Rapidly adding int to vector and clearing causes access violation - c++

Question

This is some code that is designed to be run inside a .dll. If I remove either of my calls to NumberSystem::AddZero or NumberSystem::Clear, the code executes without crashing. I have been stuck for 5 hours. I am sure that something just went right over my head. Any help is appreciated!
Code:
#include <iostream>
#include <vector>
#include <thread>
namespace NumberSystem
{
std::vector<int> Numbers = {};
void Clear()
{
Numbers.clear();
}
void AddZero()
{
NumberSystem::Numbers.push_back(0);
}
};
void NumberThread()
{
while (true)
{
NumberSystem::AddZero();
NumberSystem::Clear();
}
}
BOOL APIENTRY DllMain(HMODULE hModule, DWORD dwReason, LPVOID pReserved)
{
std::thread thMain(NumberThread);
thMain.detach();
return TRUE;
}
Callstack:
0xC0000005: Access violation reading location 0x000001CA87EE5000.
memcpy_repmovs() Line 114
std::_Copy_memmove<char const * const *,char const * *>(const char * const * _First=0x000001ca87ec5308, const char * const * _Last=0x000001ca87ec52f0, const char * * _Dest=0x000001ca8d9eef2c) Line 4366 C++
std::_Uninitialized_move<int *,std::allocator<int>>(int * const _First=0x000001ca87ec5308, int * const _Last=0x000001ca87ec52f0, int * _Dest=0x000001ca8d9eef2c, std::allocator<int> & _Al={...}) Line 1694 C++
std::vector<int,std::allocator<int>>::_Umove_if_noexcept1(int * _First=0x000001ca87ec5308, int * _Last=0x000001ca87ec52f0, int * _Dest=0x000001ca8d9eef2c, std::integral_constant<bool,1> __formal={...}) Line 1593 C++
std::vector<int,std::allocator<int>>::_Emplace_reallocate<int>(int * const _Whereptr=0x000001ca87ec5308, int && <_Val_0>=0x00000000) Line 757 C++
std::vector<int,std::allocator<int>>::emplace_back<int>(int && <_Val_0>=0x00000000) Line 708 C++
std::vector<int,std::allocator<int>>::push_back(int && _Val=0x00000000) Line 722 C++
NumberSystem::AddZero()
NumberThread()
Thanks again!

Answer 1

Your code reuses same Numbers variable from multiple threads.
I mean, DllMain callback is called multiple times with different dwReason parameter values.
You can prevent that with something like:
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD dwReason, LPVOID pReserved)
{
// Checks reason for being invoked.
if (dwReason == DLL_PROCESS_ATTACH) {
std::thread thMain(NumberThread);
thMain.detach();
}
return TRUE;
}
If you wonder how you could make it support multi-thread, then research about std::mutex usage for now (or ask different question).