Threads in C++ The interleaving pattern - c++

Question

In this program, I'm trying to print my username and then create two threads. I want each thread to print its thread id and go into a loop and display something periodically.
Here is the code I have
#include <iostream>
#include <thread>
#include <chrono>
#include <mutex>
#include <string>
void task(std::string threadNum)
{
std::thread::id this_id = std::this_thread::get_id();
std::cout << threadNum << " : " << this_id << std::endl;
for(int i=0; i<1000; i++){
if(i == 300 or i == 600 or i == 900){
std::cout << threadNum << " has reached step: " << i << std::endl;
}
}
}
int main()
{
std::cout << "Your Username Is: " << getenv("USER") << std::endl;
std::thread t1(task, "Thread 1");
std::thread t2(task, "Thread 2");
t1.join();
t2.join();
}
and I get different outputs every single time I run the program, for example
Your Username Is: gansaikhanshur
Thread 2 : Thread 1 : 0x70000741e000
Thread 2 has reached step: 0x70000739b000300
Thread 1 has reached step: 300
Thread 2 has reached step: 600
Thread 2 has reached step: 900
Thread 1 has reached step: 600
Thread 1 has reached step: 900
Thread1 and Thread 2 does not show it's thread ID as it should. Why do I get different results all the time? and Is it possible for me to make thread 1 and thread 2 to display their correct thread ids?

Answer 1

This is the way threads work -- they run independently and any side-effects they have may show up interleaved in any order. If you want to ensure that doesn't happen, you need to synchronize. For example, if you want to ensure that lines written to cout don't get mixed up, you can lock around each line output:
std::mutex cout_lock;
void task(std::string threadNum)
{
std::thread::id this_id = std::this_thread::get_id();
{
std::lock_guard<std::mutex> lock(cout_lock);
std::cout << threadNum << " : " << this_id << std::endl;
}
for(int i=0; i<1000; i++){
if(i == 300 or i == 600 or i == 900) {
std::lock_guard<std::mutex> lock(cout_lock);
std::cout << threadNum << " has reached step: " << i << std::endl;
}
}
}
lock_guard gives you a nice easy exception-safe way to manage lock/unlock operations.