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<?xml version='1.0'?> <!-- * - nxml - * -->
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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+ -->
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<refentry id= "sd_id128_get_machine" xmlns:xi= "http://www.w3.org/2001/XInclude" >
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<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>
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<refname > sd_id128_get_machine_app_specific</refname>
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<refname > sd_id128_get_boot</refname>
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<refname > sd_id128_get_boot_app_specific</refname>
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<refname > sd_id128_get_invocation</refname>
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<refpurpose > Retrieve 128-bit IDs</refpurpose>
</refnamediv>
<refsynopsisdiv >
<funcsynopsis >
<funcsynopsisinfo > #include < systemd/sd-id128.h> </funcsynopsisinfo>
<funcprototype >
<funcdef > int <function > sd_id128_get_machine</function> </funcdef>
<paramdef > sd_id128_t *<parameter > ret</parameter> </paramdef>
</funcprototype>
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<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>
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<funcprototype >
<funcdef > int <function > sd_id128_get_boot</function> </funcdef>
<paramdef > sd_id128_t *<parameter > ret</parameter> </paramdef>
</funcprototype>
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<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>
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<funcprototype >
<funcdef > int <function > sd_id128_get_invocation</function> </funcdef>
<paramdef > sd_id128_t *<parameter > ret</parameter> </paramdef>
</funcprototype>
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</funcsynopsis>
</refsynopsisdiv>
<refsect1 >
<title > Description</title>
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<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 irreversable (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
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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
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machine. The application-specific ID should be generated via a tool like <command > systemd-id128 new</command> ,
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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_machine_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>
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<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>
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<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 v4 compatible IDs. <function > sd_id128_get_machine()</function> will also return a UUID v4-compatible
ID on new installations but might not on older. It is possible to convert the machine ID into a UUID v4-compatible
one. For more information, see
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<citerefentry > <refentrytitle > machine-id</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> .</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>
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<para > Those calls return 0 on success (in which case <parameter > ret</parameter> is filled in),
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or a negative errno-style error code. In particular,
<function > sd_id128_get_machine()</function> ,
<function > sd_id128_get_machine_app_specific()</function> , and
<function > sd_id128_get_boot_app_specific()</function> return <constant > -ENOENT</constant> if
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<filename > /etc/machine-id</filename> is missing, and <constant > -ENOMEDIUM</constant> if
<filename > /etc/machine-id</filename> is empty or all zeros.</para>
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</refsect1>
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<xi:include href= "libsystemd-pkgconfig.xml" />
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<refsect1 >
<title > Examples</title>
<example >
<title > Application-specific machine ID</title>
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<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>
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<programlisting > <xi:include href= "id128-app-specific.c" parse= "text" /> </programlisting>
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</example>
</refsect1>
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<refsect1 >
<title > See Also</title>
<para >
<citerefentry > <refentrytitle > systemd</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > systemd-id128</refentrytitle> <manvolnum > 1</manvolnum> </citerefentry> ,
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<citerefentry > <refentrytitle > sd-id128</refentrytitle> <manvolnum > 3</manvolnum> </citerefentry> ,
<citerefentry > <refentrytitle > machine-id</refentrytitle> <manvolnum > 5</manvolnum> </citerefentry> ,
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<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>
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</para>
</refsect1>
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</refentry>