1<?xml version='1.0'?>
2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
5
6<refentry id="sd_event_source_set_priority" xmlns:xi="http://www.w3.org/2001/XInclude">
7
8  <refentryinfo>
9    <title>sd_event_source_set_priority</title>
10    <productname>systemd</productname>
11  </refentryinfo>
12
13  <refmeta>
14    <refentrytitle>sd_event_source_set_priority</refentrytitle>
15    <manvolnum>3</manvolnum>
16  </refmeta>
17
18  <refnamediv>
19    <refname>sd_event_source_set_priority</refname>
20    <refname>sd_event_source_get_priority</refname>
21    <refname>SD_EVENT_PRIORITY_IMPORTANT</refname>
22    <refname>SD_EVENT_PRIORITY_NORMAL</refname>
23    <refname>SD_EVENT_PRIORITY_IDLE</refname>
24
25    <refpurpose>Set or retrieve the priority of event sources</refpurpose>
26  </refnamediv>
27
28  <refsynopsisdiv>
29    <funcsynopsis>
30      <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
31
32      <funcsynopsisinfo><token>enum</token> {
33        <constant>SD_EVENT_PRIORITY_IMPORTANT</constant> = -100,
34        <constant>SD_EVENT_PRIORITY_NORMAL</constant> = 0,
35        <constant>SD_EVENT_PRIORITY_IDLE</constant> = 100,
36};</funcsynopsisinfo>
37
38      <funcprototype>
39        <funcdef>int <function>sd_event_source_set_priority</function></funcdef>
40        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
41        <paramdef>int64_t <parameter>priority</parameter></paramdef>
42      </funcprototype>
43
44      <funcprototype>
45        <funcdef>int <function>sd_event_source_get_priority</function></funcdef>
46        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
47        <paramdef>int64_t *<parameter>priority</parameter></paramdef>
48      </funcprototype>
49
50    </funcsynopsis>
51  </refsynopsisdiv>
52
53  <refsect1>
54    <title>Description</title>
55
56    <para><function>sd_event_source_set_priority()</function> may be
57    used to set the priority for the event source object specified as
58    <parameter>source</parameter>. The priority is specified as an
59    arbitrary signed 64bit integer. The priority is initialized to
60    <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) when the event
61    source is allocated with a call such as
62    <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>
63    or
64    <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
65    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
66    event sources' priorities. Event sources with smaller priority
67    values are dispatched first. As well-known points of reference,
68    the constants <constant>SD_EVENT_PRIORITY_IMPORTANT</constant>
69    (-100), <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) and
70    <constant>SD_EVENT_PRIORITY_IDLE</constant> (100) may be used to
71    indicate event sources that shall be dispatched early, normally or
72    late. It is recommended to specify priorities based on these
73    definitions, and relative to them — however, the full 64bit
74    signed integer range is available for ordering event
75    sources.</para>
76
77    <para>Priorities define the order in which event sources that have
78    seen events are dispatched. Care should be taken to ensure that
79    high-priority event sources (those with negative priority values
80    assigned) do not cause starvation of low-priority event sources
81    (those with positive priority values assigned).</para>
82
83    <para>The order in which event sources with the same priority are
84    dispatched is undefined, but the event loop generally tries to
85    dispatch them in the order it learnt about events on them. As the
86    backing kernel primitives do not provide accurate information
87    about the order in which events occurred this is not necessarily
88    reliable. However, it is guaranteed that if events are seen on
89    multiple same-priority event sources at the same time, each one is
90    not dispatched again until all others have been dispatched
91    once. This behavior guarantees that within each priority
92    particular event sources do not starve or dominate the event
93    loop.</para>
94
95    <para>The priority of event sources may be changed at any time of their lifetime, with the exception of inotify
96    event sources (i.e. those created with
97    <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>) whose
98    priority may only be changed in the time between their initial creation and the first subsequent event loop
99    iteration.</para>
100
101    <para><function>sd_event_source_get_priority()</function> may be
102    used to query the current priority assigned to the event source
103    object <parameter>source</parameter>.</para>
104  </refsect1>
105
106  <refsect1>
107    <title>Return Value</title>
108
109    <para>On success, <function>sd_event_source_set_priority()</function> and
110    <function>sd_event_source_get_priority()</function> return a non-negative integer. On failure, they
111    return a negative errno-style error code.</para>
112
113    <refsect2>
114      <title>Errors</title>
115
116      <para>Returned errors may indicate the following problems:</para>
117
118      <variablelist>
119        <varlistentry>
120          <term><constant>-EINVAL</constant></term>
121
122          <listitem><para><parameter>source</parameter> is not a valid pointer to an
123          <structname>sd_event_source</structname> object.</para></listitem>
124        </varlistentry>
125
126        <varlistentry>
127          <term><constant>-ENOMEM</constant></term>
128
129          <listitem><para>Not enough memory.</para></listitem>
130        </varlistentry>
131
132        <varlistentry>
133          <term><constant>-ESTALE</constant></term>
134
135          <listitem><para>The event loop is already terminated.</para></listitem>
136
137        </varlistentry>
138
139        <varlistentry>
140          <term><constant>-ECHILD</constant></term>
141
142          <listitem><para>The event loop has been created in a different process.</para></listitem>
143
144        </varlistentry>
145
146      </variablelist>
147    </refsect2>
148  </refsect1>
149
150  <xi:include href="libsystemd-pkgconfig.xml" />
151
152  <refsect1>
153    <title>See Also</title>
154
155    <para>
156      <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
157      <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
158      <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
159      <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
160      <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
161      <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
162      <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>
163    </para>
164  </refsect1>
165
166</refentry>
167