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_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
Returned errors may indicate the following problems:
-EINVALsource is not a valid pointer to an sd_event_source object.
The event loop is already terminated.
Not enough memory.
The event loop has been created in a different process.
The specified event source has been created with
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systemd 233 SD_EVENT_SOURCE_SET_PREPARE(3)