Are pointers private in OpenMP parallel sections?

Question

I've added OpenMP to an existing code base in order to parallelize a for loop. Several variables are created inside the scope of the parallel for region, including a pointer:

#pragma omp parallel for
for (int i = 0; i < n; i++){
    [....]
    Model *lm;
    lm->myfunc();
    lm->anotherfunc();
    [....]
}

In the resulting output files I noticed inconsistencies, presumably caused by a race condition. I ultimately resolved the race condition by using an omp critical. My question remains, though: is lm private to each thread, or is it shared?

Toby Speight · Accepted Answer · 2017-01-25 09:40:29Z

11

Yes, all variables declared inside the OpenMP region are private. This includes pointers.

Each thread will have its own copy of the pointer.

It lets you do stuff like this:

int threads = 8;
int size_per_thread = 10000000;

int *ptr = new int[size_per_thread * threads];

#pragma omp parallel num_threads(threads)
    {
        int id = omp_get_thread_num();
        int *my_ptr = ptr + size_per_thread * id;

        //  Do work on "my_ptr".
    }

edited Jan 25, 2017 at 9:40

Toby Speight

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answered Oct 12, 2011 at 4:42

Mysticial

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1 Comment

argoneus Over a year ago

Thanks for the answer. From reading the spec, it seemed as if that were the case, but it was a bit unclear.

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