man/sd_device_ref.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/4.5/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="sd_device_ref" xmlns:xi="http://www.w3.org/2001/XInclude">
   7   <refentryinfo>
   8     <title>sd_device_ref</title>
   9     <productname>systemd</productname>
  10   </refentryinfo>
  11
  12   <refmeta>
  13     <refentrytitle>sd_device_ref</refentrytitle>
  14     <manvolnum>3</manvolnum>
  15   </refmeta>
  16
  17   <refnamediv>
  18     <refname>sd_device_ref</refname>
  19     <refname>sd_device_unref</refname>
  20     <refname>sd_device_unrefp</refname>
  21
  22     <refpurpose>Create or destroy references to a device object</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <funcsynopsis>
  27       <funcsynopsisinfo>#include &lt;systemd/sd-device.h&gt;</funcsynopsisinfo>
  28
  29       <funcprototype>
  30         <funcdef>sd_device* <function>sd_device_ref</function></funcdef>
  31         <paramdef>sd_device *<parameter>device</parameter></paramdef>
  32       </funcprototype>
  33
  34       <funcprototype>
  35         <funcdef>sd_device* <function>sd_device_unref</function></funcdef>
  36         <paramdef>sd_device *<parameter>device</parameter></paramdef>
  37       </funcprototype>
  38
  39       <funcprototype>
  40         <funcdef>void <function>sd_device_unrefp</function></funcdef>
  41         <paramdef>sd_device **<parameter>device</parameter></paramdef>
  42       </funcprototype>
  43     </funcsynopsis>
  44
  45     <para><function>sd_device_ref()</function> increases the internal reference counter of
  46     <parameter>device</parameter> by one.</para>
  47
  48     <para><function>sd_device_unref()</function> decreases the internal reference counter of
  49     <parameter>device</parameter> by one. Once the reference count has dropped to zero,
  50     <parameter>device</parameter> is destroyed and cannot be used anymore, so further calls to
  51     <function>sd_device_ref()</function> or <function>sd_device_unref()</function> are illegal.</para>
  52
  53     <para><function>sd_device_unrefp()</function> is similar to <function>sd_device_unref()</function> but
  54     takes a pointer to a pointer to an <type>sd_device</type> object. This call is useful in conjunction with
  55     GCC's and LLVM's <ulink url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up
  56     Variable Attribute</ulink>. Note that this function is defined as an inline function. Use a declaration
  57     like the following, in order to allocate a device object that is freed automatically as the code block is
  58     left:</para>
  59
  60     <programlisting>{
  61   __attribute__((cleanup(sd_device_unrefp))) sd_device *device = NULL;
  62   int r;
  63   …
  64   r = sd_device_new_from_syspath(&amp;device, "…");
  65   if (r &lt; 0) {
  66     errno = -r;
  67     fprintf(stderr, "Failed to allocate device: %m\n");
  68   }
  69   …
  70 }</programlisting>
  71
  72     <para><function>sd_device_ref()</function> and <function>sd_device_unref()</function> execute no
  73     operation if the argument is <constant>NULL</constant>. <function>sd_device_unrefp()</function> will
  74     first dereference its argument, which must not be <constant>NULL</constant>, and will execute no
  75     operation if <emphasis>that</emphasis> is <constant>NULL</constant>.</para>
  76   </refsynopsisdiv>
  77
  78   <refsect1>
  79     <title>Return Value</title>
  80
  81     <para><function>sd_device_ref()</function> always returns the argument, and
  82     <function>sd_device_unref()</function> always returns <constant>NULL</constant>.
  83     </para>
  84   </refsect1>
  85 <refsect1>
  86     <title>History</title>
  87     <para><function>sd_device_ref()</function>,
  88     <function>sd_device_unref()</function>, and
  89     <function>sd_device_unrefp()</function> were added in version 251.</para>
  90   </refsect1>
  91 </refentry>