man/sd-id128.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2012 Lennart Poettering
  11 -->
  12
  13 <refentry id="sd-id128"
  14   xmlns:xi="http://www.w3.org/2001/XInclude">
  15
  16   <refentryinfo>
  17     <title>sd-id128</title>
  18     <productname>systemd</productname>
  19
  20     <authorgroup>
  21       <author>
  22         <contrib>Developer</contrib>
  23         <firstname>Lennart</firstname>
  24         <surname>Poettering</surname>
  25         <email>lennart@poettering.net</email>
  26       </author>
  27     </authorgroup>
  28   </refentryinfo>
  29
  30   <refmeta>
  31     <refentrytitle>sd-id128</refentrytitle>
  32     <manvolnum>3</manvolnum>
  33   </refmeta>
  34
  35   <refnamediv>
  36     <refname>sd-id128</refname>
  37     <refname>sd_id128_t</refname>
  38     <refname>SD_ID128_MAKE</refname>
  39     <refname>SD_ID128_MAKE_STR</refname>
  40     <refname>SD_ID128_NULL</refname>
  41     <refname>SD_ID128_CONST_STR</refname>
  42     <refname>SD_ID128_FORMAT_STR</refname>
  43     <refname>SD_ID128_FORMAT_VAL</refname>
  44     <refname>sd_id128_equal</refname>
  45     <refname>sd_id128_is_null</refname>
  46     <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  47   </refnamediv>
  48
  49   <refsynopsisdiv>
  50     <funcsynopsis>
  51       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  52     </funcsynopsis>
  53
  54     <cmdsynopsis>
  55       <command>pkg-config --cflags --libs libsystemd</command>
  56     </cmdsynopsis>
  57
  58   </refsynopsisdiv>
  59
  60   <refsect1>
  61     <title>Description</title>
  62
  63     <para><filename>sd-id128.h</filename> provides APIs to process and
  64     generate 128-bit ID values. The 128-bit ID values processed and
  65     generated by these APIs are a generalization of OSF UUIDs as
  66     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  67     4122</ulink> but use a simpler string format. These functions
  68     impose no structure on the used IDs, much unlike OSF UUIDs or
  69     Microsoft GUIDs, but are fully compatible with those types of IDs.
  70     </para>
  71
  72     <para>See
  73     <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  74     <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  75     and
  76     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  77     for more information about the implemented functions.</para>
  78
  79     <para>A 128-bit ID is implemented as the following
  80     union type:</para>
  81
  82     <programlisting>typedef union sd_id128 {
  83         uint8_t bytes[16];
  84         uint64_t qwords[2];
  85 } sd_id128_t;</programlisting>
  86
  87     <para>This union type allows accessing the 128-bit ID as 16
  88     separate bytes or two 64-bit words. It is generally safer to
  89     access the ID components by their 8-bit array to avoid endianness
  90     issues. This union is intended to be passed call-by-value (as
  91     opposed to call-by-reference) and may be directly manipulated by
  92     clients.</para>
  93
  94     <para>A couple of macros are defined to denote and decode 128-bit
  95     IDs:</para>
  96
  97     <para><function>SD_ID128_MAKE()</function> may be used to denote a
  98     constant 128-bit ID in source code. A commonly used idiom is to
  99     assign a name to a 128-bit ID using this macro:</para>
 100
 101     <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>
 102
 103     <para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
 104     bytes.</para>
 105
 106     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
 107     <type>const char*</type> expression that can be conveniently used in message formats and such:</para>
 108
 109     <programlisting>#include &lt;stdio.h&gt;
 110 #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)
 111
 112 int main(int argc, char **argv) {
 113         puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
 114 }
 115     </programlisting>
 116
 117
 118     <para><function>SD_ID128_CONST_STR()</function> may be used to
 119     convert constant 128-bit IDs into constant strings for output. The
 120     following example code will output the string
 121     "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 122     <programlisting>int main(int argc, char *argv[]) {
 123         puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 124 }</programlisting>
 125
 126     <para><function>SD_ID128_FORMAT_STR()</function> and
 127     <function>SD_ID128_FORMAT_VAL()</function> may be used to format a
 128     128-bit ID in a
 129     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 130     format string, as shown in the following example:</para>
 131
 132     <programlisting>int main(int argc, char *argv[]) {
 133         sd_id128_t id;
 134         id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 135         printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 136         return 0;
 137 }</programlisting>
 138
 139     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 140
 141     <programlisting>int main(int argc, char *argv[]) {
 142         sd_id128_t a, b, c;
 143         a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 144         b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 145         c = a;
 146         assert(sd_id128_equal(a, c));
 147         assert(!sd_id128_equal(a, b));
 148         return 0;
 149 }</programlisting>
 150
 151     <para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
 152
 153     <programlisting>int main(int argc, char *argv[]) {
 154         assert(sd_id128_is_null(SD_ID128_NULL));
 155 }</programlisting>
 156
 157     <para>Note that new, randomized IDs may be generated with
 158     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 159     <option>--new-id128</option> option.</para>
 160   </refsect1>
 161
 162   <xi:include href="libsystemd-pkgconfig.xml" />
 163
 164   <refsect1>
 165     <title>See Also</title>
 166     <para>
 167       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 168       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 169       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 170       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 171       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 172       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 173       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 174       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 175       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 176     </para>
 177   </refsect1>
 178
 179 </refentry>