A.20 Binding of barrier Directives

The directive binding rules call for a barrier directive to bind to the closest enclosing parallel directive. For more information on directive binding, see Section 2.8 on page 32.

In the following example, the call from main to sub2 is compliant because the barrier (in sub3) binds to the parallel region in sub2. The call from main to sub1 is compliant because the barrier binds to the parallel region in subroutine sub2. The call from main to sub3 is compliant because the barrier does not bind to any parallel region and is ignored. Also note that the barrier only synchronizes the team of threads in the enclosing parallel region and not all the threads created in sub1.

int main()
{
    sub1(2);
    sub2(2);
    sub3(2);
}

void sub1(int n)
{
    int i;
    #pragma omp parallel private(i) shared(n)
    {
        #pragma omp for
        for (i=0; i<n; i++)
            sub2(i);
    }
}

void sub2(int k)
{
     #pragma omp parallel shared(k)
     sub3(k);
}

void sub3(int n)
{
    work(n);
    #pragma omp barrier
    work(n);
}