man/sd-id128.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="sd-id128"
   7   xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>sd-id128</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>sd-id128</refentrytitle>
  16     <manvolnum>3</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>sd-id128</refname>
  21     <refname>sd_id128_t</refname>
  22     <refname>SD_ID128_MAKE</refname>
  23     <refname>SD_ID128_MAKE_STR</refname>
  24     <refname>SD_ID128_NULL</refname>
  25     <refname>SD_ID128_CONST_STR</refname>
  26     <refname>SD_ID128_FORMAT_STR</refname>
  27     <refname>SD_ID128_FORMAT_VAL</refname>
  28     <refname>sd_id128_equal</refname>
  29     <refname>sd_id128_is_null</refname>
  30     <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  31   </refnamediv>
  32
  33   <refsynopsisdiv>
  34     <funcsynopsis>
  35       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  36     </funcsynopsis>
  37
  38     <cmdsynopsis>
  39       <command>pkg-config --cflags --libs libsystemd</command>
  40     </cmdsynopsis>
  41
  42   </refsynopsisdiv>
  43
  44   <refsect1>
  45     <title>Description</title>
  46
  47     <para><filename>sd-id128.h</filename> provides APIs to process and
  48     generate 128-bit ID values. The 128-bit ID values processed and
  49     generated by these APIs are a generalization of OSF UUIDs as
  50     defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
  51     4122</ulink> but use a simpler string format. These functions
  52     impose no structure on the used IDs, much unlike OSF UUIDs or
  53     Microsoft GUIDs, but are fully compatible with those types of IDs.
  54     </para>
  55
  56     <para>See
  57     <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  58     <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  59     and
  60     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  61     for more information about the implemented functions.</para>
  62
  63     <para>A 128-bit ID is implemented as the following
  64     union type:</para>
  65
  66     <programlisting>typedef union sd_id128 {
  67         uint8_t bytes[16];
  68         uint64_t qwords[2];
  69 } sd_id128_t;</programlisting>
  70
  71     <para>This union type allows accessing the 128-bit ID as 16
  72     separate bytes or two 64-bit words. It is generally safer to
  73     access the ID components by their 8-bit array to avoid endianness
  74     issues. This union is intended to be passed call-by-value (as
  75     opposed to call-by-reference) and may be directly manipulated by
  76     clients.</para>
  77
  78     <para>A couple of macros are defined to denote and decode 128-bit
  79     IDs:</para>
  80
  81     <para><function>SD_ID128_MAKE()</function> may be used to denote a
  82     constant 128-bit ID in source code. A commonly used idiom is to
  83     assign a name to a 128-bit ID using this macro:</para>
  84
  85     <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>
  86
  87     <para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
  88     bytes.</para>
  89
  90     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
  91     <type>const char*</type> expression that can be conveniently used in message formats and such:</para>
  92
  93     <programlisting>#include &lt;stdio.h&gt;
  94 #define SD_MESSAGE_COREDUMP_STR SD_ID128_MAKE_STR(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)
  95
  96 int main(int argc, char **argv) {
  97         puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
  98 }
  99     </programlisting>
 100
 101     <para><function>SD_ID128_CONST_STR()</function> may be used to
 102     convert constant 128-bit IDs into constant strings for output. The
 103     following example code will output the string
 104     "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 105     <programlisting>int main(int argc, char *argv[]) {
 106         puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 107 }</programlisting>
 108
 109     <para><function>SD_ID128_FORMAT_STR()</function> and
 110     <function>SD_ID128_FORMAT_VAL()</function> may be used to format a
 111     128-bit ID in a
 112     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 113     format string, as shown in the following example:</para>
 114
 115     <programlisting>int main(int argc, char *argv[]) {
 116         sd_id128_t id;
 117         id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 118         printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 119         return 0;
 120 }</programlisting>
 121
 122     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 123
 124     <programlisting>int main(int argc, char *argv[]) {
 125         sd_id128_t a, b, c;
 126         a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 127         b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 128         c = a;
 129         assert(sd_id128_equal(a, c));
 130         assert(!sd_id128_equal(a, b));
 131         return 0;
 132 }</programlisting>
 133
 134     <para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
 135
 136     <programlisting>int main(int argc, char *argv[]) {
 137         assert(sd_id128_is_null(SD_ID128_NULL));
 138 }</programlisting>
 139
 140     <para>Note that new, randomized IDs may be generated with
 141     <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 142     <command>new</command> command.</para>
 143   </refsect1>
 144
 145   <xi:include href="libsystemd-pkgconfig.xml" />
 146
 147   <refsect1>
 148     <title>See Also</title>
 149     <para>
 150       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 151       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 152       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 153       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 154       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 155       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 156       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 157       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 159     </para>
 160   </refsect1>
 161
 162 </refentry>