By adding fflush( stdout ), you will get what you expect.  (I ran on 6.4.1)   For example,

     while(1)
     {
       printf("Printing from thread 2\n");
       fflush( stdout );
     }

Even though now you see output from both threads, your program is a very un-realistic example. That is because the 
threads require very little cpu themselves.
w
If you run the system profiler on this, you will see that your thread routines occupy the cpu maybe one-fourth of cpu 
time.  That is because when they call printf(), the thread reliinquishes control (mostly) to devc-pty... which is about 
all they do!

The attached program may be a better example of round-robin thread scheduling.  It clearly shows the "fairness" of round
-robin scheduling.  (Note the addition of pthread_barrier_wait(&barrier) which was missing in your thread routines.)

Rajorshi Dam wrote:
> Hello Attilla,
>    So this bug exists in all versions of QNX till 6.4 and there are no bug fixes for this issue. Am I right?

I only checked 6.4.1 and 6.5.0. It's possible the behavoir was inserted in 6.4.1.

However I note that putting a small busy-loop before your printf() statements causes the problem to go away.
So in general round-robin is not broken in 6.4.1. But there is something peculiar triggered when calling printf() in a 
very tight loop in 6.4.1.

-ad