std::nth_element crash问题

std::nth_element crash问题

(1) 源码：

auto less_compare = [] (const MirroringGroup& mg1, const MirroringGroup& mg2) -> bool {


return (mg1.usage() 
};


std::nth_element(mgs->begin(), mgs->begin() + (copy_count - 1), mgs->end(), less_compare); 

(2) 问题：

经常发生crash，stack如下：

#0  0x00000000004b3807 in MirroringGroup::CopyFrom (this=0x15edf20, from=...) at miuifs/miuistorage-dev/idl/proto/InternalData.pb.cc:6487


#1  0x000000000052bc71 in MirroringGroup::operator= (this=0x15edf20, from=...) at miuifs/miuistorage-dev/idl/proto/InternalData.pb.h:1797


#2  0x000000000052f7cb in std::swap (__a=..., __b=...) at /usr/local/include/c++/4.8.2/bits/move.h:177


#3  0x000000000052e0b0 in std::iter_swap<:__normal_iterator std::vector std::allocator> > >, __gnu_cxx::__normal_iterator > > > (__a=..., __b=...)


at /usr/local/include/c++/4.8.2/bits/stl_algobase.h:147


#4  0x0000000000604b11 in std::__unguarded_partition<:__normal_iterator std::vector> >, MirroringGroup, miuifs::BlockManager::ChooseWritableMirroringGroups(std::vector*, int)::__lambda101>(__gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, const MirroringGroup &, miuifs::BlockManager::__lambda101) (__first=..., __last=..., __pivot=..., __comp=...) at /usr/local/include/c++/4.8.2/bits/stl_algo.h:2270


#5  0x0000000000603c1b in std::__unguarded_partition_pivot<:__normal_iterator std::vector> >, miuifs::BlockManager::ChooseWritableMirroringGroups(std::vector*, int)::__lambda101>(__gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, miuifs::BlockManager::__lambda101) (


__first=..., __last=..., __comp=...) at /usr/local/include/c++/4.8.2/bits/stl_algo.h:2296


#6  0x0000000000603408 in std::__introselect<:__normal_iterator std::vector> >, long int, miuifs::BlockManager::ChooseWritableMirroringGroups(std::vector*, int)::__lambda101>(__gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, long, miuifs::BlockManager::__lambda101) (__first=..., __nth=..., __last=..., __depth_limit=2, 


__comp=...) at /usr/local/include/c++/4.8.2/bits/stl_algo.h:2394


#7  0x0000000000602c95 in std::nth_element<:__normal_iterator std::vector> >, miuifs::BlockManager::ChooseWritableMirroringGroups(std::vector*, int)::__lambda101>(__gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, __gnu_cxx::__normal_iterator > >, miuifs::BlockManager::__lambda101) (__first=..., __nth=..., __last=..., __comp=...)


at /usr/local/include/c++/4.8.2/bits/stl_algo.h:5417


#8  0x000000000060039c in miuifs::BlockManager::ChooseWritableMirroringGroups (this=0x118abe0 <:blockmanager::instance>, mgs=0x7fffeb9f4140, 


copy_count=2) at miuifs/miuistorage-dev/BlockManager.cc:391


#9  0x00000000005ff9cf in miuifs::BlockManager::NewBlock (this=0x118abe0 <:blockmanager::instance>) at miuifs/miuistorage-dev/BlockManager.cc:331


#10 0x00000000005fed63 in miuifs::BlockManager::AcquireBlock (this=0x118abe0 <:blockmanager::instance>, attribute=...)


at miuifs/miuistorage-dev/BlockManager.cc:243

(3) 查找问题：

问题一直出现在std::nth_element中，开始没有想到是STL的问题，一直没有很好的解决办法，后来通过阅读STL源码找到原因在/usr/local/include/c++/4.8.2/bits/stl_algo.h中：

template


inline _RandomAccessIterator


__unguarded_partition_pivot(_RandomAccessIterator __first,


_RandomAccessIterator __last, _Compare __comp)


{


_RandomAccessIterator __mid = __first + (__last - __first) / 2;


std::__move_median_to_first(__first, __first + 1, __mid, (__last - 2),


__comp);


return std::__unguarded_partition(__first + 1, __last, *__first, __comp);


} 

__move_median_to_first函数的作用是将 __first +1 , __mid, (__last - 2) 中中间大小的值和 __first交换。但是却忽略了__mid,(__last - 2) 指向相同迭代器的情况，如果输入时情况如下：

经过__move_median_to_first之后的结果如下：

此时__first指向了最大的值。然后看std::__unguarded_partition的实现，在2263行__comp(*__first, __pivot))永远返回true，导致++__first一直执行而访问了非法内存。

template


_RandomAccessIterator


__unguarded_partition(_RandomAccessIterator __first,


_RandomAccessIterator __last,


const _Tp& __pivot, _Compare __comp)


{


while (true)


{


while (__comp(*__first, __pivot))


++__first;


--__last;


while (__comp(__pivot, *__last))


--__last;


if (!(__first 
return __first;


std::iter_swap(__first, __last);


++__first;


}


} 

(4) 解决方法：

通过google找到下面这个链接，发现确实是一个STL的bug，只能通过升级C++解决了。

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=732042