systemd/man/sd_device_ref.xml

<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="sd_device_ref" xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>sd_device_ref</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_device_ref</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_device_ref</refname>
    <refname>sd_device_unref</refname>
    <refname>sd_device_unrefp</refname>

    <refpurpose>Create or destroy references to a device object</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-device.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>sd_device* <function>sd_device_ref</function></funcdef>
        <paramdef>sd_device *<parameter>device</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>sd_device* <function>sd_device_unref</function></funcdef>
        <paramdef>sd_device *<parameter>device</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>void <function>sd_device_unrefp</function></funcdef>
        <paramdef>sd_device **<parameter>device</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>

    <para><function>sd_device_ref()</function> increases the internal reference counter of
    <parameter>device</parameter> by one.</para>

    <para><function>sd_device_unref()</function> decreases the internal reference counter of
    <parameter>device</parameter> by one. Once the reference count has dropped to zero,
    <parameter>device</parameter> is destroyed and cannot be used anymore, so further calls to
    <function>sd_device_ref()</function> or <function>sd_device_unref()</function> are illegal.</para>

    <para><function>sd_device_unrefp()</function> is similar to <function>sd_device_unref()</function> but
    takes a pointer to a pointer to an <type>sd_device</type> object. This call is useful in conjunction with
    GCC's and LLVM's <ulink url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up
    Variable Attribute</ulink>. Note that this function is defined as an inline function. Use a declaration
    like the following, in order to allocate a device object that is freed automatically as the code block is
    left:</para>

    <programlisting>{
  __attribute__((cleanup(sd_device_unrefp))) sd_device *device = NULL;
  int r;
  …
  r = sd_device_new_from_syspath(&amp;device, "…");
  if (r &lt; 0) {
    errno = -r;
    fprintf(stderr, "Failed to allocate device: %m\n");
  }
  …
}</programlisting>

    <para><function>sd_device_ref()</function> and <function>sd_device_unref()</function> execute no
    operation if the argument is <constant>NULL</constant>. <function>sd_device_unrefp()</function> will
    first dereference its argument, which must not be <constant>NULL</constant>, and will execute no
    operation if <emphasis>that</emphasis> is <constant>NULL</constant>.</para>
  </refsynopsisdiv>

  <refsect1>
    <title>Return Value</title>

    <para><function>sd_device_ref()</function> always returns the argument, and
    <function>sd_device_unref()</function> always returns <constant>NULL</constant>.
    </para>
  </refsect1>
</refentry>