man/sd-id128.xml

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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_CONST_STR</refname>
  50                 <refname>SD_ID128_FORMAT_STR</refname>
  51                 <refname>SD_ID128_FORMAT_VAL</refname>
  52                 <refname>sd_id128_equal</refname>
  53                 <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  54         </refnamediv>
  55
  56         <refsynopsisdiv>
  57                 <funcsynopsis>
  58                         <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  59                 </funcsynopsis>
  60
  61                 <cmdsynopsis>
  62                         <command>pkg-config --cflags --libs libsystemd-id128</command>
  63                 </cmdsynopsis>
  64
  65         </refsynopsisdiv>
  66
  67         <refsect1>
  68                 <title>Description</title>
  69
  70                 <para><filename>sd-id128.h</filename> provides APIs to
  71                 process and generate 128-bit ID values. The 128-bit ID
  72                 values processed and generated by these APIs are a
  73                 generalization of OSF UUIDs as defined by <ulink
  74                 url="https://tools.ietf.org/html/rfc4122">RFC
  75                 4122</ulink> but use a simpler string
  76                 format. These functions impose no structure on the
  77                 used IDs, much unlike OSF UUIDs or Microsoft GUIDs,
  78                 but are fully compatible with those types of IDs.
  79                 </para>
  80
  81                 <para>See
  82                 <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  83                 <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
  84                 <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  85                 for more information about the implemented
  86                 functions.</para>
  87
  88                 <para>A 128-bit ID is implemented as the following
  89                 union type:</para>
  90
  91                 <programlisting>typedef union sd_id128 {
  92         uint8_t bytes[16];
  93         uint64_t qwords[2];
  94 } sd_id128_t;</programlisting>
  95
  96                 <para>This union type allows accessing the 128-bit ID
  97                 as 16 separate bytes or two 64-bit words. It is generally
  98                 safer to access the ID components by their 8-bit array
  99                 to avoid endianness issues. This union is intended to
 100                 be passed call-by-value (as opposed to
 101                 call-by-reference) and may be directly manipulated by
 102                 clients.</para>
 103
 104                 <para>A couple of macros are defined to denote and
 105                 decode 128-bit IDs:</para>
 106
 107                 <para><function>SD_ID128_MAKE()</function> may be used
 108                 to denote a constant 128-bit ID in source code. A
 109                 commonly used idiom is to assign a name to a 128-bit
 110                 ID using this macro:</para>
 111
 112                 <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>
 113
 114                 <para><function>SD_ID128_CONST_STR()</function> may be
 115                 used to convert constant 128-bit IDs into constant
 116                 strings for output. The following example code will
 117                 output the string
 118                 "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 119                 <programlisting>int main(int argc, char *argv[]) {
 120         puts(SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 121 }</programlisting>
 122
 123                 <para><function>SD_ID128_FORMAT_STR</function> and
 124                 <function>SD_ID128_FORMAT_VAL()</function> may be used
 125                 to format a 128-bit ID in a
 126                 <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 127                 format string, as shown in the following
 128                 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>Note that new, randomized IDs may be generated
 150                 with
 151                 <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 152                 <option>--new-id</option> option.</para>
 153         </refsect1>
 154
 155         <refsect1>
 156                 <title>Notes</title>
 157
 158                 <para>These APIs are implemented as a shared library,
 159                 which can be compiled and linked to with the
 160                 <literal>libsystemd-id128</literal> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 161                 file.</para>
 162
 163         </refsect1>
 164
 165         <refsect1>
 166                 <title>See Also</title>
 167                 <para>
 168                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 169                         <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 170                         <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 171                         <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 172                         <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 173                         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 174                         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 175                         <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 176                         <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 177                 </para>
 178         </refsect1>
 179
 180 </refentry>