#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>

// protected by g_num_mutex
int g_num = 0;  
std::mutex g_num_mutex;
 
void slow_increment(int id) 
{
    for (int i = 0; i < 3; ++i) {
        g_num_mutex.lock();
        ++g_num;
        std::cout << id << " => " << g_num << std::endl;
        g_num_mutex.unlock();
		
		//sleep for 1 second
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
}
 
int main()
{
    std::thread t1(slow_increment, 0);
    std::thread t2(slow_increment, 1);
    t1.join();
    t2.join();
	
	return 0;
}