5d13a15b1d
Double newlines (i.e. one empty lines) are great to structure code. But let's avoid triple newlines (i.e. two empty lines), quadruple newlines, quintuple newlines, …, that's just spurious whitespace. It's an easy way to drop 121 lines of code, and keeps the coding style of our sources a bit tigther.
179 lines
7 KiB
XML
179 lines
7 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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-->
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<refentry id="sd-id128"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd-id128</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd-id128</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd-id128</refname>
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<refname>sd_id128_t</refname>
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<refname>SD_ID128_MAKE</refname>
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<refname>SD_ID128_MAKE_STR</refname>
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<refname>SD_ID128_NULL</refname>
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<refname>SD_ID128_CONST_STR</refname>
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<refname>SD_ID128_FORMAT_STR</refname>
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<refname>SD_ID128_FORMAT_VAL</refname>
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<refname>sd_id128_equal</refname>
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<refname>sd_id128_is_null</refname>
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<refpurpose>APIs for processing 128-bit IDs</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-id128.h></funcsynopsisinfo>
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</funcsynopsis>
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<cmdsynopsis>
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<command>pkg-config --cflags --libs libsystemd</command>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>sd-id128.h</filename> provides APIs to process and
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generate 128-bit ID values. The 128-bit ID values processed and
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generated by these APIs are a generalization of OSF UUIDs as
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defined by <ulink url="https://tools.ietf.org/html/rfc4122">RFC
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4122</ulink> but use a simpler string format. These functions
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impose no structure on the used IDs, much unlike OSF UUIDs or
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Microsoft GUIDs, but are fully compatible with those types of IDs.
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</para>
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<para>See
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<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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for more information about the implemented functions.</para>
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<para>A 128-bit ID is implemented as the following
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union type:</para>
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<programlisting>typedef union sd_id128 {
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uint8_t bytes[16];
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uint64_t qwords[2];
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} sd_id128_t;</programlisting>
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<para>This union type allows accessing the 128-bit ID as 16
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separate bytes or two 64-bit words. It is generally safer to
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access the ID components by their 8-bit array to avoid endianness
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issues. This union is intended to be passed call-by-value (as
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opposed to call-by-reference) and may be directly manipulated by
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clients.</para>
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<para>A couple of macros are defined to denote and decode 128-bit
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IDs:</para>
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<para><function>SD_ID128_MAKE()</function> may be used to denote a
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constant 128-bit ID in source code. A commonly used idiom is to
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assign a name to a 128-bit ID using this macro:</para>
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<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>
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<para><function>SD_ID128_NULL</function> may be used to refer to the 128bit ID consisting of only NUL
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bytes.</para>
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<para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
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<type>const char*</type> expression that can be conveniently used in message formats and such:</para>
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<programlisting>#include <stdio.h>
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#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)
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int main(int argc, char **argv) {
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puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
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}
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</programlisting>
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<para><function>SD_ID128_CONST_STR()</function> may be used to
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convert constant 128-bit IDs into constant strings for output. The
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following example code will output the string
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"fc2e22bc6ee647b6b90729ab34a250b1":</para>
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<programlisting>int main(int argc, char *argv[]) {
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puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
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}</programlisting>
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<para><function>SD_ID128_FORMAT_STR()</function> and
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<function>SD_ID128_FORMAT_VAL()</function> may be used to format a
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128-bit ID in a
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<citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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format string, as shown in the following example:</para>
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<programlisting>int main(int argc, char *argv[]) {
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sd_id128_t id;
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id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
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printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
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return 0;
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}</programlisting>
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<para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
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<programlisting>int main(int argc, char *argv[]) {
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sd_id128_t a, b, c;
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a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
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b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
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c = a;
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assert(sd_id128_equal(a, c));
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assert(!sd_id128_equal(a, b));
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return 0;
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}</programlisting>
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<para>Use <function>sd_id128_is_null()</function> to check if an 128bit ID consists of only NUL bytes:</para>
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<programlisting>int main(int argc, char *argv[]) {
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assert(sd_id128_is_null(SD_ID128_NULL));
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}</programlisting>
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<para>Note that new, randomized IDs may be generated with
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
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<option>--new-id128</option> option.</para>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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