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

std::mutex mx;
int i;

void thrfunc();

int main(void)
{
   i=0;
   std::thread          thr1(thrfunc),thr2(thrfunc);

   thr1.join();
   thr2.join();

   return 0;
}

void thrfunc()
{
   mx.lock();
   i++;
   std::cout << std::this_thread::get_id() << " i: " << i << std::endl;
   mx.unlock();
}