Prime Numbers with multi-processing and multi-threding in C++

Question

I'm trying to calculate prime numbers from a given interval from CLA. The example input should be: ./program interval_min interval_max number_process number_thread

And here is the expected process and thread schema:

I coded a function for finding primes that:

int *print_prime(int min, int max)
{   
    if(min == 0){
        min += 2;
    }
    else if (min == 1)
    {
        min += 1;
    }
    
    int *list;
    list = new int;
    int listNumber = 0;
    for (size_t i = min; i <= max; i++){
        int flag = 0;

        for (size_t j = min + 1; j < i; j++){
            if(i % j == 0){
                flag++;
                continue;
            }
        }
        if (flag == 0){
            list[listNumber] = i;
            listNumber++;
        }                
    }
    return list;
}

And I handled the creating the processes like that:

int main(int argc, char **argv)
{   
    if(argc < 5){
        std::cout << "Invalid entry. Please enter 4 arguments." << std::endl;
        return EXIT_FAILURE;
    }

    std::cout << "Master: Started." << std::endl; /* The master process executed */

    pid_t pid;

    int interval_min = std::stoi(argv[1]);
    int interval_max = std::stoi(argv[2]);
    int np = std::stoi(argv[3]);
    int nt = std::stoi(argv[4]);

    for (int i = 0; i < np; i++){
        pid = fork();

        if(pid > 0){
        std::cout << "slave " << i << std::endl; 
        }
         if(pid == 0){
        std::cout << "This is a child Process" << i << std::endl;
        break;
        }
    } 

But I stuck at this point. How can I add threads under processes?

Expected output should be like this: