ev_prepare/ev_check watchers is
usually a better approach for this kind of thing.
Here are the gory details of what ev_loop does:
* If there are no active watchers (reference count is zero), return.
- - Queue prepare watchers and then call all outstanding watchers.
+ - Before the first iteration, call any pending watchers.
+ * If there are no active watchers (reference count is zero), return.
+ - Queue all prepare watchers and then call all outstanding watchers.
- If we have been forked, recreate the kernel state.
- Update the kernel state with all outstanding changes.
- Update the "event loop time".
@@ -1485,6 +1486,15 @@
of lower priority, but only once, using idle watchers to keep the event
loop from blocking if lower-priority coroutines are active, thus mapping
low-priority coroutines to idle/background tasks).
+It is recommended to give ev_check watchers highest (EV_MAXPRI)
+priority, to ensure that they are being run before any other watchers
+after the poll. Also, ev_check watchers (and ev_prepare watchers,
+too) should not activate ("feed") events into libev. While libev fully
+supports this, they will be called before other ev_check watchers did
+their job. As ev_check watchers are often used to embed other event
+loops those other event loops might be in an unusable state until their
+ev_check watcher ran (always remind yourself to coexist peacefully with
+others).