OpenMP常用指令释义

1 #pragma omp for 

int j = 0;

#pragma omp for

for ( j = 0; j < 4; j++ ){

         printf(“j = %d, ThreadId = %d
”, j, omp_get_thread_num());

     }

执行以上代码后打印出以下结果

j = 0, ThreadId = 0

j = 1, ThreadId = 0

j = 2, ThreadId = 0

j = 3, ThreadId = 0

单独使用for,后续循环在一个线程中执行。可以看出没有形成多线程的效果，所以一般不推荐使用。

2 #pragma omp parallel 

int j = 0;

#pragma omp parallel for

for ( j = 0; j < 4; j++ ){

         printf(“j = %d, ThreadId = %d
”, j, omp_get_thread_num());

     }

执行后会打印出以下结果：

j = 0, ThreadId = 0

j = 2, ThreadId = 2

j = 1, ThreadId = 1

j = 3, ThreadId = 3

区域并行化指导语句，说明后续语句按多线程方式运行，线程数默认为CPU线程数，也可以用num_threads()自行指定。

3 #pragma omp parallel  for

#pragma omp parallel

{

#pragma omp for

for ( j = 0; j < 4; j++ ){

         printf(“j = %d, ThreadId = %d
”, j, omp_get_thread_num());

     }

}

执行以上代码会打印出以下结果：

j = 1, ThreadId = 1

j = 3, ThreadId = 3

j = 2, ThreadId = 2

j = 0, ThreadId = 0

循环并行化指导语句，说明后续循环按多线程执行，线程数与循环次数相等。