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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd-id128"
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd-id128</title>
<productname>systemd</productname>
</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_MAKE_STR</refname>
<refname>SD_ID128_NULL</refname>
<refname>SD_ID128_CONST_STR</refname>
<refname>SD_ID128_FORMAT_STR</refname>
<refname>SD_ID128_UUID_FORMAT_STR</refname>
<refname>SD_ID128_FORMAT_VAL</refname>
<refname>sd_id128_equal</refname>
<refname>sd_id128_is_null</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</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_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
bytes.</para>
<para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
<type>const char*</type> expression that can be conveniently used in message formats and such:</para>
<programlisting>#include <stdio.h>
#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)
int main(int argc, char **argv) {
puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
}
</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("Match for coredumps: %s", 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 project='man-pages'><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><function>SD_ID128_UUID_FORMAT_STR()</function> is similar to
<function>SD_ID128_FORMAT_STR()</function> but includes separating hyphens to conform to the
"<ulink url="https://en.wikipedia.org/wiki/Universally_unique_identifier#Format">canonical representation</ulink>".
</para>
<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>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
<programlisting>int main(int argc, char *argv[]) {
assert(sd_id128_is_null(SD_ID128_NULL));
}</programlisting>
<para>Note that new, randomized IDs may be generated with
<citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<command>new</command> command.</para>
</refsect1>
<xi:include href="libsystemd-pkgconfig.xml" />
<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 project='man-pages'><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 project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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