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