Advanced event loops ==================== libuv provides considerable user control over event loops, and you can achieve interesting results by juggling multiple loops. You can also embed libuv's event loop into another event loop based library -- imagine a Qt based UI, and Qt's event loop driving a libuv backend which does intensive system level tasks. Stopping an event loop ~~~~~~~~~~~~~~~~~~~~~~ ``uv_stop()`` can be used to stop an event loop. The earliest the loop will stop running is *on the next iteration*, possibly later. This means that events that are ready to be processed in this iteration of the loop will still be processed, so ``uv_stop()`` can't be used as a kill switch. When ``uv_stop()`` is called, the loop **won't** block for i/o on this iteration. The semantics of these things can be a bit difficult to understand, so let's look at ``uv_run()`` where all the control flow occurs. .. rubric:: src/unix/core.c - uv_run .. literalinclude:: ../../../src/unix/core.c :language: c :linenos: :lines: 304-324 :emphasize-lines: 10,19,21 ``stop_flag`` is set by ``uv_stop()``. Now all libuv callbacks are invoked within the event loop, which is why invoking ``uv_stop()`` in them will still lead to this iteration of the loop occurring. First libuv updates timers, then runs pending timer, idle and prepare callbacks, and invokes any pending I/O callbacks. If you were to call ``uv_stop()`` in any of them, ``stop_flag`` would be set. This causes ``uv_backend_timeout()`` to return ``0``, which is why the loop does not block on I/O. If on the other hand, you called ``uv_stop()`` in one of the check handlers, I/O has already finished and is not affected. ``uv_stop()`` is useful to shutdown a loop when a result has been computed or there is an error, without having to ensure that all handlers are stopped one by one. Here is a simple example that stops the loop and demonstrates how the current iteration of the loop still takes places. .. rubric:: uvstop/main.c .. literalinclude:: ../../code/uvstop/main.c :language: c :linenos: :emphasize-lines: 11