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

 1
 2  switch (handle->type) {
 3    case UV_PREPARE:
 4    case UV_CHECK:
 5    case UV_IDLE:
 6    case UV_ASYNC:
 7    case UV_TIMER:
 8    case UV_PROCESS:
 9    case UV_FS_EVENT:
10    case UV_FS_POLL:
11    case UV_POLL:
12      break;
13
14    case UV_SIGNAL:
15      /* If there are any caught signals "trapped" in the signal pipe,
16       * we can't call the close callback yet. Reinserting the handle
17       * into the closing queue makes the event loop spin but that's
18       * okay because we only need to deliver the pending events.
19       */
20      sh = (uv_signal_t*) handle;
21      if (sh->caught_signals > sh->dispatched_signals) {

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

 1#include <stdio.h>
 2#include <uv.h>
 3
 4int64_t counter = 0;
 5
 6void idle_cb(uv_idle_t *handle) {
 7    printf("Idle callback\n");
 8    counter++;
 9
10    if (counter >= 5) {
11        uv_stop(uv_default_loop());
12        printf("uv_stop() called\n");
13    }
14}
15
16void prep_cb(uv_prepare_t *handle) {
17    printf("Prep callback\n");
18}
19
20int main() {
21    uv_idle_t idler;
22    uv_prepare_t prep;
23
24    uv_idle_init(uv_default_loop(), &idler);
25    uv_idle_start(&idler, idle_cb);
26
27    uv_prepare_init(uv_default_loop(), &prep);
28    uv_prepare_start(&prep, prep_cb);
29
30    uv_run(uv_default_loop(), UV_RUN_DEFAULT);
31
32    return 0;
33}