The first watchdog notification message is sent immediately when sd_event_set_watchdog() is invoked with a true b parameter.
The watchdog logic is designed to allow the service manager to automatically detect services that ceased processing of incoming events, and thus appear "hung". Watchdog notifications are sent out only at the beginning of each event loop iteration. If an event source dispatch function blocks for an excessively long time and does not return execution to the event loop quickly, this might hence cause the notification message to be delayed, and possibly result in abnormal program termination, as configured in the service unit file.
On success, sd_event_set_watchdog() and sd_event_get_watchdog() return a non-zero positive integer if the service manager requested watchdog support and watchdog support was successfully enabled. They return zero if the service manager did not request watchdog support, or if watchdog support was explicitly disabled with a false b parameter. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
systemd(1), sd-event(3), sd_event_new(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_child(3), sd_event_add_inotify(3), sd_event_add_defer(3), sd_watchdog_enabled(3), sd_notify(3), systemd.service(5)