man/sd_event_source_set_priority.xml

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   6   SPDX-License-Identifier: LGPL-2.1+
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   8
   9 <refentry id="sd_event_source_set_priority" xmlns:xi="http://www.w3.org/2001/XInclude">
  10
  11   <refentryinfo>
  12     <title>sd_event_source_set_priority</title>
  13     <productname>systemd</productname>
  14
  15     <authorgroup>
  16       <author>
  17         <contrib>Developer</contrib>
  18         <firstname>Lennart</firstname>
  19         <surname>Poettering</surname>
  20         <email>lennart@poettering.net</email>
  21       </author>
  22     </authorgroup>
  23   </refentryinfo>
  24
  25   <refmeta>
  26     <refentrytitle>sd_event_source_set_priority</refentrytitle>
  27     <manvolnum>3</manvolnum>
  28   </refmeta>
  29
  30   <refnamediv>
  31     <refname>sd_event_source_set_priority</refname>
  32     <refname>sd_event_source_get_priority</refname>
  33     <refname>SD_EVENT_PRIORITY_IMPORTANT</refname>
  34     <refname>SD_EVENT_PRIORITY_NORMAL</refname>
  35     <refname>SD_EVENT_PRIORITY_IDLE</refname>
  36
  37     <refpurpose>Set or retrieve the priority of event sources</refpurpose>
  38   </refnamediv>
  39
  40   <refsynopsisdiv>
  41     <funcsynopsis>
  42       <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
  43
  44       <funcsynopsisinfo><token>enum</token> {
  45         <constant>SD_EVENT_PRIORITY_IMPORTANT</constant> = -100,
  46         <constant>SD_EVENT_PRIORITY_NORMAL</constant> = 0,
  47         <constant>SD_EVENT_PRIORITY_IDLE</constant> = 100,
  48 };</funcsynopsisinfo>
  49
  50       <funcprototype>
  51         <funcdef>int <function>sd_event_source_set_priority</function></funcdef>
  52         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  53         <paramdef>int64_t <parameter>priority</parameter></paramdef>
  54       </funcprototype>
  55
  56       <funcprototype>
  57         <funcdef>int <function>sd_event_source_get_priority</function></funcdef>
  58         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  59         <paramdef>int64_t *<parameter>priority</parameter></paramdef>
  60       </funcprototype>
  61
  62     </funcsynopsis>
  63   </refsynopsisdiv>
  64
  65   <refsect1>
  66     <title>Description</title>
  67
  68     <para><function>sd_event_source_set_priority()</function> may be
  69     used to set the priority for the event source object specified as
  70     <parameter>source</parameter>. The priority is specified as an
  71     arbitrary signed 64bit integer. The priority is initialized to
  72     <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) when the event
  73     source is allocated with a call such as
  74     <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  75     or
  76     <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  77     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
  78     event sources' priorities. Event sources with smaller priority
  79     values are dispatched first. As well-known points of reference,
  80     the constants <constant>SD_EVENT_PRIORITY_IMPORTANT</constant>
  81     (-100), <constant>SD_EVENT_PRIORITY_NORMAL</constant> (0) and
  82     <constant>SD_EVENT_PRIORITY_IDLE</constant> (100) may be used to
  83     indicate event sources that shall be dispatched early, normally or
  84     late. It is recommended to specify priorities based on these
  85     definitions, and relative to them — however, the full 64bit
  86     signed integer range is available for ordering event
  87     sources.</para>
  88
  89     <para>Priorities define the order in which event sources that have
  90     seen events are dispatched. Care should be taken to ensure that
  91     high-priority event sources (those with negative priority values
  92     assigned) do not cause starvation of low-priority event sources
  93     (those with positive priority values assigned).</para>
  94
  95     <para>The order in which event sources with the same priority are
  96     dispatched is undefined, but the event loop generally tries to
  97     dispatch them in the order it learnt about events on them. As the
  98     backing kernel primitives do not provide accurate information
  99     about the order in which events occurred this is not necessarily
 100     reliable. However, it is guaranteed that if events are seen on
 101     multiple same-priority event sources at the same time, each one is
 102     not dispatched again until all others have been dispatched
 103     once. This behavior guarantees that within each priority
 104     particular event sources do not starve or dominate the event
 105     loop.</para>
 106
 107     <para>The priority of event sources may be changed at any time of their lifetime, with the exception of inotify
 108     event sources (i.e. those created with
 109     <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>) whose
 110     priority may only be changed in the time between their initial creation and the first subsequent event loop
 111     iteration.</para>
 112
 113     <para><function>sd_event_source_get_priority()</function> may be
 114     used to query the current priority assigned to the event source
 115     object <parameter>source</parameter>.</para>
 116   </refsect1>
 117
 118   <refsect1>
 119     <title>Return Value</title>
 120
 121     <para>On success,
 122     <function>sd_event_source_set_priority()</function> and
 123     <function>sd_event_source_get_priority()</function> return a
 124     non-negative integer. On failure, they return a negative
 125     errno-style error code.</para>
 126   </refsect1>
 127
 128   <refsect1>
 129     <title>Errors</title>
 130
 131     <para>Returned errors may indicate the following problems:</para>
 132
 133     <variablelist>
 134       <varlistentry>
 135         <term><constant>-EINVAL</constant></term>
 136
 137         <listitem><para><parameter>source</parameter> is not a valid
 138         pointer to an <structname>sd_event_source</structname>
 139         object.</para></listitem>
 140       </varlistentry>
 141
 142       <varlistentry>
 143         <term><constant>-ENOMEM</constant></term>
 144
 145         <listitem><para>Not enough memory.</para></listitem>
 146       </varlistentry>
 147
 148       <varlistentry>
 149         <term><constant>-ESTALE</constant></term>
 150
 151         <listitem><para>The event loop is already terminated.</para></listitem>
 152
 153       </varlistentry>
 154
 155       <varlistentry>
 156         <term><constant>-ECHILD</constant></term>
 157
 158         <listitem><para>The event loop has been created in a different process.</para></listitem>
 159
 160       </varlistentry>
 161
 162     </variablelist>
 163   </refsect1>
 164
 165   <xi:include href="libsystemd-pkgconfig.xml" />
 166
 167   <refsect1>
 168     <title>See Also</title>
 169
 170     <para>
 171       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 172       <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 173       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 174       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 175       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 176       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 177     </para>
 178   </refsect1>
 179
 180 </refentry>