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>3</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>,
  82                 <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
  83                 <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  84                 for more information about the implemented
  85                 functions.</para>
  86
  87                 <para>A 128 bit ID is implemented as the following
  88                 union type:</para>
  89
  90                 <programlisting>typedef union sd_id128 {
  91         uint8_t bytes[16];
  92         uint64_t qwords[2];
  93 } sd_id128_t;</programlisting>
  94
  95                 <para>This union type allows accessing the 128 bit ID
  96                 as 16 separate bytes or two 64 bit words. It is generally
  97                 safer to access the ID components by their 8 bit array
  98                 to avoid endianness issues. This union is intended to
  99                 be passed call-by-value (as opposed to
 100                 call-by-reference) and may be directly manipulated by
 101                 clients.</para>
 102
 103                 <para>A couple of macros are defined to denote and
 104                 decode 128 bit IDs:</para>
 105
 106                 <para><function>SD_ID128_MAKE()</function> may be used
 107                 to write a 128 bit ID in source code. A commonly used
 108                 idiom is to give 128 bit IDs names using this macro:</para>
 109
 110                 <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>
 111
 112                 <para><function>SD_ID128_FORMAT_STR</function> and
 113                 <function>SD_ID128_FORMAT_VAL()</function> may be used
 114                 to format a 128 bit ID in a
 115                 <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 116                 format string, as shown in the following
 117                 example:</para>
 118
 119                 <programlisting>int main(int argc, char *argv[]) {
 120         sd_id128_t id;
 121         id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 122         printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 123         return 0;
 124 }</programlisting>
 125
 126                 <para>Use <function>sd_id128_equal()</function> to compare two 128 bit IDs:</para>
 127
 128                 <programlisting>int main(int argc, char *argv[]) {
 129         sd_id128_t a, b, c;
 130         a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 131         b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 132         c = a;
 133         assert(sd_id128_equal(a, c));
 134         assert(!sd_id128_equal(a, b));
 135         return 0;
 136 }</programlisting>
 137
 138                 <para>Note that new, randomized IDs may be generated
 139                 with
 140                 <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 141                 <literal>--new-id</literal> option.</para>
 142         </refsect1>
 143
 144         <refsect1>
 145                 <title>Notes</title>
 146
 147                 <para>These APIs are implemented as a shared library,
 148                 which can be compiled and linked to with the
 149                 <literal>libsystemd-id128</literal>
 150                 <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 151                 file.</para>
 152
 153         </refsect1>
 154
 155         <refsect1>
 156                 <title>See Also</title>
 157                 <para>
 158                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 159                         <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 160                         <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 161                         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 162                         <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 163                         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 164                         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 165                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 166                         <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 167                 </para>
 168         </refsect1>
 169
 170 </refentry>