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92 lines
3.6 KiB
XML
92 lines
3.6 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_device_ref" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_device_ref</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_device_ref</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_device_ref</refname>
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<refname>sd_device_unref</refname>
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<refname>sd_device_unrefp</refname>
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<refpurpose>Create or destroy references to a device object</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-device.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>sd_device* <function>sd_device_ref</function></funcdef>
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<paramdef>sd_device *<parameter>device</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>sd_device* <function>sd_device_unref</function></funcdef>
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<paramdef>sd_device *<parameter>device</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>void <function>sd_device_unrefp</function></funcdef>
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<paramdef>sd_device **<parameter>device</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<para><function>sd_device_ref()</function> increases the internal reference counter of
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<parameter>device</parameter> by one.</para>
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<para><function>sd_device_unref()</function> decreases the internal reference counter of
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<parameter>device</parameter> by one. Once the reference count has dropped to zero,
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<parameter>device</parameter> is destroyed and cannot be used anymore, so further calls to
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<function>sd_device_ref()</function> or <function>sd_device_unref()</function> are illegal.</para>
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<para><function>sd_device_unrefp()</function> is similar to <function>sd_device_unref()</function> but
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takes a pointer to a pointer to an <type>sd_device</type> object. This call is useful in conjunction with
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GCC's and LLVM's <ulink url="https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html">Clean-up
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Variable Attribute</ulink>. Note that this function is defined as an inline function. Use a declaration
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like the following, in order to allocate a device object that is freed automatically as the code block is
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left:</para>
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<programlisting>{
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__attribute__((cleanup(sd_device_unrefp))) sd_device *device = NULL;
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int r;
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…
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r = sd_device_new_from_syspath(&device, "…");
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if (r < 0) {
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errno = -r;
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fprintf(stderr, "Failed to allocate device: %m\n");
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}
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…
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}</programlisting>
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<para><function>sd_device_ref()</function> and <function>sd_device_unref()</function> execute no
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operation if the argument is <constant>NULL</constant>. <function>sd_device_unrefp()</function> will
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first dereference its argument, which must not be <constant>NULL</constant>, and will execute no
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operation if <emphasis>that</emphasis> is <constant>NULL</constant>.</para>
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</refsynopsisdiv>
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<refsect1>
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<title>Return Value</title>
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<para><function>sd_device_ref()</function> always returns the argument, and
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<function>sd_device_unref()</function> always returns <constant>NULL</constant>.
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</para>
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</refsect1>
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<refsect1>
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<title>History</title>
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<para><function>sd_device_ref()</function>,
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<function>sd_device_unref()</function>, and
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<function>sd_device_unrefp()</function> were added in version 251.</para>
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</refsect1>
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</refentry>
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