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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd_event_source_set_priority" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_event_source_set_priority</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>sd_event_source_set_priority</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_event_source_set_priority</refname>
<refname>sd_event_source_get_priority</refname>
<refname>SD_EVENT_PRIORITY_IMPORTANT</refname>
<refname>SD_EVENT_PRIORITY_NORMAL</refname>
<refname>SD_EVENT_PRIORITY_IDLE</refname>
<refpurpose>Set or retrieve the priority of event sources</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
<funcsynopsisinfo><token>enum</token> {
<constant>SD_EVENT_PRIORITY_IMPORTANT</constant> = -100,
<constant>SD_EVENT_PRIORITY_NORMAL</constant> = 0,
<constant>SD_EVENT_PRIORITY_IDLE</constant> = 100,
};</funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_event_source_set_priority</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>int64_t <parameter>priority</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_priority</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>int64_t *<parameter>priority</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_event_source_set_priority()</function> may be
used to set the priority for the event source object specified as
<parameter>source</parameter>. The priority is specified as an
arbitrary signed 64-bit integer. The priority is initialized to
<constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) when the event
source is allocated with a call such as
<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>
or
<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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 <constant>SD_EVENT_PRIORITY_IMPORTANT</constant>
(-100), <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) and
<constant>SD_EVENT_PRIORITY_IDLE</constant> (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 64-bit
signed integer range is available for ordering event
sources.</para>
<para>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).</para>
<para>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.</para>
<para>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
<citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>) whose
priority may only be changed in the time between their initial creation and the first subsequent event loop
iteration.</para>
<para><function>sd_event_source_get_priority()</function> may be
used to query the current priority assigned to the event source
object <parameter>source</parameter>.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On success, <function>sd_event_source_set_priority()</function> and
<function>sd_event_source_get_priority()</function> return a non-negative integer. On failure, they
return a negative errno-style error code.</para>
<refsect2>
<title>Errors</title>
<para>Returned errors may indicate the following problems:</para>
<variablelist>
<varlistentry>
<term><constant>-EINVAL</constant></term>
<listitem><para><parameter>source</parameter> is not a valid pointer to an
<structname>sd_event_source</structname> object.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ENOMEM</constant></term>
<listitem><para>Not enough memory.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ESTALE</constant></term>
<listitem><para>The event loop is already terminated.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ECHILD</constant></term>
<listitem><para>The event loop has been created in a different process, library or module instance.</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
</refsect1>
<xi:include href="libsystemd-pkgconfig.xml" />
<refsect1>
<title>History</title>
<para><function>sd_event_source_set_priority()</function> and
<function>sd_event_source_get_priority()</function> were added in version 229.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
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