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">
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<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_ALLF</refname>
    <refname>SD_ID128_CONST_STR</refname>
    <refname>SD_ID128_FORMAT_STR</refname>
    <refname>SD_ID128_FORMAT_VAL</refname>
    <refname>SD_ID128_MAKE</refname>
    <refname>SD_ID128_MAKE_STR</refname>
    <refname>SD_ID128_MAKE_UUID_STR</refname>
    <refname>SD_ID128_NULL</refname>
    <refname>SD_ID128_UUID_FORMAT_STR</refname>
    <refname>sd_id128_equal</refname>
    <refname>sd_id128_string_equal</refname>
    <refname>sd_id128_in_set</refname>
    <refname>sd_id128_in_set_sentinel</refname>
    <refname>sd_id128_in_setv</refname>
    <refname>sd_id128_is_allf</refname>
    <refname>sd_id128_is_null</refname>
    <refname>sd_id128_t</refname>
    <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  </refnamediv>

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

      <para>
        <constant>SD_ID128_ALLF</constant>
      </para>
      <para>
        <constant>SD_ID128_NULL</constant>
      </para>
      <para>
        <constant>SD_ID128_CONST_STR(<replaceable>id</replaceable>)</constant>
      </para>
      <para>
        <constant>SD_ID128_FORMAT_STR</constant>
      </para>
      <para>
        <constant>SD_ID128_FORMAT_VAL(<replaceable>id</replaceable>)</constant>
      </para>
      <para>
        <constant>SD_ID128_MAKE(<replaceable>v0</replaceable>, <replaceable>v1</replaceable>, <replaceable>v2</replaceable>, <replaceable>v3</replaceable>, <replaceable>v4</replaceable>, <replaceable>v5</replaceable>, <replaceable>v6</replaceable>, <replaceable>v7</replaceable>, <replaceable>v8</replaceable>, <replaceable>v9</replaceable>, <replaceable>vA</replaceable>, <replaceable>vB</replaceable>, <replaceable>vC</replaceable>, <replaceable>vD</replaceable>, <replaceable>vE</replaceable>, <replaceable>vF</replaceable>)</constant>
      </para>
      <para>
        <constant>SD_ID128_MAKE_STR(<replaceable>v0</replaceable>, <replaceable>v1</replaceable>, <replaceable>v2</replaceable>, <replaceable>v3</replaceable>, <replaceable>v4</replaceable>, <replaceable>v5</replaceable>, <replaceable>v6</replaceable>, <replaceable>v7</replaceable>, <replaceable>v8</replaceable>, <replaceable>v9</replaceable>, <replaceable>vA</replaceable>, <replaceable>vB</replaceable>, <replaceable>vC</replaceable>, <replaceable>vD</replaceable>, <replaceable>vE</replaceable>, <replaceable>vF</replaceable>)</constant>
      </para>
      <para>
        <constant>SD_ID128_MAKE_UUID_STR(<replaceable>v0</replaceable>, <replaceable>v1</replaceable>, <replaceable>v2</replaceable>, <replaceable>v3</replaceable>, <replaceable>v4</replaceable>, <replaceable>v5</replaceable>, <replaceable>v6</replaceable>, <replaceable>v7</replaceable>, <replaceable>v8</replaceable>, <replaceable>v9</replaceable>, <replaceable>vA</replaceable>, <replaceable>vB</replaceable>, <replaceable>vC</replaceable>, <replaceable>vD</replaceable>, <replaceable>vE</replaceable>, <replaceable>vF</replaceable>)</constant>
      </para>
      <para>
        <constant>SD_ID128_UUID_FORMAT_STR</constant>
      </para>

      <funcprototype>
        <funcdef>int <function>sd_id128_equal</function></funcdef>
        <paramdef>sd_id128_t <parameter>a</parameter></paramdef>
        <paramdef>sd_id128_t <parameter>b</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_string_equal</function></funcdef>
        <paramdef>const char *<parameter>a</parameter></paramdef>
        <paramdef>sd_id128_t <parameter>b</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_is_null</function></funcdef>
        <paramdef>sd_id128_t <parameter>id</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_is_allf</function></funcdef>
        <paramdef>sd_id128_t <parameter>id</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_in_setv</function></funcdef>
        <paramdef>sd_id128_t <parameter>id</parameter></paramdef>
        <paramdef>va_list <parameter>ap</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_in_set_sentinel</function></funcdef>
        <paramdef>sd_id128_t <parameter>id</parameter></paramdef>
        <paramdef>…</paramdef>
        <paramdef><constant>SD_ID128_NULL</constant></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_id128_in_set</function></funcdef>
        <paramdef>sd_id128_t <parameter>id</parameter></paramdef>
        <paramdef>…</paramdef>
      </funcprototype>
    </funcsynopsis>

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

  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>sd-id128.h</filename> provides APIs to generate, convert, and compare 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
    mostly compatible with those types of IDs.
    </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 by value (as opposed to pass-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> is used to write a constant ID in source code. A commonly used
    idiom is to assign a name to an 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> defines an ID consisting of only <constant>NUL</constant> bytes
    (i.e. all bits off).</para>

    <para><constant>SD_ID128_ALLF</constant> defines an ID consisting of only <constant>0xFF</constant> bytes
    (i.e. all bits on).</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> converts constant 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> is used to
    format an 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> and <function>SD_ID128_MAKE_UUID_STR()</function>
    are similar to
    <constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_MAKE_STR()</function>,
    but include separating hyphens to conform to the
    "<ulink url="https://en.wikipedia.org/wiki/Universally_unique_identifier#Format">canonical representation</ulink>".
    They format the string based on <ulink
    url="https://tools.ietf.org/html/rfc4122">RFC4122</ulink> Variant 1 rules, i.e. converting from Big
    Endian byte order. This matches behaviour of most other Linux userspace infrastructure. It's probably
    best to avoid UUIDs of other variants, in order to avoid unnecessary ambiguities. All 128-bit IDs
    generated by the sd-id128 APIs strictly conform to Variant 1 Version 4 UUIDs, as per RFC 4122.</para>

    <para><function>sd_id128_equal()</function> compares 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><function>sd_id128_string_equal()</function> is similar to <function>sd_id128_equal()</function>,
    but the first ID is formatted as <type>const char*</type>. The same restrictions apply as to the first
    argument of <function>sd_id128_from_string()</function>.</para>

    <para><function>sd_id128_is_null()</function> checks if an ID consists of only <constant>NUL</constant>
    bytes:</para>

    <programlisting>assert(sd_id128_is_null(SD_ID128_NULL));</programlisting>

    <para>Similarly, <function>sd_id128_is_allf()</function> checks if an ID consists of only
    <constant>0xFF</constant> bytes (all bits on):</para>

    <programlisting>assert(sd_id128_is_allf(SD_ID128_ALLF));</programlisting>

    <para><function>sd_id128_in_set_sentinel()</function> takes a list of IDs and returns true if the first
    argument is equal to any of the subsequent arguments. The argument list is terminated by an
    <constant>SD_ID128_NULL</constant> sentinel, which must be present.</para>

    <para><function>sd_id128_in_set()</function> is a convenience function that takes a list of IDs and
    returns true if the first argument is equal to any of the subsequent arguments:</para>

    <programlisting>int main(int argc, char *argv[]) {
  sd_id12_t a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
  assert(sd_id128_in_set(a, a));
  assert(sd_id128_in_set(a, a, a));
  assert(!sd_id128_in_set(a));
  assert(!sd_id128_in_set(a,
                          SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e)
                          SD_ID128_MAKE(2f,88,28,5f,9c,44,09,9d,d7,15,77,04,bc,85,7e,e3)
                          SD_ID128_ALLF));
  return 0;
}
</programlisting>

    <para><function>sd_id128_in_set()</function> is defined as a macro over
    <function>sd_id128_in_set_sentinel()</function>, adding the <constant>SD_ID128_NULL</constant> sentinel
    automatically. Since <function>sd_id128_in_set_sentinel()</function> uses
    <constant>SD_ID128_NULL</constant> as the sentinel, <constant>SD_ID128_NULL</constant> cannot be
    otherwise placed in the argument list.</para>

    <para><function>sd_id128_in_setv()</function> is similar to
    <function>sd_id128_in_set_sentinel()</function>, but takes a <structname>struct varargs</structname>
    argument.</para>

    <para>New randomized IDs may be generated with
    <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
    <command>new</command> command.</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 information about other implemented functions.</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>