openmp c++ demo

#include 
int main()
{
// beginning of parallel region
    #pragma omp parallel
    {
        std::cout << "Hello World" << std::endl;
    }
// ending of parallel region
    return 0;
}

# gcc -fopenmp hello.cpp failed, don't know why
wk@ubuntu:~/cdir$ g++ -fopenmp hello.cpp
wk@ubuntu:~/cdir$ ./a.out
Hello World
Hello World
Hello World
Hello World

# 通过设置环境变量来控制线程数量
export OMP_NUM_THREADS=2
wk@ubuntu:~/cdir$ ./a.out
Hello World
Hello World