线程化时访问静态地图时发生故障。

Posted 2023-01-08

考虑以下代码片段。

myEnum stringToEnum(const std::string& enumString) 
  static const std::unordered_map<std::string, myEnum> conversionMap = 
    "enumA", myEnum::enumA, 
    "enumB", myEnum::enumB,
    "enumC", myEnum::enumC;

  if(conversionMap.count(enumString) == 0) 
     // Return some default value
  
  return conversionMap.at(enumString);



void threadedFcn() 
  // Do some things
  for (int i = 0; i < someNumber; ++i) 
    // Do some more things
    auto myEnum = stringToEnum(myString);
    // Do even more stuff
  


int main() 
  threadedFcn(); // No problem
  std::thread(threadedFcn).join(); // No problem
  std::thread(threadedFcn).detach(); // Seg fault

关于这段代码的一些事情:

1. 如果我删除对 count 就不会出现seg故障了。看来是某种原因造成的。

2. 这只发生在Linux Debian 9上(我也在Windows 10和Mac上编译过)。

3. 如果我添加一些东西，比如 std::this_thread::sleep_for(std::chrono::milliseconds(2000)); 后的detach调用，那么它就不会seg故障。

4. 如果我做了 conversionMap 而不是静态的，那么就不会出现seg故障。

我搞不清楚具体的问题是什么，但这与主线程在分离线程完成之前退出有关。

答案

正如Slava所言，问题在于你让你的 main 返回，而你的其他线程正在运行。

从 main 最终叫 exit该系统查看了所有注册的 atexit 处理程序。

当使用 libstdc++ (可能还有大多数其他的C++运行时实现)，任何已经构造好的静态C++对象都会将它的destructor用 atexit所以，在该destructor启动后，你的分离线程正在访问被销毁的东西。conversionMap 对象，结果可想而知。

你可以通过使用Address Sanitizer (-fsanitize=address)，该报告。

=================================================================
==87625==ERROR: AddressSanitizer: heap-use-after-free on address 0x603000000010 at pc 0x563122b9c6c5 bp 0x7fc54eafea20 sp 0x7fc54eafea18
READ of size 8 at 0x603000000010 thread T2
    #0 0x563122b9c6c4 in std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, std::__detail
::_Select1st, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true>
 >::_M_bucket_begin(unsigned long) const /usr/include/c++/9/bits/hashtable.h:943
    #1 0x563122b9bcf2 in std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, std::__detail
::_Select1st, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true>
 >::count(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const /usr/include/c++/9/bits/hashtable.h:1451
    #2 0x563122b9b76e in std::unordered_map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, myEnum, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::ba
sic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> > >::count(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const /usr/include/c++/9/bits/unordered_map.h:939
    #3 0x563122b9a793 in stringToEnum(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /tmp/t.cc:16
    #4 0x563122b9aa9c in threadedFcn() /tmp/t.cc:27
    #5 0x563122b9f71e in void std::__invoke_impl<void, void (*)()>(std::__invoke_other, void (*&&)()) /usr/include/c++/9/bits/invoke.h:60
    #6 0x563122b9f681 in std::__invoke_result<void (*)()>::type std::__invoke<void (*)()>(void (*&&)()) /usr/include/c++/9/bits/invoke.h:95
    #7 0x563122b9f5cb in void std::thread::_Invoker<std::tuple<void (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/include/c++/9/thread:244
    #8 0x563122b9f56c in std::thread::_Invoker<std::tuple<void (*)()> >::operator()() /usr/include/c++/9/thread:251
    #9 0x563122b9f4ed in std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)()> > >::_M_run() /usr/include/c++/9/thread:195
    #10 0x7fc552301baf  (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xcebaf)
    #11 0x7fc55202bf26 in start_thread /build/glibc-M65Gwz/glibc-2.30/nptl/pthread_create.c:479
    #12 0x7fc5521532ee in __clone (/lib/x86_64-linux-gnu/libc.so.6+0xfd2ee)

0x603000000010 is located 0 bytes inside of 24-byte region [0x603000000010,0x603000000028)
freed by thread T0 here:
    #0 0x7fc552509f97 in operator delete(void*) (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x109f97)
    #1 0x563122b9e5fb in __gnu_cxx::new_allocator<std::__detail::_Hash_node_base*>::deallocate(std::__detail::_Hash_node_base**, unsigned long) /usr/include/c++/9/ext/new_allocator.h:128
    #2 0x563122b9dbf5 in std::allocator_traits<std::allocator<std::__detail::_Hash_node_base*> >::deallocate(std::allocator<std::__detail::_Hash_node_base*>&, std::__detail::_Hash_node_base**, unsigned long) /usr/include/c++/9/bits/alloc_traits.h:470
    #3 0x563122b9d0cc in std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, true> > >::_M_deallocate_buckets(std::__detail::_Hash_node_base**, unsigned long) /usr/include/c++/9/bits/hashtable_policy.h:2148
    #4 0x563122b9c5d3 in std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, std::__detail
::_Select1st, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true>
 >::_M_deallocate_buckets(std::__detail::_Hash_node_base**, unsigned long) /usr/include/c++/9/bits/hashtable.h:370
    #5 0x563122b9bc99 in std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, std::__detail
::_Select1st, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true>
 >::_M_deallocate_buckets() /usr/include/c++/9/bits/hashtable.h:375
    #6 0x563122b9b73b in std::_Hashtable<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, std::__detail
::_Select1st, std::equal_to&l
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