Child watchers trigger when your process receives a SIGCHLD in response to some child status changes (most typically when a child of yours dies or exits). It is permissible to install a Child watcher after the child has been forked (which implies it might have already exited), as long as the event loop isn’t entered (or is continued from a watcher), i.e. forking and then immediately registering a watcher for the child is fine, but forking and registering a watcher a few event loop iterations later or in the next callback invocation is not.
You can only register Child watchers in the default loop.
Due to some design glitches inside libev, child watchers will always be handled at maximum priority (their priority is set to EV_MAXPRI by libev).
Configures the watcher.
The process id this watcher watches out for, or 0, meaning any process id.
The process id that detected a status change.