3.2.1 OMP_NESTED

可通过设置 OMP_NESTED 环境变量来启用或禁用嵌套并行操作。缺省情况下，禁用嵌套并行操作。

以下示例中的嵌套并行构造具有三个级别。

#include <omp.h>
#include <stdio.h>
void report_num_threads(int level)
{
    #pragma omp single
    {
        printf("Level %d: number of threads in the team = %d\n",
                  level, omp_get_num_threads());
    }
 }
int main()
{
    omp_set_dynamic(0);
    #pragma omp parallel num_threads(2)
    {
        report_num_threads(1);
        #pragma omp parallel num_threads(2)
        {
            report_num_threads(2);
            #pragma omp parallel num_threads(2)
            {
                report_num_threads(3);
            }
        }
    }
    return(0);
}

启用嵌套并行操作时，编译和运行此程序会产生以下（经过排序的）输出：

% setenv OMP_NESTED TRUE
% a.out | sort
Level 1: number of threads in the team = 2
Level 2: number of threads in the team = 2
Level 2: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2
Level 3: number of threads in the team = 2

在禁用嵌套并行操作的情况下运行程序会生成以下输出：

% setenv OMP_NESTED FALSE
% a.out | sort
Level 1: number of threads in the team = 2
Level 2: number of threads in the team = 1
Level 2: number of threads in the team = 1
Level 3: number of threads in the team = 1
Level 3: number of threads in the team = 1