error: expected ‘#pragma omp’ clause before ‘{’ token

Question

I have a stack. Each time I pop one element from it, handle this element and determine whether the element should be pushed back to the stack according to some results.

The code is like the following. I used the OpenMP task construct to achieve parallelism because the handling processes of different elements are independent. But I got error: expected ‘#pragma omp’ clause before ‘{’ token for both the lines of #pragma omp parallel and #pragma omp single. I have no idea about the reasons. And I am also curious about the correctness of my usage of the OpenMP task construct.

int processing_ele;

omp_set_num_threads(2);
#pragma omp parallel {
#pragma omp single {
    while (!stack_ele.empty()) {
        // pop
        int processing_ele = stack_edge_id.top();
        stack_ele.pop();

        // handle 
#pragma omp task
        Test(processing_ele);

        if (global_res[processing_ele].need_to_push) {
            stack_ele.push(processing_elec);
        }
    }
}
}

Answer 1

In OpenMP specification you can read

The syntax of an OpenMP directive is as follows:
#pragma omp directive-name [clause[ [,] clause] ... ] new-line

Please observe the new line at the end of the directive, so { have to be in a new line:

#pragma omp parallel 
{
#pragma omp single 
{

or

#pragma omp parallel 
#pragma omp single 
{