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_NULL</refname>
  51     <refname>SD_ID128_CONST_STR</refname>
  52     <refname>SD_ID128_FORMAT_STR</refname>
  53     <refname>SD_ID128_FORMAT_VAL</refname>
  54     <refname>sd_id128_equal</refname>
  55     <refname>sd_id128_is_null</refname>
  56     <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  57   </refnamediv>
  58
  59   <refsynopsisdiv>
  60     <funcsynopsis>
  61       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  62     </funcsynopsis>
  63
  64     <cmdsynopsis>
  65       <command>pkg-config --cflags --libs libsystemd</command>
  66     </cmdsynopsis>
  67
  68   </refsynopsisdiv>
  69
  70   <refsect1>
  71     <title>Description</title>
  72
  73     <para><filename>sd-id128.h</filename> provides APIs to process and
  74     generate 128-bit ID values. The 128-bit ID values processed and
  75     generated by these APIs are a generalization of OSF UUIDs as
  76     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  77     4122</ulink> but use a simpler string format. These functions
  78     impose no structure on the used IDs, much unlike OSF UUIDs or
  79     Microsoft GUIDs, 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>
  85     and
  86     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  87     for more information about the implemented 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 as 16
  98     separate bytes or two 64-bit words. It is generally safer to
  99     access the ID components by their 8-bit array to avoid endianness
 100     issues. This union is intended to be passed call-by-value (as
 101     opposed to call-by-reference) and may be directly manipulated by
 102     clients.</para>
 103
 104     <para>A couple of macros are defined to denote and decode 128-bit
 105     IDs:</para>
 106
 107     <para><function>SD_ID128_MAKE()</function> may be used to denote a
 108     constant 128-bit ID in source code. A commonly used idiom is to
 109     assign a name to a 128-bit ID using this macro:</para>
 110
 111     <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>
 112
 113     <para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
 114     bytes.</para>
 115
 116     <para><function>SD_ID128_CONST_STR()</function> may be used to
 117     convert constant 128-bit IDs into constant strings for output. The
 118     following example code will 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 to format a
 126     128-bit ID in a
 127     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 128     format string, as shown in the following example:</para>
 129
 130     <programlisting>int main(int argc, char *argv[]) {
 131         sd_id128_t id;
 132         id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 133         printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 134         return 0;
 135 }</programlisting>
 136
 137     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 138
 139     <programlisting>int main(int argc, char *argv[]) {
 140         sd_id128_t a, b, c;
 141         a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 142         b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 143         c = a;
 144         assert(sd_id128_equal(a, c));
 145         assert(!sd_id128_equal(a, b));
 146         return 0;
 147 }</programlisting>
 148
 149     <para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
 150
 151     <programlisting>int main(int argc, char *argv[]) {
 152         assert(sd_id128_is_null(SD_ID128_NULL));
 153 }</programlisting>
 154
 155     <para>Note that new, randomized IDs may be generated with
 156     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 157     <option>--new-id</option> option.</para>
 158   </refsect1>
 159
 160   <xi:include href="libsystemd-pkgconfig.xml" />
 161
 162   <refsect1>
 163     <title>See Also</title>
 164     <para>
 165       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 166       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 167       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 168       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 169       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 170       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 171       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 172       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 173       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 174     </para>
 175   </refsect1>
 176
 177 </refentry>