man/sd_event_source_set_priority.xml

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   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2015 Lennart Poettering
  11 -->
  12
  13 <refentry id="sd_event_source_set_priority" xmlns:xi="http://www.w3.org/2001/XInclude">
  14
  15   <refentryinfo>
  16     <title>sd_event_source_set_priority</title>
  17     <productname>systemd</productname>
  18
  19     <authorgroup>
  20       <author>
  21         <contrib>Developer</contrib>
  22         <firstname>Lennart</firstname>
  23         <surname>Poettering</surname>
  24         <email>lennart@poettering.net</email>
  25       </author>
  26     </authorgroup>
  27   </refentryinfo>
  28
  29   <refmeta>
  30     <refentrytitle>sd_event_source_set_priority</refentrytitle>
  31     <manvolnum>3</manvolnum>
  32   </refmeta>
  33
  34   <refnamediv>
  35     <refname>sd_event_source_set_priority</refname>
  36     <refname>sd_event_source_get_priority</refname>
  37     <refname>SD_EVENT_PRIORITY_IMPORTANT</refname>
  38     <refname>SD_EVENT_PRIORITY_NORMAL</refname>
  39     <refname>SD_EVENT_PRIORITY_IDLE</refname>
  40
  41     <refpurpose>Set or retrieve the priority of event sources</refpurpose>
  42   </refnamediv>
  43
  44   <refsynopsisdiv>
  45     <funcsynopsis>
  46       <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
  47
  48       <funcsynopsisinfo><token>enum</token> {
  49         <constant>SD_EVENT_PRIORITY_IMPORTANT</constant> = -100,
  50         <constant>SD_EVENT_PRIORITY_NORMAL</constant> = 0,
  51         <constant>SD_EVENT_PRIORITY_IDLE</constant> = 100,
  52 };</funcsynopsisinfo>
  53
  54       <funcprototype>
  55         <funcdef>int <function>sd_event_source_set_priority</function></funcdef>
  56         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  57         <paramdef>int64_t <parameter>priority</parameter></paramdef>
  58       </funcprototype>
  59
  60       <funcprototype>
  61         <funcdef>int <function>sd_event_source_get_priority</function></funcdef>
  62         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  63         <paramdef>int64_t *<parameter>priority</parameter></paramdef>
  64       </funcprototype>
  65
  66     </funcsynopsis>
  67   </refsynopsisdiv>
  68
  69   <refsect1>
  70     <title>Description</title>
  71
  72     <para><function>sd_event_source_set_priority()</function> may be
  73     used to set the priority for the event source object specified as
  74     <parameter>source</parameter>. The priority is specified as an
  75     arbitrary signed 64bit integer. The priority is initialized to
  76     <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) when the event
  77     source is allocated with a call such as
  78     <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  79     or
  80     <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  81     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
  82     event sources' priorities. Event sources with smaller priority
  83     values are dispatched first. As well-known points of reference,
  84     the constants <constant>SD_EVENT_PRIORITY_IMPORTANT</constant>
  85     (-100), <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) and
  86     <constant>SD_EVENT_PRIORITY_IDLE</constant> (100) may be used to
  87     indicate event sources that shall be dispatched early, normally or
  88     late. It is recommended to specify priorities based on these
  89     definitions, and relative to them — however, the full 64bit
  90     signed integer range is available for ordering event
  91     sources.</para>
  92
  93     <para>Priorities define the order in which event sources that have
  94     seen events are dispatched. Care should be taken to ensure that
  95     high-priority event sources (those with negative priority values
  96     assigned) do not cause starvation of low-priority event sources
  97     (those with positive priority values assigned).</para>
  98
  99     <para>The order in which event sources with the same priority are
 100     dispatched is undefined, but the event loop generally tries to
 101     dispatch them in the order it learnt about events on them. As the
 102     backing kernel primitives do not provide accurate information
 103     about the order in which events occurred this is not necessarily
 104     reliable. However, it is guaranteed that if events are seen on
 105     multiple same-priority event sources at the same time, each one is
 106     not dispatched again until all others have been dispatched
 107     once. This behavior guarantees that within each priority
 108     particular event sources do not starve or dominate the event
 109     loop.</para>
 110
 111     <para><function>sd_event_source_get_priority()</function> may be
 112     used to query the current priority assigned to the event source
 113     object <parameter>source</parameter>.</para>
 114   </refsect1>
 115
 116   <refsect1>
 117     <title>Return Value</title>
 118
 119     <para>On success,
 120     <function>sd_event_source_set_priority()</function> and
 121     <function>sd_event_source_get_priority()</function> return a
 122     non-negative integer. On failure, they return a negative
 123     errno-style error code.</para>
 124   </refsect1>
 125
 126   <refsect1>
 127     <title>Errors</title>
 128
 129     <para>Returned errors may indicate the following problems:</para>
 130
 131     <variablelist>
 132       <varlistentry>
 133         <term><constant>-EINVAL</constant></term>
 134
 135         <listitem><para><parameter>source</parameter> is not a valid
 136         pointer to an <structname>sd_event_source</structname>
 137         object.</para></listitem>
 138       </varlistentry>
 139
 140       <varlistentry>
 141         <term><constant>-ENOMEM</constant></term>
 142
 143         <listitem><para>Not enough memory.</para></listitem>
 144       </varlistentry>
 145
 146       <varlistentry>
 147         <term><constant>-ESTALE</constant></term>
 148
 149         <listitem><para>The event loop is already terminated.</para></listitem>
 150
 151       </varlistentry>
 152
 153       <varlistentry>
 154         <term><constant>-ECHILD</constant></term>
 155
 156         <listitem><para>The event loop has been created in a different process.</para></listitem>
 157
 158       </varlistentry>
 159
 160     </variablelist>
 161   </refsect1>
 162
 163   <xi:include href="libsystemd-pkgconfig.xml" />
 164
 165   <refsect1>
 166     <title>See Also</title>
 167
 168     <para>
 169       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 170       <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 171       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 172       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 173       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 174       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 175     </para>
 176   </refsect1>
 177
 178 </refentry>