OpenMP and STL vector

Question

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

std::vector v(1000);
# pragma omp parallel for
for (int i = 0; i <          


        

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;
}

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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.

Answer 2

Multiple reads are safe but I would recommend to avoid multiple writes to the same container. But you can write to memory you manage on your own. The difference to a vector would be that you can be sure that the memory would not be changed or reallocated at the same time. Otherwise you can also use a semaphore but this would probably decrease the efficiency and if you use several it can even cause deadlocks if you don't work properly.