Question

I’ve got some code for which I’d like to use OpenMP in the following way:

std::vector<int> v(1000);
# pragma omp parallel for
for (int i = 0; i < 1000; ++i) {
    v[i] = i;
}

I have read that STL vector container is not thread-safe in the situation where multiple threads write to a single container, which would imply that I’d need to lock the vector before making any writes; however, I’ve also been told that the write operation above is somehow “atomic”, and so there is no race condition above. Could someone clarify this?

Answer 1

In this particular example, it will be safe.

The reason is that you are not using operations that could cause a reallocation. (such as push_back()). You are only changing the contents of the individual elements.

Note that you can just as legally do this:

std::vector<int> v(1000);
int *ptr = &v[0];

# pragma omp parallel for
for (int i = 0; i < 1000; ++i) {
    ptr[i] = i;
}

---

It becomes not-thread-safe when you start calling methods like push_back(), pop_back(), insert(), etc… from multiple threads.

I’ll also add that this particular example isn’t well-suited for parallelism since there’s hardly any work to be done. But I suppose it’s just a dumbed-down example for the purpose of asking this question.