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)