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Re: malloc and maximum size

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On 10/14/2011 03:46 PM, Lowell Gilbert wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
> 
>> On 10/14/2011 03:00 PM, Lowell Gilbert wrote:
>>> James Kuyper <jameskuyper@verizon.net> writes:
>>>
>>>> On 10/14/2011 11:10 AM, Lowell Gilbert wrote:
>>>> ...
>>>>> Does SIZE_MAX have to be the largest value representable in a size_t?  
>>>>> The standard just says that it's the "maximum value" for the type,
>>>>
>>>> What distinction do you see between "largest value representable" if a
>>>> type and "maximum value" for a type? The phrases seem synonymous to me.
>>>
>>> Certainly there's no difference in *most* cases.
>>>
>>> For size_t, the type is just defined as the output type for sizeof,
>>> which is defined in turn as yielding the size of its operand.  So I can
>>> see an argument that the "maximum value" of a size_t could be viewed as
>>> the largest value that sizeof could possibly yield on the particular
>>> implementation.
>>
>> Well, the maximum value returnable by sizeof has to be the same as the
>> maximum value representable by size_t:
>>
>> 	sizeof(char[(size_t)-1]) == (size_t)(-1)
> 
> Assuming I understand what you're getting at, you're begging the
> question here.  [But the expression doesn't make sense to me, so 
> the assumption may well be wrong.]

I had not intended to beg the question. Per 6.3.1.3p2, (size_t)(-1) is
precisely "the maximum value that can be represented" as a size_t.

char[(size_t)(-1)] is an array type whose size in bytes is (size_t)(-1).
sizeof(char[(size_t)(-1)]) must therefore yield a value which is
(size_t)(-1). Therefore, the largest value that sizeof could possibly
yield must be (size_t)(-1), the same as the maximum representable value.

...
>> However, there's no possible basis that I'm aware of for rejecting
>> sizeof(char[(size_t)-1]).
> 
> I assume you mean SIZE_MAX rather than (size_t),

No, in don't mean SIZE_MAX, since I was trying to address your claim
that it can be less than maximum representable value. I didn't mean
(size_t), either, which is no more than a parenthesized type - I presume
that was an editing error? I meant (size_t)(-1), which is guaranteed by
the standard to be the maximum representable value.

> ... in which case I agree.  
> But that doesn't mean SIZE_MAX has to be the largest value a size_t can
> hold, if there are other limits on how big an object can be.

The key point is that sizeof acts on types, not objects. Even when you
write "sizeof expression", and expression is an lvalue referring to an
object, it doesn't look into the expression to determine which object it
refers to; it just looks at the type of the expression. It doesn't care
whether or not there's any actual object of that type. sizeof(int) ==
sizeof *(int*)0, even though the standard prohibits (int*)0 from
pointing at any actual object.

>>> ...  Of course, that can be (and, indeed, often is) smaller
>>> than a size_t could represent.
>>
>> With respect to your claim "often is" - can you cite any implementation
>> where the equality expression given above is NOT true?
> 
> I think you're misreading my claim.  There certainly are systems with
> architectural limitations on how big an object can be, well short of
> what fits in a size_t.

And sizeof(char[(size_t)(-1)]) exceeds none of those limitations,
because it doesn't cause the creation of any object of the specified type.

> For example, the machine I'm posting from has 32-bit words, from which
> it gets a 4-gigabyte range for its size_t.  However, the operating
> system reserves some of the address space, so no program can ever have
> more than 3 gigabytes.  As a result, sizeof can *never* return more than
> 3 gigabytes.

Not even sizeof(char[3500000000])? For an implementation where
(size_t)(-1) is 4294967295, what gives that implementation permission to
do anything with that expression other than yield a value of
(size_t)3500000000? The implementation is free to issue a diagnostic, of
course, but not to reject the code.

> ... This prompted me to wonder whether it would be legitimate
> for the system's SIZE_MAX to be 3 gigabytes.
> 
> Am I making more sense now?

Not yet. You seem to have missed the point I was making, while I think I
understand yours perfectly. Is my point clearer now?

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Thread

malloc and maximum size "V.Subramanian, India" <subramanian100in@yahoo.com> - 2011-10-14 06:31 -0700
  Re: malloc and maximum size gordonb.pey1d@burditt.org (Gordon Burditt) - 2011-10-14 09:45 -0500
    Re: malloc and maximum size Lowell Gilbert <lgusenet@be-well.ilk.org> - 2011-10-14 11:10 -0400
      Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 11:40 -0400
        Re: malloc and maximum size Lowell Gilbert <lgusenet@be-well.ilk.org> - 2011-10-14 15:00 -0400
          Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 15:17 -0400
            Re: malloc and maximum size Lowell Gilbert <lgusenet@be-well.ilk.org> - 2011-10-14 15:46 -0400
              Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 16:38 -0400
                Re: malloc and maximum size Ike Naar <ike@iceland.freeshell.org> - 2011-10-14 21:59 +0000
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 18:26 -0400
                Re: malloc and maximum size Lowell Gilbert <lgusenet@be-well.ilk.org> - 2011-10-14 18:42 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 16:27 -0700
                Re: malloc and maximum size Richard Damon <news.x.richarddamon@xoxy.net> - 2011-10-14 20:41 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 18:31 -0700
                Re: malloc and maximum size Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-10-15 02:58 +0100
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 22:11 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 22:59 -0400
                Re: malloc and maximum size Richard Damon <news.x.richarddamon@xoxy.net> - 2011-10-14 23:44 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 22:56 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-15 07:52 -0400
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:54 -0800
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:51 -0800
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 22:51 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 22:48 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-15 07:54 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-15 11:25 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-15 21:21 -0400
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 17:41 -0800
                Re: malloc and maximum size Kaz Kylheku <kaz@kylheku.com> - 2012-01-25 02:11 +0000
                Re: malloc and maximum size Richard Damon <news.x.richarddamon@xoxy.net> - 2011-10-15 11:50 -0400
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-15 21:38 -0400
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 17:33 -0800
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 17:06 -0800
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:58 -0800
                Re: malloc and maximum size Kaz Kylheku <kaz@kylheku.com> - 2012-01-25 01:23 +0000
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 16:31 -0800
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 22:20 -0400
                Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 16:03 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 23:01 -0400
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 17:35 -0800
                Re: malloc and maximum size Kaz Kylheku <kaz@kylheku.com> - 2012-01-25 02:33 +0000
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 17:14 -0800
              Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:45 -0800
            Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:32 -0800
              Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2012-01-25 02:40 -0800
                Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 17:27 -0800
          Re: malloc and maximum size Richard Damon <news.x.richarddamon@xoxy.net> - 2011-10-14 16:17 -0400
            Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 16:52 -0400
              Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 16:12 -0700
                Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 23:04 -0400
          Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:36 -0800
    Re: malloc and maximum size Ike Naar <ike@iceland.freeshell.org> - 2011-10-14 16:49 +0000
  Re: malloc and maximum size James Kuyper <jameskuyper@verizon.net> - 2011-10-14 11:05 -0400
  Re: malloc and maximum size Nobody <nobody@nowhere.com> - 2011-10-14 16:11 +0100
  Re: malloc and maximum size Kenneth Brody <kenbrody@spamcop.net> - 2011-10-14 12:24 -0400
  Re: malloc and maximum size jt@toerring.de (Jens Thoms Toerring) - 2011-10-14 17:10 +0000
  Re: malloc and maximum size Stephen Sprunk <stephen@sprunk.org> - 2011-10-14 13:43 -0500
    Re: malloc and maximum size Keith Thompson <kst-u@mib.org> - 2011-10-14 12:03 -0700
      Re: malloc and maximum size jt@toerring.de (Jens Thoms Toerring) - 2011-10-14 20:36 +0000
    Re: malloc and maximum size Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:26 -0800
  Re: malloc and maximum size Seebs <usenet-nospam@seebs.net> - 2011-10-17 17:21 +0000
  Re: malloc and maximum size Shao Miller <sha0.miller@gmail.com> - 2012-01-24 23:49 -0500
    Re: malloc and maximum size Shao Miller <sha0.miller@gmail.com> - 2012-01-24 23:55 -0500

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