Puzzle – Easy : C++ Multithreading

Below is a small and easy puzzle on multi-threading, ideal for beginners. And to be honest, title should have been “C Multithreading” since it uses pthreads, and not C++11 for threading.

#include <pthread.h>
#include <unistd.h>
#include <iostream>

using namespace std;

int i;

void* f1(void*)
{
    for (i = 0; i < 10; i+=2){
        cout << i << endl;
        sleep(1);
    }
    return 0;
}

void* f2(void*)
{
    for (i = 1; i < 10; i+=2){
        cout << i << endl;
        sleep(1);
    }
    return 0;
}

int main()
{
    pthread_t t1, t2;
    pthread_create(&t1, 0, f1, 0);
    pthread_create(&t2, 0, f2, 0);
    pthread_join(t1, 0);
    pthread_join(t2, 0);
}

So what would be the output? In what order numbers will be printed? That’s the small puzzle for you.

To make you really think, I won’t give answer. You are free to share your answer in comments though. And hey running the program without thinking would be considered cheating!

Don’t you cheat!