sd_event_source_get_pending man page
sd_event_source_get_pending — Determine pending state of event sources
int sd_event_source_get_pending(sd_event_source *source);
sd_event_source_get_pending() may be used to determine whether the event source object specified as source has seen events but has not been dispatched yet (and thus is marked "pending").
Event source objects initially are not marked pending, when they are created with calls such as sd_event_add_io(3), sd_event_add_time(3), with the exception of those created with sd_event_add_defer(3) which are immediately marked pending, and sd_event_add_exit(3) for which the "pending" concept is not defined. For details see the respective manual pages.
In each event loop iteration one event source of those marked pending is dispatched, in the order defined by the event source priority, as set with sd_event_source_set_priority(3).
For I/O event sources, as created with sd_event_add_io(3), the call sd_event_source_get_io_revents(3) may be used to query the type of event pending in more detail.
On success, sd_event_source_get_pending() returns an integer greater than zero when the event source is marked pending, and zero when the event source is not marked pending. On failure, it returns a negative errno-style error code.
Returned errors may indicate the following problems:
source is not a valid pointer to an sd_event_source object.
source refers to an event source object created with sd_event_add_exit(3).
Not enough memory.
The event loop is already terminated.
The event loop has been created in a different process.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
sd-event(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_add_child(3), sd_event_add_signal(3), sd_event_add_defer(3), sd_event_source_unref(3)
sd-event(3), sd_event_add_io(3), systemd.directives(7), systemd.index(7).