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-or-later -->
   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_ALLF</refname>
  22     <refname>SD_ID128_CONST_STR</refname>
  23     <refname>SD_ID128_FORMAT_STR</refname>
  24     <refname>SD_ID128_FORMAT_VAL</refname>
  25     <refname>SD_ID128_MAKE</refname>
  26     <refname>SD_ID128_MAKE_STR</refname>
  27     <refname>SD_ID128_MAKE_UUID_STR</refname>
  28     <refname>SD_ID128_NULL</refname>
  29     <refname>SD_ID128_UUID_FORMAT_STR</refname>
  30     <refname>sd_id128_equal</refname>
  31     <refname>sd_id128_in_set</refname>
  32     <refname>sd_id128_in_set_sentinel</refname>
  33     <refname>sd_id128_in_setv</refname>
  34     <refname>sd_id128_is_allf</refname>
  35     <refname>sd_id128_is_null</refname>
  36     <refname>sd_id128_t</refname>
  37     <refpurpose>APIs for processing 128-bit IDs</refpurpose>
  38   </refnamediv>
  39
  40   <refsynopsisdiv>
  41     <funcsynopsis>
  42       <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
  43     </funcsynopsis>
  44
  45     <cmdsynopsis>
  46       <command>pkg-config --cflags --libs libsystemd</command>
  47     </cmdsynopsis>
  48
  49   </refsynopsisdiv>
  50
  51   <refsect1>
  52     <title>Description</title>
  53
  54     <para><filename>sd-id128.h</filename> provides APIs to process and generate 128-bit ID values. The
  55     128-bit ID values processed and generated by these APIs are a generalization of OSF UUIDs as defined by
  56     <ulink url="https://tools.ietf.org/html/rfc4122">RFC 4122</ulink> but use a simpler string format. These
  57     functions impose no structure on the used IDs, much unlike OSF UUIDs or Microsoft GUIDs, but are mostly
  58     compatible with those types of IDs.
  59     </para>
  60
  61     <para>See
  62     <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  63     <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  64     and
  65     <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  66     for more information about the implemented functions.</para>
  67
  68     <para>A 128-bit ID is implemented as the following
  69     union type:</para>
  70
  71     <programlisting>typedef union sd_id128 {
  72   uint8_t bytes[16];
  73   uint64_t qwords[2];
  74 } sd_id128_t;</programlisting>
  75
  76     <para>This union type allows accessing the 128-bit ID as 16
  77     separate bytes or two 64-bit words. It is generally safer to
  78     access the ID components by their 8-bit array to avoid endianness
  79     issues. This union is intended to be passed call-by-value (as
  80     opposed to call-by-reference) and may be directly manipulated by
  81     clients.</para>
  82
  83     <para>A couple of macros are defined to denote and decode 128-bit
  84     IDs:</para>
  85
  86     <para><function>SD_ID128_MAKE()</function> may be used to denote a
  87     constant 128-bit ID in source code. A commonly used idiom is to
  88     assign a name to a 128-bit ID using this macro:</para>
  89
  90     <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>
  91
  92     <para><constant>SD_ID128_NULL</constant> may be used to refer to the 128-bit ID consisting of only
  93     <constant>NUL</constant> bytes (i.e. all bits off).</para>
  94
  95     <para><constant>SD_ID128_ALLF</constant> may be used to refer to the 128-bit ID consisting of only
  96     <constant>0xFF</constant> bytes (i.e. all bits on).</para>
  97
  98     <para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
  99     <type>const char*</type> expression that can be conveniently used in message formats and such:</para>
 100
 101     <programlisting>#include &lt;stdio.h&gt;
 102 #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)
 103
 104 int main(int argc, char **argv) {
 105   puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
 106 }</programlisting>
 107
 108     <para><function>SD_ID128_CONST_STR()</function> may be used to
 109     convert constant 128-bit IDs into constant strings for output. The
 110     following example code will output the string
 111     "fc2e22bc6ee647b6b90729ab34a250b1":</para>
 112     <programlisting>int main(int argc, char *argv[]) {
 113   puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
 114 }</programlisting>
 115
 116     <para><constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_FORMAT_VAL()</function> may
 117     be used to format a 128-bit ID in a
 118     <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 119     format string, as shown in the following example:</para>
 120
 121     <programlisting>int main(int argc, char *argv[]) {
 122   sd_id128_t id;
 123   id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 124   printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
 125   return 0;
 126 }</programlisting>
 127
 128     <para><constant>SD_ID128_UUID_FORMAT_STR</constant> and <function>SD_ID128_MAKE_UUID_STR()</function>
 129     are similar to
 130     <constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_MAKE_STR()</function>,
 131     but include separating hyphens to conform to the
 132     "<ulink url="https://en.wikipedia.org/wiki/Universally_unique_identifier#Format">canonical representation</ulink>".
 133     They format the string based on <ulink
 134     url="https://tools.ietf.org/html/rfc4122">RFC4122</ulink> Variant 1 rules, i.e. converting from Big
 135     Endian byte order. This matches behaviour of most other Linux userspace infrastructure. It's probably
 136     best to avoid UUIDs of other variants, in order to avoid unnecessary ambiguities. All 128-bit IDs
 137     generated by the sd-id128 APIs strictly conform to Variant 1 Version 4 UUIDs, as per RFC 4122.</para>
 138
 139     <para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
 140
 141     <programlisting>int main(int argc, char *argv[]) {
 142   sd_id128_t a, b, c;
 143   a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 144   b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
 145   c = a;
 146   assert(sd_id128_equal(a, c));
 147   assert(!sd_id128_equal(a, b));
 148   return 0;
 149 }</programlisting>
 150
 151     <para>Use <function>sd_id128_is_null()</function> to check if an 128-bit ID consists of only
 152     <constant>NUL</constant> bytes:</para>
 153
 154     <programlisting>assert(sd_id128_is_null(SD_ID128_NULL));</programlisting>
 155
 156     <para>Similarly, use <function>sd_id128_is_allf()</function> to check if an 128-bit ID consists of only
 157     <constant>0xFF</constant> bytes (all bits on):</para>
 158
 159     <programlisting>assert(sd_id128_is_allf(SD_ID128_ALLF));</programlisting>
 160
 161     <para>For convenience, <function>sd_id128_in_set()</function> takes a list of IDs and
 162     returns true if any are equal to the first argument:</para>
 163
 164     <programlisting>int main(int argc, char *argv[]) {
 165   sd_id12_t a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
 166   assert(sd_id128_in_set(a, a));
 167   assert(sd_id128_in_set(a, a, a));
 168   assert(!sd_id128_in_set(a));
 169   assert(!sd_id128_in_set(a,
 170                           SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e)
 171                           SD_ID128_MAKE(2f,88,28,5f,9c,44,09,9d,d7,15,77,04,bc,85,7e,e3)
 172                           SD_ID128_ALLF));
 173   return 0;
 174 }
 175 </programlisting>
 176
 177     <para><function>sd_id128_in_set()</function> is defined as a macro over
 178     <function>sd_id128_in_set_sentinel()</function>, adding the <constant>SD_ID128_NULL</constant>
 179     sentinel. Since <function>sd_id128_in_set_sentinel()</function> uses <constant>SD_ID128_NULL</constant>
 180     as the sentinel, <constant>SD_ID128_NULL</constant> cannot be otherwise placed in the argument list.
 181     </para>
 182
 183     <para><function>sd_id128_in_setv()</function> is similar to
 184     <function>sd_id128_in_set_sentinel()</function>, but takes a <structname>struct varargs</structname>
 185     argument.</para>
 186
 187     <para>Note that new, randomized IDs may be generated with
 188     <citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
 189     <command>new</command> command.</para>
 190   </refsect1>
 191
 192   <xi:include href="libsystemd-pkgconfig.xml" />
 193
 194   <refsect1>
 195     <title>See Also</title>
 196     <para>
 197       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 198       <citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 199       <citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 200       <citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 201       <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 202       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 203       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 204       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 205       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 206     </para>
 207   </refsect1>
 208
 209 </refentry>