man/sd-id128.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   8   Copyright 2012 Lennart Poettering
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  23
  24 <refentry id="sd-id128">
  25
  26         <refentryinfo>
  27                 <title>sd-id128</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>sd-id128</refentrytitle>
  42                 <manvolnum>7</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>sd-id128</refname>
  47                 <refname>sd_id128_t</refname>
  48                 <refname>SD_ID128_MAKE</refname>
  49                 <refname>SD_ID128_FORMAT_STR</refname>
  50                 <refname>SD_ID128_FORMAT_VAL</refname>
  51                 <refname>sd_id128_equal</refname>
  52                 <refpurpose>APIs for processing 128 bit IDs</refpurpose>
  53         </refnamediv>
  54
  55         <refsynopsisdiv>
  56                 <funcsynopsis>
  57                         <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  58                 </funcsynopsis>
  59
  60                 <cmdsynopsis>
  61                         <command>pkg-config --cflags --libs libsystemd-id128</command>
  62                 </cmdsynopsis>
  63
  64         </refsynopsisdiv>
  65
  66         <refsect1>
  67                 <title>Description</title>
  68
  69                 <para><filename>sd-id128.h</filename> provides APIs to
  70                 process and generate 128 bit ID values. The 128 bit ID
  71                 values processed and generated by these APIs are a
  72                 generalization of OSF UUIDs as defined by <ulink
  73                 url="http://tools.ietf.org/html/rfc4122">RFC
  74                 4122</ulink>, though use a simpler string
  75                 formatting. These functions impose no structure on the
  76                 used IDs, much unlike OSF UUIDs or Microsoft GUIDs,
  77                 but are fully compatible with those types of IDs.
  78                 </para>
  79
  80                 <para>See
  81                 <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
  82                 <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  83                 for more information about the implemented
  84                 functions.</para>
  85
  86                 <para>A 128 bit ID is implemented as the following
  87                 union type:</para>
  88
  89                 <programlisting>typedef union sd_id128 {
  90         uint8_t bytes[16];
  91         uint64_t qwords[2];
  92 } sd_id128_t;</programlisting>
  93
  94                 <para>This union type allows accessing the 128 bit ID
  95                 as 16 separate bytes or two 64 bit words. It is generally
  96                 safer to access the ID components by their 8 bit array
  97                 to avoid endianess issues. This union is intended to
  98                 be passed call-by-value (as opposed to
  99                 call-by-reference) and may be directly manipulated by
 100                 clients.</para>
 101
 102                 <para>A couple of macros are defined to denote and
 103                 decode 128 bit IDs:</para>
 104
 105                 <para><function>SD_ID128_MAKE()</function> may be used
 106                 to write a 128 bit ID in source code. A commonly used
 107                 idiom is to give 128 bit IDs names using this macro:</para>
 108
 109                 <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>
 110
 111                 <para><function>SD_ID128_FORMAT_STR</function> and
 112                 <function>SD_ID128_FORMAT_VAL()</function> may be used
 113                 to format a 128 bit ID in a
 114                 <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 115                 format string, as shown in the following
 116                 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>Note that new, randomized IDs may be generated
 138                 with
 139                 <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 140                 <literal>--new-id</literal> option.</para>
 141         </refsect1>
 142
 143         <refsect1>
 144                 <title>Notes</title>
 145
 146                 <para>These APIs are implemented as a shared library,
 147                 which can be compiled and linked to with the
 148                 <literal>libsystemd-id128</literal>
 149                 <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 150                 file.</para>
 151
 152         </refsect1>
 153
 154         <refsect1>
 155                 <title>See Also</title>
 156                 <para>
 157                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 158                         <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 159                         <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 160                         <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 161                         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 162                         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 163                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 164                         <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 165                 </para>
 166         </refsect1>
 167
 168 </refentry>