线程时访问静态映射时出现段错误

Question

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
}

关于这个的一些事情：

如果我删除对 count 的调用，则它不会出现段错误。看来是不知何故造成的。
它只发生在 Linux Debian 9 上（我也使用 Windows 10 和 Mac 构建）。
如果我在 detach 调用后添加类似 std::this_thread::sleep_for(std::chrono::milliseconds(2000)); 的内容，那么它不会出现段错误。
如果我让conversionMap不是静态的，那么它就不会出现段错误。

我不知道确切的问题是什么，但这与主线程在分离线程完成之前退出有关。

Answer 1

正如 Slava 所说的那样，问题是您让 main return，而您的另一个线程是运行。

从 main 返回最终会调用 exit，它会遍历所有已注册的 atexit 处理程序。

当使用 libstdc++（可能还有大多数其他 C++ 运行时实现）时，任何已构造的静态 C++ 对象都会向 atexit 注册其析构函数，因此在该析构函数触发后，您的分离线程是访问已销毁的 conversionMap 对象，结果可预测。

您可以使用 Address Sanitizer (-fsanitize=address) 观察这一点，它报告：

=================================================================
==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<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>
 >::~_Hashtable() /usr/include/c++/9/bits/hashtable.h:1353
    #7 0x563122b9b4ff 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> > >::~unordered_map() /usr/include/c++/9/bits/unordered_map.h:102
    #8 0x7fc552093e26 in __run_exit_handlers /build/glibc-M65Gwz/glibc-2.30/stdlib/exit.c:108

previously allocated by thread T0 here:
    #0 0x7fc55250919f in operator new(unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10919f)
    #1 0x563122b9e828 in __gnu_cxx::new_allocator<std::__detail::_Hash_node_base*>::allocate(unsigned long, void const*) /usr/include/c++/9/ext/new_allocator.h:114
    #2 0x563122b9defc in std::allocator_traits<std::allocator<std::__detail::_Hash_node_base*> >::allocate(std::allocator<std::__detail::_Hash_node_base*>&, unsigned long) /usr/include/c++/9/bits/alloc_traits.h:444
    #3 0x563122b9d6d4 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_allocate_buckets(unsigned long) /usr/include/c++/9/bits/hashtable_policy.h:2134
    #4 0x563122b9ce5b 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_allocate_buckets(unsigned long) /usr/include/c++/9/bits/hashtable.h:361
    #5 0x563122b9c3db 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>
 >::_Hashtable<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> const*>(std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> const*, std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> const*, unsigned l
ong, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&, std::__detail::_Mod_range_hashing const&, std::__detail::_Default_ranged_hash const&, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&, std::__detail::_Select1st const&, std::allocator<std::pair<std::__cxx
11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> > const&) /usr/include/c++/9/bits/hashtable.h:989
    #6 0x563122b9bab5 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>
 >::_Hashtable(std::initializer_list<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, unsigned long, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&,
std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> > const&) /usr/include/c++/9/bits/hashtable.h:466
    #7 0x563122b9b6a3 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> > >::unordered_map(std::initializer_list<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> >, unsigned long, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&, std::equal_to<std::__
cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, myEnum> > const&) /usr/include/c++/9/bits/unordered_map.h:231
    #8 0x563122b9a678 in stringToEnum(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /tmp/t.cc:14
    #9 0x563122b9aa9c in threadedFcn() /tmp/t.cc:27
    #10 0x563122b9ac0f in main /tmp/t.cc:33
    #11 0x7fc55207ce0a in __libc_start_main ../csu/libc-start.c:308

Thread T2 created by T0 here:
    #0 0x7fc5524399b2 in pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x399b2)
    #1 0x7fc552301e24 in std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xcee24)
    #2 0x563122b9ac75 in main /tmp/t.cc:35
    #3 0x7fc55207ce0a in __libc_start_main ../csu/libc-start.c:308

更新：

I guess an easy fix is to just make my variable not-static (functionally it will be the same but with an obvious performance hit).

有一种简单的方法可以避免在每次调用时都必须构造一个新的 unordered_map，同时还可以避免全局破坏问题：使用静态指针：

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

  if (conversionMap->count(enumString) == 0) {
     // Return some default value
  }
  return conversionMap->at(enumString);
}

线程时访问静态映射时出现段错误

Segfault when accessing static map when threading

c++

static

dictionary

segmentation-fault

stdthread