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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"

"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<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_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 &lt;systemd/sd-id128.h&gt;</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="http://tools.ietf.org/html/rfc4122">RFC

4122</ulink>, though use a simpler string

formatting. 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 write a 128 bit ID in source code. A commonly used

idiom is to give 128 bit IDs names 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_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

<literal>--new-id</literal> 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>