man: document a batch of bus functions

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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 2014 Zbigniew Jędrzejewski-Szmek
+
+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_bus_open_user">
+
+  <refentryinfo>
+    <title>sd_bus_open_user</title>
+    <productname>systemd</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>A monkey with a typewriter</contrib>
+        <firstname>Zbigniew</firstname>
+        <surname>Jędrzejewski-Szmek</surname>
+        <email>zbyszek@in.waw.pl</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_bus_open_user</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_bus_open_user</refname>
+    <refname>sd_bus_open_system</refname>
+    <refname>sd_bus_open_system_remote</refname>
+    <refname>sd_bus_open_system_container</refname>
+
+    <refname>sd_bus_default_user</refname>
+    <refname>sd_bus_default_system</refname>
+
+    <refpurpose>Open a connection to the system or user bus</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_open_user</function></funcdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_open_system</function></funcdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
+        <paramdef>const char *<parameter>host</parameter></paramdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_open_system_container</function></funcdef>
+        <paramdef>const char *<parameter>machine</parameter></paramdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_default_user</function></funcdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_default_system</function></funcdef>
+        <paramdef>sd_bus** <parameter>bus</parameter></paramdef>
+      </funcprototype>
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_bus_open_user()</function> creates a new bus
+    object and opens a connection to the the user bus.
+    <function>sd_bus_open_system()</function> does the same, but
+    connects to the system bus.</para>
+
+    <para>If <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
+    variable is set
+    (c.f. <citerefentry><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
+    it will be used as the address of the user bus. This variable can
+    contain multiple addresses separated by <literal>;</literal>. If
+    this variable is not set, a suitable default for the default user
+    D-Bus instance will be used.</para>
+
+    <para>If <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname> environment
+    variable is set, it will be used as the address of the system
+    bus. This variable uses the same syntax as
+    <varname>$DBUS_SESSION_BUS_ADDRESS</varname>/. If this variable is
+    not set, a suitable default for the default system D-Bus instance
+    will be used.</para>
+
+    <para><function>sd_bus_open_system_remote()</function> connects to
+    the system bus on the specified <parameter>host</parameter> using
+    SSH. <parameter>host</parameter> consists of an optional user name
+    followed by the <literal>@</literal> symbol, and the hostname.
+    </para>
+
+    <para><function>sd_bus_open_system_remote()</function> connects to
+    the system bus in the specified <parameter>machine</parameter>,
+    where <parameter>machine</parameter> is the name of a container.
+    See
+    <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    for more information about "machines".</para>
+
+    <para><function>sd_bus_default_user()</function> returns a bus
+    object connected to the user bus. This bus object is shared
+    per-thread. It is created on the first invocation of
+    <function>sd_bus_default_user()</function>, and subsequent
+    invocations returns a reference to the same object.</para>
+
+    <para><function>sd_bus_default_system()</function> is similar to
+    <function>sd_bus_default_user()</function>, but connects to the
+    system bus.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>On success, these calls return 0 or a positive
+    integer. On failure, these calls return a negative
+    errno-style error code.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Reference ownership</title>
+    <para>Functions <function>sd_bus_open_user()</function>,
+    <function>sd_bus_open_system()</function>,
+    <function>sd_bus_open_system_remote()</function>, and
+    <function>sd_bus_open_system_machine()</function> return a new
+    object and the caller owns the sole reference. When not needed
+    anymore, this reference should be destroyed with
+    <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+    </para>
+
+    <para>Functions <function>sd_bus_default_user()</function> and
+    <function>sd_bus_default_system()</function> do not create a new
+    reference.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Errors</title>
+
+    <para>Returned errors may indicate the following problems:</para>
+
+    <variablelist>
+
+      <varlistentry>
+        <term><varname>-EINVAL</varname></term>
+
+        <listitem><para>Specified parameter is invalid
+        (<constant>NULL</constant> in case of output
+        parameters).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>-ENOMEM</varname></term>
+
+        <listitem><para>Memory allocation failed.</para></listitem>
+      </varlistentry>
+
+      <para>In addition, any further connection-related errors may be
+      by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes</title>
+
+    <para><function>sd_bus_open_user()</function> and other functions
+    described here are available as a shared library, which can be
+    compiled and linked to with the
+    <constant>libsystemd-bus</constant> <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-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>