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.

src/unix/core.c - uv_run

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  }
}


int uv_is_closing(const uv_handle_t* handle) {
  return uv__is_closing(handle);
}


int uv_backend_fd(const uv_loop_t* loop) {
  return loop->backend_fd;
}


int uv_backend_timeout(const uv_loop_t* loop) {
  if (loop->stop_flag != 0)
    return 0;

  if (!uv__has_active_handles(loop) && !uv__has_active_reqs(loop))
    return 0;

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.

uvstop/main.c

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#include <stdio.h>
#include <uv.h>

int64_t counter = 0;

void idle_cb(uv_idle_t *handle) {
    printf("Idle callback\n");
    counter++;

    if (counter >= 5) {
        uv_stop(uv_default_loop());
        printf("uv_stop() called\n");
    }
}

void prep_cb(uv_prepare_t *handle) {
    printf("Prep callback\n");
}

int main() {
    uv_idle_t idler;
    uv_prepare_t prep;

    uv_idle_init(uv_default_loop(), &idler);
    uv_idle_start(&idler, idle_cb);

    uv_prepare_init(uv_default_loop(), &prep);
    uv_prepare_start(&prep, prep_cb);

    uv_run(uv_default_loop(), UV_RUN_DEFAULT);

    return 0;
}