C ++-thread_local向量导致join（）崩溃

Question

EDIT: in the original question, thread_array was declared as vector<thread> thread_array(4); instead of vector<thread> thread_array; , which caused an error - this has been edited now, but the problem persists.

Coming from this question: C++ program crashes sometimes when join()-ing threads

I managed to narrow it down to this very simple program which hopefully you can compile and run easily:

#include <thread>
#include <vector>
using namespace std;

thread_local vector<int> v;

void foo(int n) 
{  
    for(int i=0 ; i<n ; i++)
        v.push_back(i);
}

int main() 
{
    vector<thread> thread_array;
    for(int i=0 ; i<4 ; i++)
        thread_array.push_back(thread(foo,100));
    for(int i=0 ; i<4 ; i++)
        thread_array.at(i).join();
    return 0;
}

Why does this program crash after reaching the second for loop (the joining one)? Is this a MinGW bug? As far as I know, I shouldn't be doing anything extra to the thread_local vector. I can post specifics if needed.

Answer 1

thread_array actually contains 8 objects. The 4 default constructed std::thread added by vector<thread> thread_array(4); and the 4 you push_back after. In your second loop, you try to join on the default constructed ones which aren't joinable.

To solve the problem, simply don't add 4 default constructed threads instead of using push_back :

int main() 
{
    vector<thread> thread_array; // <-- remove (4) from here
    for(int i=0 ; i<4 ; i++)
        thread_array.push_back(thread(foo,100));
    for(int i=0 ; i<4 ; i++)
        thread_array.at(i).join();
    return 0;
}

Alternatively, you can assign to the 4 default constructed ones :

int main()
{
    vector<thread> thread_array(4);
    for (auto & worker : thread_array)
        worker = thread(foo, 100);
    for (auto & worker : thread_array)
        worker.join();
    return 0;
}