It's leaky in one extremely minor way that affects almost no programs whatsoever. It's extremely rare for anyone to do fork-without-exec, and anyone doing that

It's leaky in one extremely minor way that affects almost no programs whatsoever. It's extremely rare for anyone to do fork-without-exec, and anyone doing that already has a large number of restrictions they must be aware of. And no, the user can't threadify that themselves. Your sample code is putting the library call on a thread. That's not at all what I was talking about. I was talking about a library using multiple threads (or, more likely, some abstraction over a thread pool that handles scheduling chunks of work on threads for you, such as libdispatch on Apple platforms) in order to process a bunch of data in parallel, data that only the library knows about. The user makes a single call into the library, the library processes a bunch of data in parallel, and then returns to the user. From the user's perspective it's a synchronous call, but it runs in a fraction of the time it would if the library wasn't allowed to use threading internally.