Thread: New threading question

What actually happens when I make a thread and pass a vector reference to it which i fill in the thread. example : let say I have a function that takes a vector reference and adds numbers to it. and i thread that function into two threads. is the acces to the passed vector somehow coordinated or is there a possibility ffor two threads to write to the same location in that vector:

Code:

void Func(vector<int>& xx){
   for(...)
     xx.push_back(i);
}


void main(){

      threads ...
      vec ...

       for (int i = 0; i < 2; ++i) 
         threads.push_back(thread(Func, ref(vec)));

       for(auto &t : threads)
          t.join();
}

will two threads sinc their access and push back or is it up to me to do that ??

thnx


PS the above is a form of pseudo-code