C++ generating random numbers inside loop

I want to generate random numbers inside loop but results are always same numbers.
What I'm doing wrong? Thanks.

Code

#include <fstream>
#include <ctime>
#include <cstdlib>

using namespace std;

const char duom[] = "U1.txt";
const char rez[] = "U1_rez.txt";

void num_gen(int & x, int & y);

int main(){
    srand(time(NULL));

    int x, y;

    ifstream fd(duom);
    fd >> x >> y;
    fd.close();

    ofstream fr(rez);
    for(int j = 1; j <= 4; j++){
        num_gen(x, y);
        fr << x << " + " << y << " = "<< x + y << endl;
        fr << x << " - " << y << " = "<< x - y << endl;
        fr << x << " * " << y << " = "<< x * y << endl;
        fr << x << " / " << y << " = "<< x / y << endl;
        fr << "************" << endl;
    }
    fr.close();
    return 0;
}

void num_gen(int & x, int & y){
     x = 3 + (rand() % 10);
     y = 3 + (rand() % 10);
}

Result

4 + 8= 12
4 - 8= -4
4 * 8= 32
4 / 8= 0
************
4 + 9= 13
4 - 9= -5
4 * 9= 36
4 / 9= 0
************
9 + 11= 20
9 - 11= -2
9 * 11= 99
9 / 11= 0
************
12 + 8= 20
12 - 8= 4
12 * 8= 96
12 / 8= 1
************