уведомлять сервисного менеджера о завершении запуска и других изменениях статуса сервиса (notify service manager about start-up completion and other service status changes)
Возвращаемое значение (Return value)
On failure, these calls return a negative errno-style error code.
If $NOTIFY_SOCKET was not set and hence no status message could
be sent, 0 is returned. If the status was sent, these functions
return a positive value. In order to support both service
managers that implement this scheme and those which do not, it is
generally recommended to ignore the return value of this call.
Note that the return value simply indicates whether the
notification message was enqueued properly, it does not reflect
whether the message could be processed successfully.
Specifically, no error is returned when a file descriptor is
attempted to be stored using FDSTORE=1 but the service is not
actually configured to permit storing of file descriptors (see
above).
