Reputation: 385
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?
Upvotes: 23
Views: 13023
Reputation: 471567
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.
Upvotes: 35
Reputation: 994
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.
Upvotes: 0