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.
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  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>The priority of event sources may be changed at any time of their lifetime, with the exception of inotify
 112     event sources (i.e. those created with
 113     <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>) whose
 114     priority may only be changed in the time between their initial creation and the first subsequent event loop
 115     iteration.</para>
 116
 117     <para><function>sd_event_source_get_priority()</function> may be
 118     used to query the current priority assigned to the event source
 119     object <parameter>source</parameter>.</para>
 120   </refsect1>
 121
 122   <refsect1>
 123     <title>Return Value</title>
 124
 125     <para>On success,
 126     <function>sd_event_source_set_priority()</function> and
 127     <function>sd_event_source_get_priority()</function> return a
 128     non-negative integer. On failure, they return a negative
 129     errno-style error code.</para>
 130   </refsect1>
 131
 132   <refsect1>
 133     <title>Errors</title>
 134
 135     <para>Returned errors may indicate the following problems:</para>
 136
 137     <variablelist>
 138       <varlistentry>
 139         <term><constant>-EINVAL</constant></term>
 140
 141         <listitem><para><parameter>source</parameter> is not a valid
 142         pointer to an <structname>sd_event_source</structname>
 143         object.</para></listitem>
 144       </varlistentry>
 145
 146       <varlistentry>
 147         <term><constant>-ENOMEM</constant></term>
 148
 149         <listitem><para>Not enough memory.</para></listitem>
 150       </varlistentry>
 151
 152       <varlistentry>
 153         <term><constant>-ESTALE</constant></term>
 154
 155         <listitem><para>The event loop is already terminated.</para></listitem>
 156
 157       </varlistentry>
 158
 159       <varlistentry>
 160         <term><constant>-ECHILD</constant></term>
 161
 162         <listitem><para>The event loop has been created in a different process.</para></listitem>
 163
 164       </varlistentry>
 165
 166     </variablelist>
 167   </refsect1>
 168
 169   <xi:include href="libsystemd-pkgconfig.xml" />
 170
 171   <refsect1>
 172     <title>See Also</title>
 173
 174     <para>
 175       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 176       <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 177       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 178       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 179       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 180       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 181     </para>
 182   </refsect1>
 183
 184 </refentry>