Stl container vector push_back with OpenMP multithreading

Question

I want to push_back an object into a vector from different threads. The no. of threads depends on the machine.

#pragma omp parallel shared(Spaces, LookUpTable) private(LutDistribution, tid)
{
    tid = omp_get_thread_num();

    BestCoreSpaces.push_back( computeBestCoreSpace(tid, &Spaces, &LookUpTable, LutDistribution));
}

The problem is, that I'm not sure if it's working. I don't get crashes. I'm using openMP. Is openMP queuing something? Maybe its enough to reserve memory for the container with BestCoreSpaces.reserve(tid) or to assign the amount of elements with BestCoreSpaces.assign(tid, Space). Can somebody help me?

score 4 · Answer 1 · answered Aug 17 '11 at 23:17

You just get away with it - you have a race condition that might or might not manifest itself depending on the optimization level at compile time, the thread execution and/or the alignment of the stars.

You have to make the push_back() a critical section (i.e use a mutex). For example:

#pragma omp parallel shared(Spaces, LookUpTable, BestCoreSpaces) private(LutDistribution, tid)
{
  tid = omp_get_thread_num();
#pragma omp critical
  BestCoreSpaces.push_back( 
    computeBestCoreSpace(tid, &Spaces, &LookUpTable, LutDistribution)
  );
}

Stl container vector push_back with OpenMP multithreading

1 Answers1