sd_event_source_set_priority, sd_event_source_get_priority, SD_EVENT_PRIORITY_IMPORTANT, SD_EVENT_PRIORITY_NORMAL, SD_EVENT_PRIORITY_IDLE — Set or retrieve the priority of event sources
SD_EVENT_PRIORITY_IDLE= 100, };
sd_event_source_set_priority() may be
used to set the priority for the event source object specified as
source. The priority is specified as an
arbitrary signed 64bit integer. The priority is initialized to
SD_EVENT_PRIORITY_NORMAL (0) when the event
source is allocated with a call such as
and may be changed with this call. If multiple event sources have seen events at the same time, they are dispatched in the order indicated by the
event sources' priorities. Event sources with smaller priority
values are dispatched first. As well-known points of reference,
SD_EVENT_PRIORITY_NORMAL (0) and
SD_EVENT_PRIORITY_IDLE (100) may be used to
indicate event sources that shall be dispatched early, normally or
late. It is recommended to specify priorities based on these
definitions, and relative to them — however, the full 64bit
signed integer range is available for ordering event
Priorities define the order in which event sources that have seen events are dispatched. Care should be taken to ensure that high-priority event sources (those with negative priority values assigned) do not cause starvation of low-priority event sources (those with positive priority values assigned).
The order in which event sources with the same priority are dispatched is undefined, but the event loop generally tries to dispatch them in the order it learnt about events on them. As the backing kernel primitives do not provide accurate information about the order in which events occurred this is not necessarily reliable. However, it is guaranteed that if events are seen on multiple same-priority event sources at the same time, each one is not dispatched again until all others have been dispatched once. This behavior guarantees that within each priority particular event sources do not starve or dominate the event loop.
The priority of event sources may be changed at any time of their lifetime, with the exception of inotify event sources (i.e. those created with sd_event_add_inotify(3)) whose priority may only be changed in the time between their initial creation and the first subsequent event loop iteration.
sd_event_source_get_priority() may be
used to query the current priority assigned to the event source
sd_event_source_get_priority() return a non-negative integer. On failure, they
return a negative errno-style error code.
These APIs are implemented as a shared
library, which can be compiled and linked to with the