New threading question

**baxy** · 07-03-2015

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

Guest · 07-03-2015

I'm pretty sure you'll have to avoid trying that, because push_back will result in new allocations and such, which are not designed to be thread safe.

If the math you do is complex enough (I suspect not), you can have each thread fill its own vector with the computed values and then append these vectors to build your final vector. The appending operation would be protected by a lock:

Code:

{
    tempVec.reserve(N);
    for(size_t i = 0; i < N; ++i)
    {
        tempVec[i] = baxyInt;
    }
    std::lock_guard<std::mutex> lock(baxyMutex);
    xx.insert(xx.cend(), tempVec.cbegin(), tempVec.cend());
}

Something like the above, I haven't checked for errors. But if you really just want to fill a vector with integers, multiple threads might not be the fastest solution.

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