sd_event_source_set_prepare — Set a preparation callback for event sources
sd_event_source_set_prepare() may be
used to set a preparation callback for the event source object
source. The callback function
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.
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_io(3), sd_event_add_time(3). Preparation callback functions are supported for all event source types with the exception of those created with sd_event_add_exit(3). Preparation callback functions are dispatched in the order indicated by the event source's priority field, as set with sd_event_source_set_priority(3). Preparation callbacks of disabled event sources (see sd_event_source_set_enabled(3)) are not invoked.
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:
These APIs are implemented as a shared
library, which can be compiled and linked to with the