systemd/man/sd-id128.xml

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<!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"
  xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd-id128</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd-id128</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd-id128</refname>
    <refname>sd_id128_t</refname>
    <refname>SD_ID128_MAKE</refname>
    <refname>SD_ID128_MAKE_STR</refname>
    <refname>SD_ID128_NULL</refname>
    <refname>SD_ID128_CONST_STR</refname>
    <refname>SD_ID128_FORMAT_STR</refname>
    <refname>SD_ID128_UUID_FORMAT_STR</refname>
    <refname>SD_ID128_FORMAT_VAL</refname>
    <refname>sd_id128_equal</refname>
    <refname>sd_id128_is_null</refname>
    <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
    </funcsynopsis>

    <cmdsynopsis>
      <command>pkg-config --cflags --libs libsystemd</command>
    </cmdsynopsis>

  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>sd-id128.h</filename> provides APIs to process and
    generate 128-bit ID values. The 128-bit ID values processed and
    generated by these APIs are a generalization of OSF UUIDs as
    defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
    4122</ulink> but use a simpler string format. These functions
    impose no structure on the used IDs, much unlike OSF UUIDs or
    Microsoft GUIDs, but are fully compatible with those types of IDs.
    </para>

    <para>See
    <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    and
    <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for more information about the implemented functions.</para>

    <para>A 128-bit ID is implemented as the following
    union type:</para>

    <programlisting>typedef union sd_id128 {
        uint8_t bytes[16];
        uint64_t qwords[2];
} sd_id128_t;</programlisting>

    <para>This union type allows accessing the 128-bit ID as 16
    separate bytes or two 64-bit words. It is generally safer to
    access the ID components by their 8-bit array to avoid endianness
    issues. This union is intended to be passed call-by-value (as
    opposed to call-by-reference) and may be directly manipulated by
    clients.</para>

    <para>A couple of macros are defined to denote and decode 128-bit
    IDs:</para>

    <para><function>SD_ID128_MAKE()</function> may be used to denote a
    constant 128-bit ID in source code. A commonly used idiom is to
    assign a name to a 128-bit ID using this macro:</para>

    <programlisting>#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)</programlisting>

    <para><constant>SD_ID128_NULL</constant> may be used to refer to the 128bit ID consisting of only
    <constant>NUL</constant> bytes.</para>

    <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
    <type>const char*</type> expression that can be conveniently used in message formats and such:</para>

    <programlisting>#include &lt;stdio.h&gt;
#define SD_MESSAGE_COREDUMP_STR SD_ID128_MAKE_STR(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)

int main(int argc, char **argv) {
        puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
}
    </programlisting>

    <para><function>SD_ID128_CONST_STR()</function> may be used to
    convert constant 128-bit IDs into constant strings for output. The
    following example code will output the string
    "fc2e22bc6ee647b6b90729ab34a250b1":</para>
    <programlisting>int main(int argc, char *argv[]) {
        puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
}</programlisting>

    <para><constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_FORMAT_VAL()</function> may
    be used to format a 128-bit ID in a
    <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    format string, as shown in the following example:</para>

    <programlisting>int main(int argc, char *argv[]) {
        sd_id128_t id;
        id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
        printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
        return 0;
}</programlisting>

    <para><constant>SD_ID128_UUID_FORMAT_STR</constant> is similar to
    <constant>SD_ID128_FORMAT_STR</constant> but includes separating hyphens to conform to the
    "<ulink url="https://en.wikipedia.org/wiki/Universally_unique_identifier#Format">canonical representation</ulink>".
    </para>

    <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>

    <programlisting>int main(int argc, char *argv[]) {
        sd_id128_t a, b, c;
        a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
        b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
        c = a;
        assert(sd_id128_equal(a, c));
        assert(!sd_id128_equal(a, b));
        return 0;
}</programlisting>

    <para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only
    <constant>NUL</constant> bytes:</para>

    <programlisting>int main(int argc, char *argv[]) {
        assert(sd_id128_is_null(SD_ID128_NULL));
}</programlisting>

    <para>Note that new, randomized IDs may be generated with
    <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
    <command>new</command> command.</para>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>