NAME

       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

SYNOPSIS

       #include <systemd/sd-event.h>

       enum {
               SD_EVENT_PRIORITY_IMPORTANT = -100,
               SD_EVENT_PRIORITY_NORMAL = 0,
               SD_EVENT_PRIORITY_IDLE = 100,
       };

       int sd_event_source_set_priority(sd_event_source *source, int64_t priority);

       int sd_event_source_get_priority(sd_event_source *source, int64_t *priority);

DESCRIPTION

       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 sd_event_add_io(3) or sd_event_add_time(3), 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, the
       constants SD_EVENT_PRIORITY_IMPORTANT (-100), 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 sources.

       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.

       sd_event_source_get_priority() may be used to query the current priority assigned to the
       event source object source.

RETURN VALUE

       On success, sd_event_source_set_priority() and sd_event_source_get_priority() return a
       non-negative integer. On failure, they return a negative errno-style error code.

ERRORS

       Returned errors may indicate the following problems:

       -EINVAL
           source is not a valid pointer to an sd_event_source object.

       -ENOMEM
           Not enough memory.

       -ESTALE
           The event loop is already terminated.

       -ECHILD
           The event loop has been created in a different process.

NOTES

       These APIs are implemented as a shared library, which can be compiled and linked to with
       the libsystemd pkg-config(1) file.

SEE ALSO

       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)