I had thought every fork was a thread.... And a call to fork() started a child process and returned the PID of that process.

fork starts a process. Not a thread.

A process consists of at least one thread and can contain multiple threads.

Each process has its own address space. The threads of a single process share that same address space.

I had heard fork() leakes like a sieve even in 12.

Where did you hear that?

If it is true, and I'm not aware that it is, it will (almost certainly) be platform dependant. Which OS are you using?

However I am not proficient enough to follow your code completely.... I am assuming @threads now holds the 10 results for each child process. Correct?

No. @threads contains handles to the 10 threads (objects). The values returned by the threads--if any--are not accessible to you until you call the method join() on those threads handles (objects). The values returned from the threads are stored internally until you fetch them using the join() method.

BTW: Embedding part of your question within the <code></code> tags is not helpful. They can easily be missed. Please use code tags for code; not for (parts of) the question.