4.3 在嵌套并行区域中使用 OpenMP 库例程

#include <stdio.h>
#include <omp.h>
int main()
{
    omp_set_nested(1);
    omp_set_dynamic(0);
    #pragma omp parallel num_threads(2)
    {
        if (omp_get_thread_num() == 0)
            omp_set_num_threads(4);       /* line A */
        else
            omp_set_num_threads(6);       /* line B */

        /* The following statement will print out
         *
         * 0: 2 4
         * 1: 2 6
         *
         * omp_get_num_threads() returns the number
         * of the threads in the team, so it is
         * the same for the two threads in the team.
         */
        printf("%d: %d %d\n", omp_get_thread_num(),
               omp_get_num_threads(),
               omp_get_max_threads());

        /* Two inner parallel regions will be created
         * one with a team of 4 threads, and the other
         * with a team of 6 threads.
         */
        #pragma omp parallel
        {
            #pragma omp master
            {
                /* The following statement will print out
                 *
                 * Inner: 4
                 * Inner: 6
                 */
                printf("Inner: %d\n", omp_get_num_threads());
            }
            omp_set_num_threads(7);      /* line C */
        }

        /* Again two inner parallel regions will be created,
         * one with a team of 4 threads, and the other
         * with a team of 6 threads.
         *
         * The omp_set_num_threads(7) call at line C
         * has no effect here, since it affects only
         * parallel regions at the same or inner nesting
         * level as line C.
         */

        #pragma omp parallel
        {
            printf("count me.\n");
        }
    }
    return(0);
}

% a.out
0: 2 4
Inner: 4
1: 2 6
Inner: 6
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.
count me.