If you can accept cooperative scheduling that runs tasks only on a single core, then yes, lightweight solutions exist. This is more like "event-driven coroutine

If you can accept cooperative scheduling that runs tasks only on a single core, then yes, lightweight solutions exist. This is more like "event-driven coroutines" than "green threads" though. If you want "full" green threads with preemptive scheduling and parallel execution across multiple cores, then an active runtime is needed.