How to implement this script about concurrency in c++ 11 - c++11

Question

I implement an concurrency script of multi thread in c++ 11 . But i stuck .
int product_val = 0;
- thread 1 : increase product_val to vector in thread 2 , notify thread 2 and waiting for thread 2 print product_val;
- thread 2 : wait and decrease product_val , print product_val
1 #include <iostream>
2 #include <thread>
3 #include <condition_variable>
4 #include <mutex>
5 #include <chrono>
6 #include <queue>
7 using namespace std;
8 int product_val = 0;
9 std::condition_variable cond;
10 std::mutex sync;
11 int main() {
12 //thread 2
13 std::thread con = std::thread([&](){
14 while (1)
15 {
16 std::unique_lock<std::mutex> l(sync);
17 cond.wait(l);
18 product_val--;
19 printf("Consumer product_val = %d \n", product_val);
20 l.unlock();
21 }
22 });
23 //thread 1 (main thread) process
24 for (int i = 0; i < 5; i++)
25 {
26 std::unique_lock<std::mutex> l(sync);
27 product_val++;
28 std::cout << "producer product val " << product_val;
29 cond.notify_one();
30 l.unlock();
31 l.lock();
32 while (product_val)
33 {
34
35 }
36 std::cout << "producer product val " << product_val;
37 l.unlock();
38 }
39 return 0;
40 }