OpenMP和不同的STL向量

Question

Is this safe as is? Or we need to uncomment the "critical" pragma?

std::vector< std::vector<int> > vv(10);
#pragma omp parallel for
for (int i=0; i<10; ++i)
    for (int j=0; j<100; ++j)
        // Should we uncomment the following pragma?
        // #pragma omp critical
        vv[i].push_back(j);

Answer 1

The given code is safe as is because all threads will operate on different elements of vv . std::vector is weakly thread safe in the sense that you can operate concurrently on different vectors as you wish.

Answer 2

Short answer

Short answer: do not uncomment, it is safe without critical section

Long Answer

There is no need for a critical section as echo thread is given a unique subset of iterations from the for loop (there are different strategies that you can apply for distributing them). What is important for you is not having race conditions which happen when concurrenlty adding elements on the same vector ( push_back is not thread-safe per se).

In your case each thread always and only access its own set of vectors, so there is no chance two thread write the same vector .