mutlprocess and multithread program with c++

#include 
#include 
#include 
#include 
#define THREAD_NUM 2
int main()
{
        pid_t pid = fork();
// beginning of parallel region
// omp_set_num_threads(2);
    #pragma omp parallel num_threads(2)
    {
        std::cout << "Hello World" << std::endl;
    }
// ending of parallel region
    return 0;
}