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