Check inside std::vector::_Reserve() triggers integer underflow and fails to detect invalid parameter passed to std::vector::resize() - by Dmitry Me

Status : 

  Fixed<br /><br />
		This item has been fixed in the current or upcoming version of this product.<br /><br />
		A more detailed explanation for the resolution of this particular item may have been provided in the comments section.

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ID 694663 Comments
Status Closed Workarounds
Type Bug Repros 0
Opened 10/13/2011 12:45:29 AM
Access Restriction Public


As of version 10.0.40219.1 SP1Rel std::vector::_Reserve() contains the following code:

size_type _Size = size();
if (max_size() - _Count < _Size)

which has potential for arithmetic undeflow. For example, in this code:

int _tmain(int /*argc*/, _TCHAR* /*argv*/[])
	std::vector<int> vec;
	vec.resize( vec.max_size() + 1 );

restriction "_Size + _Count <= max_size()" is clearly violated, yet _Xlen() is not called from that code - instead code proceeds to "else if", then to the "else", then reserve() is called and _Xlen() is called from inside reserve().

The reason is the following. The intention was to compute (_Size+_Count) is such way that it doesn't overflow. There were two options to do that - either check that "max_size()-_Size<_Count" or check that "max_size()-_Count < Size".

The difference is that when checking "max_size()-_Size<_Count" there's a guarantee that "max_size()>= _Size" and therefore there's no risk of underflow. Meanwhile when checking  "max_size()-_Count < Size" there's no such guarantee - _Count can have whatever value and that value can exceed "max_size()" and cause an underflow during subtraction.

Because of that the check doesn't work in certain cases, including one in the code snippet above. This check should be rewrtitten.
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Posted by Dmitry Me on 11/6/2011 at 9:51 PM
Thanks a lot for fixing this and other items in std::vector source code.
Posted by Microsoft on 11/4/2011 at 9:05 PM

Thanks for reporting this bug. We've fixed it, and the fix will be available in VC11.

If you're curious, here's my new implementation of _Reserve():

size_type _Unused_capacity() const _NOEXCEPT
    {    // micro-optimization for capacity() - size()
    return (this->_Myend - this->_Mylast);

void _Reserve(size_type _Count)
    {    // ensure room for _Count new elements, grow exponentially
    if (_Unused_capacity() < _Count)
        if (max_size() - size() < _Count)
        _Reallocate(_Grow_to(size() + _Count));

If you have any further questions, feel free to E-mail me at .

Stephan T. Lavavej
Visual C++ Libraries Developer
Posted by MS-Moderator10 [Feedback Moderator] on 10/13/2011 at 1:56 AM
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Posted by MS-Moderator01 on 10/13/2011 at 1:45 AM
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