20:01 +0200
            Re: Reference counting Keith Thompson <kst-u@mib.org> - 2011-06-19 11:07 -0700
            Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-19 23:15 -0500
              Re: Reference counting "io_x" <a@b.c.invalid> - 2011-06-20 08:21 +0200
        Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 19:05 -0500
          Re: Reference counting Ian Collins <ian-news@hotmail.com> - 2011-06-17 12:09 +1200
  Re: Reference counting luser- -droog <mijoryx@yahoo.com> - 2011-06-15 08:40 -0700
  Re: Reference counting "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2011-06-16 11:08 -0700
    Re: Reference counting Shao Miller <sha0.miller@gmail.com> - 2011-06-16 14:42 -0400

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