c - OpenMP始终在同一线程上工作

Question

我正在使用OpenMP和C进行大学交付，并且正在尝试执行以下代码，我唯一想做的就是查看每个部分在每个不同线程中的工作方式:

#include <omp.h>
#include <stdio.h>
int main() {                             
    int id, np;     

    printf("Max threads number: %d\n",omp_get_max_threads());
    omp_set_num_threads(omp_get_max_threads());

    #pragma omp parallel sections private(id, np)
    {
        np = omp_get_num_threads(); 
        #pragma omp section
        { 
            id = omp_get_thread_num();
            printf("Hello from thread %d out of %d threads\n", id, np);
        }
        #pragma omp section
        { 
            id = omp_get_thread_num();
            printf("Hello from thread %d out of %d threads\n", id, np);
        }
        #pragma omp section
        { 
            id = omp_get_thread_num();
            printf("Hello from thread %d out of %d threads\n", id, np);
        }
        #pragma omp section
        { 
            id = omp_get_thread_num();
            printf("Hello from thread %d out of %d threads\n", id, np);
        }
    }
}

我正在Linux上工作，因此在编译时会说:

g++ prueba.c -lgomp -o prueba

我得到下一个输出:

Max threads number: 4
Hello from thread 0 out of 1 threads
Hello from thread 0 out of 1 threads
Hello from thread 0 out of 1 threads
Hello from thread 0 out of 1 threads

谁能告诉我为什么它总是在线程号0上工作，为什么omp_get_num_threads()总是为1？

我要实现的输出是:

Max threads number: 4
Hello from thread 0 out of 3 threads
Hello from thread 1 out of 3 threads
Hello from thread 2 out of 3 threads
Hello from thread 3 out of 3 threads

提前致谢!

Answer 1

首先，您缺少-fopenmp编译器标志，因此忽略了编译指示。在所有编译和链接步骤上指定fopenmp，除非必要，请勿手动与-lgomp链接。

第一句话

np = omp_get_max_threads();

如另一个答案所解释的， sections构造中的“内部”仍将被视为仅由单个线程执行的结构化块。因此，您的代码等于:

 #pragma omp parallel sections private(id)
 {
     #pragma omp section
     {
         np = omp_get_max_threads();
     }
     #pragma omp section
     { 
     id = omp_get_thread_num();
     printf("Hello from thread %d out of %d threads\n", id, np);
     }
....

Note that you could also split the `parallel` / `sections` and initalize the private variables for each thread:

很好该代码由所有并行线程执行，并且 np(每个线程具有相同的值)通过工作共享 omp section保留。您甚至可以移动:

#pragma omp parallel
{
    // Declaring the variables within makes them implicitly private
    // and avoids stupid mistakes
    int np = omp_get_max_threads();
    int id = omp_get_thread_num();
    #pragma omp sections
    {
        #pragma omp section
        {
            printf("Hello from thread %d out of %d threads\n", id, np);
        }
        ...
    }
}