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?

Answer 1

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".
    }