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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2012 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="sd-id128">
<refentryinfo>
<title>sd-id128</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>sd-id128</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd-id128</refname>
<refname>sd_id128_t</refname>
<refname>SD_ID128_MAKE</refname>
<refname>SD_ID128_CONST_STR</refname>
<refname>SD_ID128_FORMAT_STR</refname>
<refname>SD_ID128_FORMAT_VAL</refname>
<refname>sd_id128_equal</refname>
<refpurpose>APIs for processing 128-bit IDs</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-id128.h></funcsynopsisinfo>
</funcsynopsis>
<cmdsynopsis>
<command>pkg-config --cflags --libs libsystemd-id128</command>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><filename>sd-id128.h</filename> provides APIs to
process and generate 128-bit ID values. The 128-bit ID
values processed and generated by these APIs are a
generalization of OSF UUIDs as defined by <ulink
url="https://tools.ietf.org/html/rfc4122">RFC
4122</ulink> but use a simpler string
format. These functions impose no structure on the
used IDs, much unlike OSF UUIDs or Microsoft GUIDs,
but are fully compatible with those types of IDs.
</para>
<para>See
<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
for more information about the implemented
functions.</para>
<para>A 128-bit ID is implemented as the following
union type:</para>
<programlisting>typedef union sd_id128 {
uint8_t bytes[16];
uint64_t qwords[2];
} sd_id128_t;</programlisting>
<para>This union type allows accessing the 128-bit ID
as 16 separate bytes or two 64-bit words. It is generally
safer to access the ID components by their 8-bit array
to avoid endianness issues. This union is intended to
be passed call-by-value (as opposed to
call-by-reference) and may be directly manipulated by
clients.</para>
<para>A couple of macros are defined to denote and
decode 128-bit IDs:</para>
<para><function>SD_ID128_MAKE()</function> may be used
to denote a constant 128-bit ID in source code. A
commonly used idiom is to assign a name to a 128-bit
ID using this macro:</para>
<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>
<para><function>SD_ID128_CONST_STR()</function> may be
used to convert constant 128-bit IDs into constant
strings for output. The following example code will
output the string
"fc2e22bc6ee647b6b90729ab34a250b1":</para>
<programlisting>int main(int argc, char *argv[]) {
puts(SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
}</programlisting>
<para><function>SD_ID128_FORMAT_STR</function> and
<function>SD_ID128_FORMAT_VAL()</function> may be used
to format a 128-bit ID in a
<citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
format string, as shown in the following
example:</para>
<programlisting>int main(int argc, char *argv[]) {
sd_id128_t id;
id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
return 0;
}</programlisting>
<para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
<programlisting>int main(int argc, char *argv[]) {
sd_id128_t a, b, c;
a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
c = a;
assert(sd_id128_equal(a, c));
assert(!sd_id128_equal(a, b));
return 0;
}</programlisting>
<para>Note that new, randomized IDs may be generated
with
<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<option>--new-id</option> option.</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>These APIs are implemented as a shared library,
which can be compiled and linked to with the
<literal>libsystemd-id128</literal> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
file.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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