man/sd_bus_track_new.xml

   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+ -->
   5
   6 <refentry id="sd_bus_track_new" xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sd_bus_track_new</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sd_bus_track_new</refentrytitle>
  15     <manvolnum>3</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sd_bus_track_new</refname>
  20     <refname>sd_bus_track_ref</refname>
  21     <refname>sd_bus_track_unref</refname>
  22     <refname>sd_bus_track_unrefp</refname>
  23     <refname>sd_bus_track_set_recursive</refname>
  24     <refname>sd_bus_track_get_recursive</refname>
  25     <refname>sd_bus_track_get_bus</refname>
  26     <refname>sd_bus_track_get_userdata</refname>
  27     <refname>sd_bus_track_set_userdata</refname>
  28
  29     <refpurpose>Track bus peers</refpurpose>
  30   </refnamediv>
  31
  32   <refsynopsisdiv>
  33     <funcsynopsis>
  34       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  35
  36       <funcprototype>
  37         <funcdef>int <function>sd_bus_track_new</function></funcdef>
  38         <paramdef>sd_bus* <parameter>bus</parameter></paramdef>
  39         <paramdef>sd_bus_track** <parameter>ret</parameter></paramdef>
  40         <paramdef>sd_bus_track_handler_t <parameter>handler</parameter></paramdef>
  41         <paramdef>void* <parameter>userdata</parameter></paramdef>
  42       </funcprototype>
  43
  44       <funcprototype>
  45         <funcdef>sd_bus_track *<function>sd_bus_track_ref</function></funcdef>
  46         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  47       </funcprototype>
  48
  49       <funcprototype>
  50         <funcdef>sd_bus_track *<function>sd_bus_track_unref</function></funcdef>
  51         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  52       </funcprototype>
  53
  54       <funcprototype>
  55         <funcdef>void <function>sd_bus_track_unrefp</function></funcdef>
  56         <paramdef>sd_bus_track **<parameter>t</parameter></paramdef>
  57       </funcprototype>
  58
  59       <funcprototype>
  60         <funcdef>int <function>sd_bus_track_get_recursive</function></funcdef>
  61         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  62       </funcprototype>
  63
  64       <funcprototype>
  65         <funcdef>int <function>sd_bus_track_set_recursive</function></funcdef>
  66         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  67         <paramdef>int <parameter>b</parameter></paramdef>
  68       </funcprototype>
  69
  70       <funcprototype>
  71         <funcdef>sd_bus* <function>sd_bus_track_get_bus</function></funcdef>
  72         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  73       </funcprototype>
  74
  75       <funcprototype>
  76         <funcdef>void* <function>sd_bus_track_get_userdata</function></funcdef>
  77         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  78       </funcprototype>
  79
  80       <funcprototype>
  81         <funcdef>void* <function>sd_bus_track_set_userdata</function></funcdef>
  82         <paramdef>sd_bus_track *<parameter>t</parameter></paramdef>
  83         <paramdef>void *userdata</paramdef>
  84       </funcprototype>
  85
  86     </funcsynopsis>
  87   </refsynopsisdiv>
  88
  89   <refsect1>
  90     <title>Description</title>
  91
  92     <para><function>sd_bus_track_new()</function> creates a new bus peer tracking object. The object is allocated for
  93     the specified bus, and returned in the <parameter>*ret</parameter> parameter. After use, the object should be freed
  94     again by dropping the acquired reference with <function>sd_bus_track_unref()</function> (see below). A bus peer
  95     tracking object may be used to keep track of peers on a specific IPC bus, for cases where peers are making use of
  96     one or more local objects, in order to control the lifecycle of the local objects and ensure they stay around as
  97     long as the peers needing them are around, and unreferenced (and possibly destroyed) as soon as all relevant peers
  98     have vanished. Each bus peer tracking object may be used to track zero, one or more peers add a time. References to
  99     specific bus peers are added via
 100     <citerefentry><refentrytitle>sd_bus_track_add_name</refentrytitle><manvolnum>3</manvolnum></citerefentry> or
 101     <function>sd_bus_track_add_sender()</function>. They may be dropped again via
 102     <function>sd_bus_track_remove_name()</function> and
 103     <function>sd_bus_track_remove_sender()</function>. Alternatively, references on peers are removed automatically
 104     when they disconnect from the bus.  If non-NULL the <parameter>handler</parameter> may specify a function that is
 105     invoked whenever the last reference is dropped, regardless whether the reference is dropped explicitly via
 106     <function>sd_bus_track_remove_name()</function> or implicitly because the peer disconnected from the bus. The final
 107     argument <parameter>userdata</parameter> may be used to attach a generic user data pointer to the object. This
 108     pointer is passed to the handler callback when it is invoked.</para>
 109
 110     <para><function>sd_bus_track_ref()</function> creates a new reference to a bus peer tracking object. This object
 111     will not be destroyed until <function>sd_bus_track_unref()</function> has been called as many times plus once
 112     more. Once the reference count has dropped to zero, the specified object cannot be used anymore, further calls to
 113     <function>sd_bus_track_ref()</function> or <function>sd_bus_track_unref()</function> on the same object are
 114     illegal.</para>
 115
 116     <para><function>sd_bus_track_unref()</function> destroys a reference to a bus peer tracking object.</para>
 117
 118     <para><function>sd_bus_track_unrefp()</function> is similar to <function>sd_bus_track_unref()</function> but takes
 119     a pointer to a pointer to an <type>sd_bus_track</type> object. This call is useful in conjunction with GCC's and
 120     LLVM's <ulink url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up Variable
 121     Attribute</ulink>. Note that this function is defined as inline function.</para>
 122
 123     <para><function>sd_bus_track_ref()</function>, <function>sd_bus_track_unref()</function> and
 124     <function>sd_bus_track_unrefp()</function> execute no operation if the passed in bus peer tracking object is
 125     <constant>NULL</constant>.</para>
 126
 127     <para>Bus peer tracking objects may exist in two modes: by default they operate in non-recursive mode, but may
 128     optionally be switched into recursive mode. If operating in the default non-recursive mode a peer is either tracked
 129     or not tracked. In this mode invoking <function>sd_bus_track_add_name()</function> multiple times in a row for the
 130     same peer is fully equivalent to calling it just once, as the call adds the peer to the set of tracked peers if
 131     necessary, and executes no operation if the peer is already being tracked. A single invocation of
 132     <function>sd_bus_track_remove_name()</function> removes the reference on the peer again, regardless how many times
 133     <function>sd_bus_track_add_name()</function> was called before. If operating in recursive mode, the number of times
 134     <function>sd_bus_track_add_name()</function> is invoked for the same peer name is counted and
 135     <function>sd_bus_track_remove_name()</function> must be called the same number of times before the peer is not
 136     tracked anymore, with the exception when the tracked peer vanishes from the bus, in which case the count is
 137     irrelevant and the tracking of the specific peer is immediately
 138     removed. <function>sd_bus_track_get_recursive()</function> may be used to determine whether the bus peer tracking
 139     object is operating in recursive mode. <function>sd_bus_track_set_recursive()</function> may be used to enable or
 140     disable recursive mode. By default a bus peer tracking object operates in non-recursive mode, and
 141     <function>sd_bus_track_get_recursive()</function> for a newly allocated object hence returns a value equal to
 142     zero. Use <function>sd_bus_track_set_recursive()</function> to enable recursive mode, right after allocation. It
 143     takes a boolean argument to enable or disable recursive mode. Note that tracking objects for which
 144     <function>sd_bus_track_add_name()</function> was already invoked at least once (and which hence track already one
 145     or more peers) may not be switched from recursive to non-recursive mode anymore.</para>
 146
 147     <para><function>sd_bus_track_get_bus()</function> returns the bus object the bus peer tracking object belongs
 148     to. It returns the bus object initially passed to <function>sd_bus_track_new()</function> when the object was
 149     allocated.</para>
 150
 151     <para><function>sd_bus_track_get_userdata()</function> returns the generic user data pointer set on the bus peer
 152     tracking object at the time of creation using <function>sd_bus_track_new()</function> or at a later time, using
 153     <function>sd_bus_track_set_userdata()</function>.</para>
 154   </refsect1>
 155
 156   <refsect1>
 157     <title>Return Value</title>
 158
 159     <para>On success, <function>sd_bus_track_new()</function> and <function>sd_bus_track_set_recursive()</function>
 160     return 0 or a positive integer. On failure, they return a negative errno-style error code.</para>
 161
 162     <para><function>sd_bus_track_ref()</function> always returns the argument.</para>
 163
 164     <para><function>sd_bus_track_unref()</function> always returns <constant>NULL</constant>.</para>
 165
 166     <para><function>sd_bus_track_get_recursive()</function> returns 0 if non-recursive mode is selected (default), and
 167     greater than 0 if recursive mode is selected. On failure a negative errno-style error code is returned.</para>
 168
 169     <para><function>sd_bus_track_get_bus()</function> returns the bus object associated to the bus peer tracking
 170     object.</para>
 171
 172     <para><function>sd_bus_track_get_userdata()</function> returns the generic user data pointer associated with the
 173     bus peer tracking object. <function>sd_bus_track_set_userdata()</function> returns the previous user data pointer
 174     set.</para>
 175
 176   </refsect1>
 177
 178   <refsect1>
 179     <title>Reference ownership</title>
 180
 181     <para>The <function>sd_bus_track_new()</function> function creates a new object and the caller owns the sole
 182     reference. When not needed anymore, this reference should be destroyed with
 183     <function>sd_bus_track_unref()</function>.
 184     </para>
 185
 186     <refsect2>
 187       <title>Errors</title>
 188
 189       <para>Returned errors may indicate the following problems:</para>
 190
 191       <variablelist>
 192
 193         <varlistentry>
 194           <term><constant>-EBUSY</constant></term>
 195
 196           <listitem><para>Bus peers have already been added to the bus peer tracking object and
 197           <function>sd_bus_track_set_recursive()</function> was called to change tracking mode.
 198           </para></listitem>
 199         </varlistentry>
 200
 201         <varlistentry>
 202           <term><constant>-EINVAL</constant></term>
 203
 204           <listitem><para>Specified parameter is invalid
 205           (<constant>NULL</constant> in case of output
 206           parameters).</para></listitem>
 207         </varlistentry>
 208
 209         <varlistentry>
 210           <term><constant>-ENOMEM</constant></term>
 211
 212           <listitem><para>Memory allocation failed.</para></listitem>
 213         </varlistentry>
 214
 215       </variablelist>
 216     </refsect2>
 217   </refsect1>
 218
 219   <xi:include href="libsystemd-pkgconfig.xml" />
 220
 221   <refsect1>
 222     <title>See Also</title>
 223
 224     <para>
 225       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 226       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 227       <citerefentry><refentrytitle>sd_bus_track_add_name</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 228     </para>
 229   </refsect1>
 230
 231 </refentry>