man/sd-id128.xml

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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 process and
  72     generate 128-bit ID values. The 128-bit ID values processed and
  73     generated by these APIs are a generalization of OSF UUIDs as
  74     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  75     4122</ulink> but use a simpler string format. These functions
  76     impose no structure on the used IDs, much unlike OSF UUIDs or
  77     Microsoft GUIDs, 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>
  83     and
  84     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  85     for more information about the implemented 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 as 16
  96     separate bytes or two 64-bit words. It is generally safer to
  97     access the ID components by their 8-bit array to avoid endianness
  98     issues. This union is intended to be passed call-by-value (as
  99     opposed to call-by-reference) and may be directly manipulated by
 100     clients.</para>
 101
 102     <para>A couple of macros are defined to denote and decode 128-bit
 103     IDs:</para>
 104
 105     <para><function>SD_ID128_MAKE()</function> may be used to denote a
 106     constant 128-bit ID in source code. A commonly used idiom is to
 107     assign a name to a 128-bit ID 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_CONST_STR()</function> may be used to
 112     convert constant 128-bit IDs into constant strings for output. The
 113     following example code will output the string
 114     "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 115     <programlisting>int main(int argc, char *argv[]) {
 116   puts(SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 117 }</programlisting>
 118
 119     <para><function>SD_ID128_FORMAT_STR</function> and
 120     <function>SD_ID128_FORMAT_VAL()</function> may be used to format a
 121     128-bit ID in a
 122     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 123     format string, as shown in the following example:</para>
 124
 125     <programlisting>int main(int argc, char *argv[]) {
 126   sd_id128_t id;
 127   id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 128   printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 129   return 0;
 130 }</programlisting>
 131
 132     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 133
 134     <programlisting>int main(int argc, char *argv[]) {
 135   sd_id128_t a, b, c;
 136   a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 137   b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 138   c = a;
 139   assert(sd_id128_equal(a, c));
 140   assert(!sd_id128_equal(a, b));
 141   return 0;
 142 }</programlisting>
 143
 144     <para>Note that new, randomized IDs may be generated with
 145     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 146     <option>--new-id</option> option.</para>
 147   </refsect1>
 148
 149   <xi:include href="libsystemd-pkgconfig.xml" />
 150
 151   <refsect1>
 152     <title>See Also</title>
 153     <para>
 154       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 155       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 156       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 157       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 158       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 159       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 160       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 161       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 162       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 163     </para>
 164   </refsect1>
 165
 166 </refentry>