man/sd-id128.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7 -->
   8
   9 <refentry id="sd-id128"
  10   xmlns:xi="http://www.w3.org/2001/XInclude">
  11
  12   <refentryinfo>
  13     <title>sd-id128</title>
  14     <productname>systemd</productname>
  15   </refentryinfo>
  16
  17   <refmeta>
  18     <refentrytitle>sd-id128</refentrytitle>
  19     <manvolnum>3</manvolnum>
  20   </refmeta>
  21
  22   <refnamediv>
  23     <refname>sd-id128</refname>
  24     <refname>sd_id128_t</refname>
  25     <refname>SD_ID128_MAKE</refname>
  26     <refname>SD_ID128_MAKE_STR</refname>
  27     <refname>SD_ID128_NULL</refname>
  28     <refname>SD_ID128_CONST_STR</refname>
  29     <refname>SD_ID128_FORMAT_STR</refname>
  30     <refname>SD_ID128_FORMAT_VAL</refname>
  31     <refname>sd_id128_equal</refname>
  32     <refname>sd_id128_is_null</refname>
  33     <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  34   </refnamediv>
  35
  36   <refsynopsisdiv>
  37     <funcsynopsis>
  38       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  39     </funcsynopsis>
  40
  41     <cmdsynopsis>
  42       <command>pkg-config --cflags --libs libsystemd</command>
  43     </cmdsynopsis>
  44
  45   </refsynopsisdiv>
  46
  47   <refsect1>
  48     <title>Description</title>
  49
  50     <para><filename>sd-id128.h</filename> provides APIs to process and
  51     generate 128-bit ID values. The 128-bit ID values processed and
  52     generated by these APIs are a generalization of OSF UUIDs as
  53     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  54     4122</ulink> but use a simpler string format. These functions
  55     impose no structure on the used IDs, much unlike OSF UUIDs or
  56     Microsoft GUIDs, but are fully compatible with those types of IDs.
  57     </para>
  58
  59     <para>See
  60     <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  61     <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  62     and
  63     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  64     for more information about the implemented functions.</para>
  65
  66     <para>A 128-bit ID is implemented as the following
  67     union type:</para>
  68
  69     <programlisting>typedef union sd_id128 {
  70         uint8_t bytes[16];
  71         uint64_t qwords[2];
  72 } sd_id128_t;</programlisting>
  73
  74     <para>This union type allows accessing the 128-bit ID as 16
  75     separate bytes or two 64-bit words. It is generally safer to
  76     access the ID components by their 8-bit array to avoid endianness
  77     issues. This union is intended to be passed call-by-value (as
  78     opposed to call-by-reference) and may be directly manipulated by
  79     clients.</para>
  80
  81     <para>A couple of macros are defined to denote and decode 128-bit
  82     IDs:</para>
  83
  84     <para><function>SD_ID128_MAKE()</function> may be used to denote a
  85     constant 128-bit ID in source code. A commonly used idiom is to
  86     assign a name to a 128-bit ID using this macro:</para>
  87
  88     <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>
  89
  90     <para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
  91     bytes.</para>
  92
  93     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
  94     <type>const char*</type> expression that can be conveniently used in message formats and such:</para>
  95
  96     <programlisting>#include &lt;stdio.h&gt;
  97 #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)
  98
  99 int main(int argc, char **argv) {
 100         puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
 101 }
 102     </programlisting>
 103
 104     <para><function>SD_ID128_CONST_STR()</function> may be used to
 105     convert constant 128-bit IDs into constant strings for output. The
 106     following example code will output the string
 107     "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 108     <programlisting>int main(int argc, char *argv[]) {
 109         puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 110 }</programlisting>
 111
 112     <para><function>SD_ID128_FORMAT_STR()</function> and
 113     <function>SD_ID128_FORMAT_VAL()</function> may be used to format a
 114     128-bit ID in a
 115     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 116     format string, as shown in the following example:</para>
 117
 118     <programlisting>int main(int argc, char *argv[]) {
 119         sd_id128_t id;
 120         id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 121         printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 122         return 0;
 123 }</programlisting>
 124
 125     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 126
 127     <programlisting>int main(int argc, char *argv[]) {
 128         sd_id128_t a, b, c;
 129         a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 130         b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 131         c = a;
 132         assert(sd_id128_equal(a, c));
 133         assert(!sd_id128_equal(a, b));
 134         return 0;
 135 }</programlisting>
 136
 137     <para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
 138
 139     <programlisting>int main(int argc, char *argv[]) {
 140         assert(sd_id128_is_null(SD_ID128_NULL));
 141 }</programlisting>
 142
 143     <para>Note that new, randomized IDs may be generated with
 144     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 145     <option>--new-id128</option> option.</para>
 146   </refsect1>
 147
 148   <xi:include href="libsystemd-pkgconfig.xml" />
 149
 150   <refsect1>
 151     <title>See Also</title>
 152     <para>
 153       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 154       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 155       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 156       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 157       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 159       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 160       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 161       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 162     </para>
 163   </refsect1>
 164
 165 </refentry>