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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!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_id128_get_machine" xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>sd_id128_get_machine</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>sd_id128_get_machine</refentrytitle>
+ <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>sd_id128_get_machine</refname>
+ <refname>sd_id128_get_machine_app_specific</refname>
+ <refname>sd_id128_get_boot</refname>
+ <refname>sd_id128_get_boot_app_specific</refname>
+ <refname>sd_id128_get_invocation</refname>
+ <refpurpose>Retrieve 128-bit IDs</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_get_machine</function></funcdef>
+ <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_get_machine_app_specific</function></funcdef>
+ <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
+ <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_get_boot</function></funcdef>
+ <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_get_boot_app_specific</function></funcdef>
+ <paramdef>sd_id128_t <parameter>app_id</parameter></paramdef>
+ <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_get_invocation</function></funcdef>
+ <paramdef>sd_id128_t *<parameter>ret</parameter></paramdef>
+ </funcprototype>
+
+ </funcsynopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para><function>sd_id128_get_machine()</function> returns the machine ID of the executing host. This reads and
+ parses the <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ file. This function caches the machine ID internally to make retrieving the machine ID a cheap operation. This ID
+ may be used wherever a unique identifier for the local system is needed. However, it is recommended to use this ID
+ as-is only in trusted environments. In untrusted environments it is recommended to derive an application specific
+ ID from this machine ID, in an irreversible (cryptographically secure) way. To make this easy
+ <function>sd_id128_get_machine_app_specific()</function> is provided, see below.</para>
+
+ <para><function>sd_id128_get_machine_app_specific()</function> is similar to
+ <function>sd_id128_get_machine()</function>, but retrieves a machine ID that is specific to the application that is
+ identified by the indicated application ID. It is recommended to use this function instead of
+ <function>sd_id128_get_machine()</function> when passing an ID to untrusted environments, in order to make sure
+ that the original machine ID may not be determined externally. This way, the ID used by the application remains
+ stable on a given machine, but cannot be easily correlated with IDs used in other applications on the same
+ machine. The application-specific ID should be generated via a tool like <command>systemd-id128 new</command>,
+ and may be compiled into the application. This function will return the same application-specific ID for each
+ combination of machine ID and application ID. Internally, this function calculates HMAC-SHA256 of the application
+ ID, keyed by the machine ID.</para>
+
+ <para><function>sd_id128_get_boot()</function> returns the boot ID of the executing kernel. This reads and parses
+ the <filename>/proc/sys/kernel/random/boot_id</filename> file exposed by the kernel. It is randomly generated early
+ at boot and is unique for every running kernel instance. See <citerefentry
+ project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more
+ information. This function also internally caches the returned ID to make this call a cheap operation. It is
+ recommended to use this ID as-is only in trusted environments. In untrusted environments it is recommended to
+ derive an application specific ID using <function>sd_id128_get_boot_app_specific()</function>, see below.</para>
+
+ <para><function>sd_id128_get_boot_app_specific()</function> is analogous to
+ <function>sd_id128_get_machine_app_specific()</function> but returns an ID that changes between boots. Some
+ machines may be used for a long time without rebooting, hence the boot ID may remain constant for a long time, and
+ has properties similar to the machine ID during that time.</para>
+
+ <para><function>sd_id128_get_invocation()</function> returns the invocation ID of the currently executed
+ service. In its current implementation, this reads and parses the <varname>$INVOCATION_ID</varname> environment
+ variable that the service manager sets when activating a service, see
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
+ ID is cached internally. In future a different mechanism to determine the invocation ID may be added.</para>
+
+ <para>Note that <function>sd_id128_get_machine_app_specific()</function>,
+ <function>sd_id128_get_boot()</function>, <function>sd_id128_get_boot_app_specific()</function>, and
+ <function>sd_id128_get_invocation()</function> always return UUID Variant 1 Version 4 compatible IDs.
+ <function>sd_id128_get_machine()</function> will also return a UUID Variant 1 Version 4 compatible ID on
+ new installations but might not on older. It is possible to convert the machine ID non-reversibly into a
+ UUID Variant 1 Version 4 compatible one. For more information, see
+ <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>. It is
+ hence guaranteed that these functions will never return the ID consisting of all zero or all one bits
+ (<constant>SD_ID128_NULL</constant>, <constant>SD_ID128_ALLF</constant>) — with the possible exception of
+ <function>sd_id128_get_machine()</function>, as mentioned.</para>
+
+ <para>For more information about the <literal>sd_id128_t</literal>
+ type see
+ <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Return Value</title>
+
+ <para>Those calls return 0 on success (in which case <parameter>ret</parameter> is filled in),
+ or a negative errno-style error code.</para>
+
+ <refsect2>
+ <title>Errors</title>
+ <para>Returned errors may indicate the following problems:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term><constant>-ENOENT</constant></term>
+
+ <listitem><para>Returned by <function>sd_id128_get_machine()</function>,
+ <function>sd_id128_get_machine_app_specific()</function>, and
+ <function>sd_id128_get_boot_app_specific()</function> when <filename>/etc/machine-id</filename> is
+ missing.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><constant>-ENOMEDIUM</constant></term>
+
+ <listitem><para>Returned by <function>sd_id128_get_machine()</function>,
+ <function>sd_id128_get_machine_app_specific()</function>, and
+ <function>sd_id128_get_boot_app_specific()</function> when <filename>/etc/machine-id</filename> is
+ empty or all zeros.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><constant>-ENXIO</constant></term>
+
+ <listitem><para>Returned by <function>sd_id128_get_invocation()</function> if no invocation ID is
+ set.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><constant>-EIO</constant></term>
+
+ <listitem><para>Returned by any of the functions described here when the configured value has
+ invalid format.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><constant>-EPERM</constant></term>
+
+ <listitem><para>Requested information could not be retrieved because of insufficient permissions.
+ </para></listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect2>
+ </refsect1>
+
+ <xi:include href="libsystemd-pkgconfig.xml" />
+
+ <refsect1>
+ <title>Examples</title>
+
+ <example>
+ <title>Application-specific machine ID</title>
+
+ <para>First, generate the application ID:</para>
+ <programlisting>$ systemd-id128 -p new
+As string:
+c273277323db454ea63bb96e79b53e97
+
+As UUID:
+c2732773-23db-454e-a63b-b96e79b53e97
+
+As man:sd-id128(3) macro:
+#define MESSAGE_XYZ SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)
+...
+</programlisting>
+
+ <para>Then use the new identifier in an example application:</para>
+
+ <programlisting><xi:include href="id128-app-specific.c" parse="text" /></programlisting>
+ </example>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>