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May the size argument of operator new overflow?

 
 
Angel Tsankov
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      06-18-2008
>>>> Does the C++ standard define what happens when the size
>>>> argument of void* operator new(size_t size) cannot represent
>>>> the total number of bytes to be allocated?
>>>
>>>> For example:
>>>
>>>> struct S
>>>> {
>>>> char a[64];
>>>> };
>>>
>>>> S* allocate(int size)
>>>> {
>>>> return new S[size]; // What happens here?
>>>> }
>>>
>>>> int main()
>>>> {
>>>> allocate(0x7FFFFFFF);
>>>> }
>>>
>>> Supposing that all values in an int can be represented in a
>>> size_t (i.e. that size_t is unsigned int or larger---very, very
>>> probably), then you should either get the memory, or get a
>>> bad_alloc exception (which you don't catch). That's according
>>> to the standard; a lot of implementations seem to have bugs
>>> here.

>>
>> I think, you are missing a twist that the OP has hidden within his
>> posting: the size of S is at least 64. The number of S objects that
>> he requests is close to numeric_limits<size_t>::max(). So when new
>> S[size] is translated into raw memory allocation, the number of
>> bytes (not the number of S objects) requested might exceed
>> numeric_limits<size_t>::max().


Thanks for pointing this out, I though it would be obvious to everyone.
The following example might be a little bit less confusing:

struct S
{
char a[64]; // Any size greater than 1 would do.
};

S* allocate(std::size_t size)
{
return new S[size]; // How many bytes of memory must the new operator
allocate if size equals std::numeric_limits<size_t>::max()?
}

>> I think (based on my understanding of [5.3.4/12]) that in such a
>> case, the unsigned arithmetic will just silently overflow and you
>> end up allocating a probably unexpected amount of memory.

>
> Here is what one compiler does - catch the overflow and wrap it back to
> numeric_limits<size_t>::max().
>
> int main()
> {
> allocate(0x7FFFFFFF);
> 00401000 xor ecx,ecx
> 00401002 mov eax,7FFFFFFFh
> 00401007 mov edx,40h
> 0040100C mul eax,edx
> 0040100E seto cl
> 00401011 neg ecx
> 00401013 or ecx,eax
> 00401015 push ecx
> 00401016 call operator new[] (401021h)
> 0040101B add esp,4
> }
> 0040101E xor eax,eax
> 00401020 ret


Yes, the size requested is rounded to the maximum allocatable size, but is
this standard-compliant behavior? And if it is, how is client code notified
of the rounding?


 
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Ian Collins
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      06-18-2008
Angel Tsankov wrote:
>>> Hello!
>>>
>>> Does the C++ standard define what happens when the size argument of void*
>>> operator new(size_t size) cannot represent the total number of bytes to
>>> be
>>> allocated? For example:
>>>

>> size_t will always be wide enough to represent the maximum memory range
>> on a given system.
>>
>> If the system can't supply the requested size, new throws std::bad_alloc.

>
> This is not an answer to the question what happens in the example you have
> cut off.
>

What more is there to say other than "If the system can't supply the
requested size, new throws std::bad_alloc"? If the system had 32GB
free, new would succeed, otherwise it would fail.

--
Ian Collins.
 
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Ian Collins
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      06-18-2008
Angel Tsankov wrote:

[please don't snip attributions]

> Bo Persson wrote:


>> Here is what one compiler does - catch the overflow and wrap it back to
>> numeric_limits<size_t>::max().
>>
>> int main()
>> {
>> allocate(0x7FFFFFFF);
>> 00401000 xor ecx,ecx
>> 00401002 mov eax,7FFFFFFFh
>> 00401007 mov edx,40h
>> 0040100C mul eax,edx
>> 0040100E seto cl
>> 00401011 neg ecx
>> 00401013 or ecx,eax
>> 00401015 push ecx
>> 00401016 call operator new[] (401021h)
>> 0040101B add esp,4
>> }
>> 0040101E xor eax,eax
>> 00401020 ret

>
> Yes, the size requested is rounded to the maximum allocatable size, but is
> this standard-compliant behavior? And if it is, how is client code notified
> of the rounding?
>

Your question has nothing to do with operator new() and everything to do
with integer overflow.

The reason some of us answered the way we did is probably because we are
used to systems where sizeof(int) == 4 and sizeof(size_t) == 8, so your
original code would simply have requested 32GB, not a lot on some systems.

--
Ian Collins.
 
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Bo Persson
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      06-18-2008
Angel Tsankov wrote:
>>>>> Does the C++ standard define what happens when the size
>>>>> argument of void* operator new(size_t size) cannot represent
>>>>> the total number of bytes to be allocated?
>>>>
>>>>> For example:
>>>>
>>>>> struct S
>>>>> {
>>>>> char a[64];
>>>>> };
>>>>
>>>>> S* allocate(int size)
>>>>> {
>>>>> return new S[size]; // What happens here?
>>>>> }
>>>>
>>>>> int main()
>>>>> {
>>>>> allocate(0x7FFFFFFF);
>>>>> }
>>>>
>>>> Supposing that all values in an int can be represented in a
>>>> size_t (i.e. that size_t is unsigned int or larger---very, very
>>>> probably), then you should either get the memory, or get a
>>>> bad_alloc exception (which you don't catch). That's according
>>>> to the standard; a lot of implementations seem to have bugs
>>>> here.
>>>
>>> I think, you are missing a twist that the OP has hidden within his
>>> posting: the size of S is at least 64. The number of S objects
>>> that he requests is close to numeric_limits<size_t>::max(). So
>>> when new S[size] is translated into raw memory allocation, the
>>> number of bytes (not the number of S objects) requested might
>>> exceed numeric_limits<size_t>::max().

>
> Thanks for pointing this out, I though it would be obvious to
> everyone. The following example might be a little bit less
> confusing:
> struct S
> {
> char a[64]; // Any size greater than 1 would do.
> };
>
> S* allocate(std::size_t size)
> {
> return new S[size]; // How many bytes of memory must the new
> operator allocate if size equals std::numeric_limits<size_t>::max()?
> }
>
>>> I think (based on my understanding of [5.3.4/12]) that in such a
>>> case, the unsigned arithmetic will just silently overflow and you
>>> end up allocating a probably unexpected amount of memory.

>>
>> Here is what one compiler does - catch the overflow and wrap it
>> back to numeric_limits<size_t>::max().
>>
>> int main()
>> {
>> allocate(0x7FFFFFFF);
>> 00401000 xor ecx,ecx
>> 00401002 mov eax,7FFFFFFFh
>> 00401007 mov edx,40h
>> 0040100C mul eax,edx
>> 0040100E seto cl
>> 00401011 neg ecx
>> 00401013 or ecx,eax
>> 00401015 push ecx
>> 00401016 call operator new[] (401021h)
>> 0040101B add esp,4
>> }
>> 0040101E xor eax,eax
>> 00401020 ret

>
> Yes, the size requested is rounded to the maximum allocatable size,
> but is this standard-compliant behavior? And if it is, how is
> client code notified of the rounding?


Requesting a numeric_limits<size_t>::max() allocation size is pretty
much assured to fail with a std::bad_alloc exception.


Bo Persson


 
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James Kanze
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      06-18-2008
On Jun 18, 5:40 pm, Kai-Uwe Bux <(E-Mail Removed)> wrote:
> James Kanze wrote:
> > On Jun 18, 11:44 am, "Angel Tsankov" <(E-Mail Removed)-sofia.bg> wrote:
> >> Does the C++ standard define what happens when the size
> >> argument of void* operator new(size_t size) cannot represent
> >> the total number of bytes to be allocated?


> >> For example:


> >> struct S
> >> {
> >> char a[64];
> >> };


> >> S* allocate(int size)
> >> {
> >> return new S[size]; // What happens here?
> >> }


> >> int main()
> >> {
> >> allocate(0x7FFFFFFF);
> >> }


> > Supposing that all values in an int can be represented in a
> > size_t (i.e. that size_t is unsigned int or larger---very, very
> > probably), then you should either get the memory, or get a
> > bad_alloc exception (which you don't catch). That's according
> > to the standard; a lot of implementations seem to have bugs
> > here.


> I think, you are missing a twist that the OP has hidden within
> his posting: the size of S is at least 64. The number of S
> objects that he requests is close to
> numeric_limits<size_t>::max().


It's not on the systems I usually use, but that's not the point.

> So when new S[size] is translated into raw memory allocation,
> the number of bytes (not the number of S objects) requested
> might exceed numeric_limits<size_t>::max().


And? That's the implementation's problem, not mine. I don't
see anything in the standard which authorizes special behavior
in this case.

> I think (based on my understanding of [5.3.4/12]) that in such
> a case, the unsigned arithmetic will just silently overflow
> and you end up allocating a probably unexpected amount of
> memory.


Could you please point to something in §5.3.4/12 (or elsewhere)
that says anything about "unsigned arithmetic". I only have a
recent draft here, but it doesn't say anything about using
unsigned arithmetic, or that the rules of unsigned arithmetic
apply for this calcule, or even that there is a calcule. (It is
a bit vague, I'll admit, since it says "A new-expression passes
the amount of space requested to the allocation function as the
first argument of type std:: size_t." It doesn't really say
what happens if the "amount of space" isn't representable in a
size_t. But since it's clear that the request can't be honored,
the only reasonable interpretation is that you get a bad_alloc.)

--
James Kanze (GABI Software) email:(E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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James Kanze
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      06-18-2008
On Jun 18, 9:16 pm, Ian Collins <(E-Mail Removed)> wrote:
> Angel Tsankov wrote:


> > Bo Persson wrote:
> >> Here is what one compiler does - catch the overflow and
> >> wrap it back to numeric_limits<size_t>::max().


> >> int main()
> >> {
> >> allocate(0x7FFFFFFF);
> >> 00401000 xor ecx,ecx
> >> 00401002 mov eax,7FFFFFFFh
> >> 00401007 mov edx,40h
> >> 0040100C mul eax,edx
> >> 0040100E seto cl
> >> 00401011 neg ecx
> >> 00401013 or ecx,eax
> >> 00401015 push ecx
> >> 00401016 call operator new[] (401021h)
> >> 0040101B add esp,4
> >> }
> >> 0040101E xor eax,eax
> >> 00401020 ret


> > Yes, the size requested is rounded to the maximum
> > allocatable size, but is this standard-compliant behavior?


If the implementation can be sure that the call to operator
new[] will fail, it's probably the best solution. (This would
be the case, for example, if it really was impossible to
allocate that much memory.)

> > And if it is, how is client code notified of the rounding?


It doesn't have to be.

> Your question has nothing to do with operator new() and
> everything to do with integer overflow.


His question concerned operator new. Not unsigned integral
arithmetic.

> The reason some of us answered the way we did is probably
> because we are used to systems where sizeof(int) == 4 and
> sizeof(size_t) == 8, so your original code would simply have
> requested 32GB, not a lot on some systems.


Or because we take the standard literally.

--
James Kanze (GABI Software) email:(E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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James Kanze
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      06-18-2008
On Jun 18, 7:53 pm, Jerry Coffin <(E-Mail Removed)> wrote:
> In article <g3alej$(E-Mail Removed)>, (E-Mail Removed)-sofia.bg says...
> > Does the C++ standard define what happens when the size
> > argument of void* operator new(size_t size) cannot represent
> > the total number of bytes to be allocated? For example:


> > struct S
> > {
> > char a[64];
> > };


> > S* allocate(int size)
> > {
> > return new S[size]; // What happens here?
> > }


> > int main()
> > {
> > allocate(0x7FFFFFFF);
> > }


> Chances are pretty good that at some point, you get something
> like:


> void *block = ::new(0x7FFFFFFF*64);


There are a lot of implementations that do that. Luckily,
there's nothing in the standard which allows it.

--
James Kanze (GABI Software) email:(E-Mail Removed)
Conseils en informatique orientée objet/
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James Kanze
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      06-18-2008
On Jun 18, 9:24 pm, Paavo Helde <(E-Mail Removed)> wrote:
> Jerry Coffin <(E-Mail Removed)> kirjutas:


[...]
> The standard says that for too large allocations
> std::bad_alloc must be thrown. In the user code there is no
> unsigned arithmetic done, thus no wraparound can occur. I
> would say that if the implementation does not check for the
> overflow and silently wraps the result, the implementation
> does not conform to the standard. It is irrelevant if the
> implementation uses unsigned arithmetics inside, or e.g.
> double.


> I have not studied the standard in detail, so this is just my
> opinion how it should work.


I have studied the standard in some detail, and your analysis is
clearly correct. Whether this is actually what the authors
meant to say is another question, but it is clearly what the
standard says. It is also obviously how it should work, from a
quality of implementation point of view. Anything else more or
less makes array new unusable. (On the other hand: who cares?
In close to twenty years of C++ programming, I've yet to find a
use for array new.)

--
James Kanze (GABI Software) email:(E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
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Kai-Uwe Bux
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      06-18-2008
James Kanze wrote:

> On Jun 18, 5:40 pm, Kai-Uwe Bux <(E-Mail Removed)> wrote:
>> James Kanze wrote:
>> > On Jun 18, 11:44 am, "Angel Tsankov" <(E-Mail Removed)-sofia.bg> wrote:
>> >> Does the C++ standard define what happens when the size
>> >> argument of void* operator new(size_t size) cannot represent
>> >> the total number of bytes to be allocated?

>
>> >> For example:

>
>> >> struct S
>> >> {
>> >> char a[64];
>> >> };

>
>> >> S* allocate(int size)
>> >> {
>> >> return new S[size]; // What happens here?
>> >> }

>
>> >> int main()
>> >> {
>> >> allocate(0x7FFFFFFF);
>> >> }

>
>> > Supposing that all values in an int can be represented in a
>> > size_t (i.e. that size_t is unsigned int or larger---very, very
>> > probably), then you should either get the memory, or get a
>> > bad_alloc exception (which you don't catch). That's according
>> > to the standard; a lot of implementations seem to have bugs
>> > here.

>
>> I think, you are missing a twist that the OP has hidden within
>> his posting: the size of S is at least 64. The number of S
>> objects that he requests is close to
>> numeric_limits<size_t>::max().

>
> It's not on the systems I usually use, but that's not the point.
>
>> So when new S[size] is translated into raw memory allocation,
>> the number of bytes (not the number of S objects) requested
>> might exceed numeric_limits<size_t>::max().

>
> And? That's the implementation's problem, not mine. I don't
> see anything in the standard which authorizes special behavior
> in this case.


The question is what behavior is "special". I do not see which behavior the
standard requires in this case.


>> I think (based on my understanding of [5.3.4/12]) that in such
>> a case, the unsigned arithmetic will just silently overflow
>> and you end up allocating a probably unexpected amount of
>> memory.

>
> Could you please point to something in §5.3.4/12 (or elsewhere)
> that says anything about "unsigned arithmetic".


I qualified my statement by "I think" simply because the standard is vague
to me. However, it says for instance

new T[5] results in a call of operator new[](sizeof(T)*5+x),

and operator new takes its argument at std::size_t. Now, whenever any
arithmetic type is converted to std::size_t, I would expect [4.7/2] to
apply since size_t is unsigned. When the standard does not say that usual
conversion rules do not apply in the evaluation of the expression

sizeof(T)*5+x

what am I to conclude?

> I only have a
> recent draft here, but it doesn't say anything about using
> unsigned arithmetic, or that the rules of unsigned arithmetic
> apply for this calcule, or even that there is a calcule.


It gives the formula above. It does not really matter whether you interpret

sizeof(T)*5+x

as unsigned arithmetic or as plain math. A conversion to std::size_t has to
happen at some point because of the signature of the allocation function.
If [4.7/2] is not meant to apply to that conversion, the standard should
say that somewhere.

> (It is
> a bit vague, I'll admit, since it says "A new-expression passes
> the amount of space requested to the allocation function as the
> first argument of type std:: size_t." It doesn't really say
> what happens if the "amount of space" isn't representable in a
> size_t.


So you see: taken litterally, the standard guarantees something impossible
to happen.

> But since it's clear that the request can't be honored,
> the only reasonable interpretation is that you get a bad_alloc.)


Hm, that is a mixure of common sense and wishfull thinking

I agree that a bad_alloc is clearly what I would _want_ to get. I do not
see, however, how to argue from the wording of the standard that I _will_
get that.


Best

Kai-Uwe Bux
 
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Jerry Coffin
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      06-19-2008
In article <Xns9AC1E3EAF7668nobodyebiee@216.196.97.131>, http://www.velocityreviews.com/forums/(E-Mail Removed)
says...

[ ... ]

> The standard says that for too large allocations std::bad_alloc must be
> thrown. In the user code there is no unsigned arithmetic done, thus no
> wraparound can occur. I would say that if the implementation does not
> check for the overflow and silently wraps the result, the implementation
> does not conform to the standard. It is irrelevant if the implementation
> uses unsigned arithmetics inside, or e.g. double.
>
> I have not studied the standard in detail, so this is just my opinion how
> it should work.


Though it's in a non-normative note, the standard says ($5.3.4/12):

new T[5] results in a call of operator new[](sizeof(T)*5+x)

Even though that's a note, I think it's going to be hard to say it's
_wrong_ for an implementation to do exactly what that says -- and if
sizeof(T) is the maximum value for size_t, the expression above will
clearly wraparound...

--
Later,
Jerry.

The universe is a figment of its own imagination.
 
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