Only a single handler may be installed for a specific child process. The handler is enabled for a single event (SD_EVENT_ONESHOT), but this may be changed with sd_event_source_set_enabled(3). If the handler function returns a negative error code, it will be disabled after the invocation, even if the SD_EVENT_ON mode was requested before.

To destroy an event source object use sd_event_source_unref(3), but note that the event source is only removed from the event loop when all references to the event source are dropped. To make sure an event source does not fire anymore, even when there's still a reference to it kept, consider setting the event source to SD_EVENT_OFF with sd_event_source_set_enabled(3).

If the second parameter of sd_event_add_child() is passed as NULL no reference to the event source object is returned. In this case the event source is considered "floating", and will be destroyed implicitly when the event loop itself is destroyed.

Note that the handler function is invoked at a time where the child process is not reaped yet (and thus still is exposed as a zombie process by the kernel). However, the child will be reaped automatically after the function returns. Child processes for which no child process state change event sources are installed will not be reaped by the event loop implementation.

If both a child process state change event source and a SIGCHLD signal event source is installed in the same event loop, the configured event source priorities decide which event source is dispatched first. If the signal handler is processed first, it should leave the child processes for which child process state change event sources are installed unreaped.

sd_event_source_get_child_pid() retrieves the configured PID of a child process state change event source created previously with sd_event_add_child(). It takes the event source object as the source parameter and a pointer to a pid_t variable to return the process ID in.