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  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<refentry id="sd_id128_get_machine">

  <refentryinfo>
    <title>sd_id128_get_machine</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Lennart</firstname>
        <surname>Poettering</surname>
        <email>lennart@poettering.net</email>
      </author>
    </authorgroup>
  </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_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_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 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
    that the original machine ID may not be determined externally. The application-specific ID should be generated via
    a tool like <command>journalctl --new-id128</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.</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>
    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
    <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>

    <para>The two calls return 0 on success (in which case
    <parameter>ret</parameter> is filled in), or a negative
    errno-style error code.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para>The <function>sd_id128_get_machine()</function>, <function>sd_id128_get_machine_app_specific()</function>
    <function>sd_id128_get_boot()</function> and <function>sd_id128_get_invocation()</function> interfaces are
    available as a shared library, which can be compiled and linked to with the
    <literal>libsystemd</literal> <citerefentry
    project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> file.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Application-specific machine ID</title>

      <para>Here's a simple example for an application specific machine ID:</para>

      <programlisting>#include &lt;systemd/sd-id128.h&gt;
#include &lt;stdio.h&gt;

#define OUR_APPLICATION_ID SD_ID128_MAKE(c2,73,27,73,23,db,45,4e,a6,3b,b9,6e,79,b5,3e,97)

int main(int argc, char *argv[]) {
        sd_id128_t id;
        sd_id128_get_machine_app_specific(OUR_APPLICATION_ID, &amp;id);
        printf("Our application ID: " SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(id));
        return 0;
}</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</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>