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<?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 <systemd/sd-device.h></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(&device, "…");
if (r < 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>
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