Std::nth_element Crash question _php tutorial

Std::nth_element crash problems

(1) Source code:

 
  
  

  
   
   
Auto Less_compare = [] (const mirroringgroup& MG1, const mirroringgroup& MG2), bool {

  
   
   
Return (Mg1.usage () < Mg2.usage ());

  
   
   
};

  
   
   

 
  
  

(2) Questions:

Often occurs crash,stack as follows:

 
 

  
  

  
#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<__gnu_cxx::__normal_iterator    >, __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<__gnu_cxx::__normal_iterator  , Mirroringgroup, miuifs::blockmanager::choosewritablemirroringgroups (std::vector) mirroringgroup> *, int)::__lambda101> (__gnu_cxx::__normal_iterator  >, __gnu_cxx::__normal_iterator  >, const mirroringgroup &, miuifs::blockmanager::__lambda101) (__fi Rst= ..., __last= ..., __pivot= ..., __comp= ...) at/usr/local/include/c++/4.8.2/bits/stl_algo.h:2270 
    
  
  

  
#5 0x0000000000603c1b in Std::__unguarded_partition_pivot<__gnu_cxx::__normal_iterator   ;, 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<__gnu_cxx::__normal_iterator   ;, long int, miuifs::blo Ckmanager::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<__gnu_cxx::__normal_iterator   ;, Miuifs::blockmanager:: Choosewritablemirroringgroups (std::vector  *, int)::__lambda101> (__gnu_cxx::__normal_iterator     >, __gnu_cxx::__normal_iterator    >;, __gnu_cxx::__normal_ite Rator    >, 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  , mgs=0x7fffeb9f414 0, 
 
  
  

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

  
  

  
#9 0x00000000005ff9cf in Miuifs::blockmanager::newblock (THIS=0X118ABE0  ) at Miuifs/miuistorage-dev/blockmanager . cc:331 
 
  
  

  
#10 0x00000000005fed63 in Miuifs::blockmanager::acquireblock (THIS=0X118ABE0  , attribute= ...) 
 
  
  

  
At miuifs/miuistorage-dev/blockmanager.cc:243
 
 

(3) Find the problem:

Problem has been in the std::nth_element, began to think of STL is not the problem, has not been a good solution, and then read the STL source code to find the reason in/usr/local/include/c++/4.8.2/bits/stl_ In Algo.h:

 
  
  

  
   
   
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);

  
   
   

 
  
  

The role of the __move_median_to_first function is to Exchange __first +1, __mid, (__last-2) in the middle size of the value and __first. But ignoring the __mid, (__last-2) points to the same iterator, if the input is as follows:

After __move_median_to_first, the results are as follows:

At this point, __first points to the maximum value. Then look at the implementation of Std::__unguarded_partition, in 2263 rows __comp (*__first, __pivot)) forever returns True, causing ++__first to execute and access the illegal memory.

 
  
  

  
   
   
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 < __last))

  
   
   
return __first;

  
   
   
Std::iter_swap (__first, __last);

  
   
   
++__first;

  
   
   
}

  
   
   

 
  
  

(4) Workaround:

Through Google find the following link, found that is really a STL bug, can only be resolved by upgrading C + +.

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

http://www.bkjia.com/PHPjc/1071768.html www.bkjia.com true http://www.bkjia.com/PHPjc/1071768.html techarticle std::nth_element crash problem (1) Source: Auto Less_compare = [] (const mirroringgroup MG2), bool {return (Mg1.usage ()}; Std::nth_element (Mgs->begin (), Mgs->begin () + (Copy_co ...

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