systemd sd_event_source_set_prepare 3 sd_event_source_set_prepare Set a preparation callback for event sources #include <systemd/sd-event.h> int sd_event_source_set_prepare sd_event_source *source sd_event_handler_t callback typedef int (*sd_event_handler_t) sd_event_source *s void *userdata sd_event_source_set_prepare() may be used to set a preparation callback for the event source object specified as source. The callback function specified as callback will be invoked immediately before the event loop goes to sleep to wait for incoming events. It is invoked with the user data pointer passed when the event source was created. The event source will be disabled if the callback function returns a negative error code. The callback function may be used to reconfigure the precise events to wait for. If the callback parameter is passed as NULL the callback function is reset. Event source objects have no preparation callback associated when they are first created with calls such as sd_event_add_io3, sd_event_add_time3. Preparation callback functions are supported for all event source types with the exception of those created with sd_event_add_exit3. Preparation callback functions are dispatched in the order indicated by the event source's priority field, as set with sd_event_source_set_priority3. Preparation callbacks of disabled event sources (see sd_event_source_set_enabled3) are not invoked. On success, sd_event_source_set_prepare() returns a non-negative integer. On failure, it returns a negative errno-style error code. Returned errors may indicate the following problems: -EINVAL source is not a valid pointer to an sd_event_source object. -ESTALE The event loop is already terminated. -ENOMEM Not enough memory. -ECHILD The event loop has been created in a different process, library or module instance. -EDOM The specified event source has been created with sd_event_add_exit3. sd-event3, sd_event_add_io3, sd_event_add_time3, sd_event_add_signal3, sd_event_add_child3, sd_event_add_inotify3, sd_event_add_defer3, sd_event_source_set_enabled3, sd_event_source_set_priority3, sd_event_source_set_userdata3