This #pragma omp parallel constructor will create a parallel region, with as many threads as you have set it up. Hence, when you do:
#pragma omp parallel
{
for(int i=0; i<num_threads; i++)
{
...
}
}
every thread in the parallel region will execute all the iterations of the loop. That is why you have 16 outputted lines (i.e., 4 threads x 4 loop iterations).
If you want to distribute the iterations of a loop among threads you should use the #pragma omp for instead. So in your code you can either do:
#pragma omp parallel
{
#pragma omp for
for(int i=0; i<num_threads; i++)
{
...
}
}
or
#pragma omp parallel for
for(int i=0; i<num_threads; i++)
{
...
}
Since, you only want to distribute the iterations of the loop among threads, you can use the latter (i.e., #pragma omp parallel for).
It looks as though you are using
#pragma omp critical
{
std::cout << "Thread id: " << omp_get_thread_num() << " loop id: " << i << std::endl;
}
for debugging purposes. Bear in mind, however, that even with the critical region, the order in which threads will output is non-deterministic. If you rather that threads would output deterministically, use #pragma omp ordered instead of critical. The ordered constructor will enforce that the chunk of code that it wraps around will be executed in the same order that would have been executed if the code was executed sequentially.