C++：使用 OpenMP 插入 std::vector

Question

I'm getting a segmentation fault in the following function that creates a grid of points in parallel with OpenMP using vector insert.

std::vector<n_point_t> fill_points(size_t Nn1, size_t Nn2) {
    std::vector<n_point_t> grid;
    grid.reserve(Nn1*Nn2);
    #pragma omp parallel for
    for (size_t i=0; i<Nn1; i++) {
        std::vector<n_point_t> subgrid = get_subgrid(Nn2);
        grid.insert(grid.begin()+i*Nn2, subgrid.begin(), subgrid.end());
    }
    return grid;
}

n_point_t is defined as

union n_point_t {
    double coords[6];
    struct {
        double n1x;
        double n1y;
        double n1z;
        double n2x;
        double n2y;
        double n2z;
    };
};

and get_subgrid(size_t Nn2) creates a grid of n_point_t of size Nn2 .

The insert is definitely responsible for the segmentation fault. I don't understand the problem here. Each thread should be inserting into a different part of the grid because of the insert indexing.

I get a segmentation fault even if I protect the insert with #pragma omp critical .

Answer 1

Since you do call reserve() in advance, no reallocation will happen here. But you are passing a dangerous argument grid.begin()+i*Nn2 to insert . It is not guaranteed to be a valid iterator.

What if the length of subgrid is less than N2 ? Will you get an uncontinuous vector? Please do not do so. It works in a single thread, just because grid.begin()+i*Nn2 happens to be valid. In another word, do not try to touch the unused memory of a vector.

One suggested solution maybe be resize() the vector and assign them if you must use multiple threads.