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173 lines
7.3 KiB
XML
173 lines
7.3 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_event_source_set_priority" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_source_set_priority</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_source_set_priority</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_event_source_set_priority</refname>
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<refname>sd_event_source_get_priority</refname>
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<refname>SD_EVENT_PRIORITY_IMPORTANT</refname>
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<refname>SD_EVENT_PRIORITY_NORMAL</refname>
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<refname>SD_EVENT_PRIORITY_IDLE</refname>
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<refpurpose>Set or retrieve the priority of event sources</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
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<funcsynopsisinfo><token>enum</token> {
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<constant>SD_EVENT_PRIORITY_IMPORTANT</constant> = -100,
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<constant>SD_EVENT_PRIORITY_NORMAL</constant> = 0,
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<constant>SD_EVENT_PRIORITY_IDLE</constant> = 100,
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};</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_priority</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int64_t <parameter>priority</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_priority</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int64_t *<parameter>priority</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_event_source_set_priority()</function> may be
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used to set the priority for the event source object specified as
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<parameter>source</parameter>. The priority is specified as an
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arbitrary signed 64-bit integer. The priority is initialized to
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<constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) when the event
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source is allocated with a call such as
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<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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or
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<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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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
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event sources' priorities. Event sources with smaller priority
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values are dispatched first. As well-known points of reference,
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the constants <constant>SD_EVENT_PRIORITY_IMPORTANT</constant>
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(-100), <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) and
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<constant>SD_EVENT_PRIORITY_IDLE</constant> (100) may be used to
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indicate event sources that shall be dispatched early, normally or
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late. It is recommended to specify priorities based on these
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definitions, and relative to them — however, the full 64-bit
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signed integer range is available for ordering event
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sources.</para>
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<para>Priorities define the order in which event sources that have
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seen events are dispatched. Care should be taken to ensure that
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high-priority event sources (those with negative priority values
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assigned) do not cause starvation of low-priority event sources
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(those with positive priority values assigned).</para>
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<para>The order in which event sources with the same priority are
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dispatched is undefined, but the event loop generally tries to
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dispatch them in the order it learnt about events on them. As the
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backing kernel primitives do not provide accurate information
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about the order in which events occurred this is not necessarily
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reliable. However, it is guaranteed that if events are seen on
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multiple same-priority event sources at the same time, each one is
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not dispatched again until all others have been dispatched
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once. This behavior guarantees that within each priority
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particular event sources do not starve or dominate the event
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loop.</para>
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<para>The priority of event sources may be changed at any time of their lifetime, with the exception of inotify
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event sources (i.e. those created with
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<citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>) whose
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priority may only be changed in the time between their initial creation and the first subsequent event loop
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iteration.</para>
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<para><function>sd_event_source_get_priority()</function> may be
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used to query the current priority assigned to the event source
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object <parameter>source</parameter>.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>sd_event_source_set_priority()</function> and
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<function>sd_event_source_get_priority()</function> return a non-negative integer. On failure, they
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return a negative errno-style error code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para><parameter>source</parameter> is not a valid pointer to an
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<structname>sd_event_source</structname> object.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ENOMEM</constant></term>
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<listitem><para>Not enough memory.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ESTALE</constant></term>
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<listitem><para>The event loop is already terminated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The event loop has been created in a different process, library or module instance.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>History</title>
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<para><function>sd_event_source_set_priority()</function> and
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<function>sd_event_source_get_priority()</function> were added in version 229.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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