OpenMP and STL vector

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?

Check for concurrent_vector which allows multiple reads, writes — user, Jun 09 '13 at 08:46

score 35 · Accepted Answer · edited Nov 18 '17 at 14:08

35

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.

edited Nov 18 '17 at 14:08

Tom de Geus

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answered Feb 13 '12 at 22:42

Mysticial

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"not modifying the vector itself" -> "not using operations that could cause reallocation". But +1 anyway. – Fred Foo Feb 13 '12 at 22:45
@larsmans Yes, that's a better wording that what I used. :) I'll change that. Thanks. – Mysticial Feb 13 '12 at 22:47

score 0 · Answer 2 · answered Feb 13 '12 at 22:48

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.

OpenMP and STL vector

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