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b1de39dec8
Logically, this is better, because we're describing a subset of possible return values. Visually this also looks quite good because groff renders refsect2 much less prominently. Also rewrap things, add <constant> in various places, fix some typos.
323 lines
16 KiB
XML
323 lines
16 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="sd_bus_default" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_bus_default</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_bus_default</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_bus_default</refname>
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<refname>sd_bus_default_user</refname>
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<refname>sd_bus_default_system</refname>
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<refname>sd_bus_open</refname>
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<refname>sd_bus_open_with_description</refname>
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<refname>sd_bus_open_user</refname>
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<refname>sd_bus_open_user_with_description</refname>
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<refname>sd_bus_open_system</refname>
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<refname>sd_bus_open_system_with_description</refname>
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<refname>sd_bus_open_system_remote</refname>
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<refname>sd_bus_open_system_machine</refname>
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<refpurpose>Acquire a connection to a system or user bus</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_bus_default</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_default_user</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_default_system</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_with_description</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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<paramdef>const char *<parameter>description</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_user</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_user_with_description</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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<paramdef>const char *<parameter>description</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_system</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_system_with_description</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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<paramdef>const char *<parameter>description</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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<paramdef>const char *<parameter>host</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
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<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
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<paramdef>const char *<parameter>machine</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_bus_default()</function> acquires a bus
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connection object to the user bus when invoked in user context, or
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to the system bus otherwise. The connection object is associated
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with the calling thread. Each time the function is invoked from
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the same thread, the same object is returned, but its reference
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count is increased by one, as long as at least one reference is
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kept. When the last reference to the connection is dropped (using
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the
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<citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call), the connection is terminated. Note that the connection is
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not automatically terminated when the associated thread ends. It
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is important to drop the last reference to the bus connection
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explicitly before the thread ends, as otherwise, the connection will
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leak. Also, queued but unread or unwritten messages keep the
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bus referenced, see below.</para>
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<para><function>sd_bus_default_user()</function> returns a user
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bus connection object associated with the calling thread.
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<function>sd_bus_default_system()</function> is similar, but
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connects to the system bus. Note that
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<function>sd_bus_default()</function> is identical to these two
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calls, depending on the execution context.</para>
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<para><function>sd_bus_open()</function> creates a new,
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independent bus connection to the user bus when invoked in user
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context, or the system bus
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otherwise. <function>sd_bus_open_user()</function> is similar, but
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connects only to the user bus.
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<function>sd_bus_open_system()</function> does the same, but
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connects to the system bus. In contrast to
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<function>sd_bus_default()</function>,
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<function>sd_bus_default_user()</function>, and
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<function>sd_bus_default_system()</function>, these calls return
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new, independent connection objects that are not associated with
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the invoking thread and are not shared between multiple
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invocations. It is recommended to share connections per thread to
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efficiently make use the available resources. Thus, it is
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recommended to use <function>sd_bus_default()</function>,
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<function>sd_bus_default_user()</function> and
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<function>sd_bus_default_system()</function> to connect to the
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user or system buses.</para>
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<para><function>sd_bus_open_with_description()</function>,
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<function>sd_bus_open_user_with_description()</function>, and
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<function>sd_bus_open_system_with_description()</function> are similar to
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<function>sd_bus_open()</function>, <function>sd_bus_open_user()</function>, and
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<function>sd_bus_open_system()</function>, but allow a description string to be set, see
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<citerefentry><refentrytitle>sd_bus_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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<parameter>description</parameter> may be <constant>NULL</constant>, in which case this function
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is equivalent to <function>sd_bus_open()</function>. This description string is used in log
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messages about the bus object, and including a "name" for the bus makes them easier to
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understand. Some messages are emitted during bus initialization, hence using this function is
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prefereable to setting the description later with
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<function>sd_bus_open_with_description()</function>. The argument is copied internally and will
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not be referenced after the function returns.</para>
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<para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
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variable is set
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(cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
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it will be used as the address of the user bus. This variable can
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contain multiple addresses separated by <literal>;</literal>. If
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this variable is not set, a suitable default for the default user
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D-Bus instance will be used.</para>
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<para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
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environment variable is set, it will be used as the address of the
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system bus. This variable uses the same syntax as
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<varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
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not set, a suitable default for the default system D-Bus instance
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will be used.</para>
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<para><function>sd_bus_open_system_remote()</function> connects to the system bus on
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the specified host using
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<citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
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<parameter>host</parameter> consists of an optional user name followed by the
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<literal>@</literal> symbol, and the hostname, optionally followed by a
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<literal>:</literal> and a port, optionally followed by a
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<literal>/</literal> and a machine name. If the machine name is given, a connection
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is created to the system bus in the specified container on the remote machine, and
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otherwise a connection to the system bus on the specified host is created.</para>
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<para>Note that entering a container is a privileged operation, and will likely only
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work for the root user on the remote machine.</para>
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<para><function>sd_bus_open_system_machine()</function> connects
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to the system bus in the specified <parameter>machine</parameter>,
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where <parameter>machine</parameter> is the name of a local
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container. See
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<citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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for more information about the "machine" concept. Note that
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connections into local containers are only available to privileged
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processes at this time.</para>
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<para>These calls allocate a bus connection object and initiate
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the connection to a well-known bus of some form. An alternative to
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using these high-level calls is to create an unconnected bus
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object with
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<citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and to connect it with
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<citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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</para>
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</refsect1>
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<refsect1>
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<title>Reference ownership</title>
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<para>The functions <function>sd_bus_open()</function>,
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<function>sd_bus_open_user()</function>,
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<function>sd_bus_open_system()</function>,
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<function>sd_bus_open_system_remote()</function>, and
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<function>sd_bus_open_system_machine()</function> return a new
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connection object and the caller owns the sole reference. When not
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needed anymore, this reference should be destroyed with
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<citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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</para>
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<para>The functions <function>sd_bus_default()</function>,
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<function>sd_bus_default_user()</function> and
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<function>sd_bus_default_system()</function> do not necessarily
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create a new object, but increase the connection reference of an
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existing connection object by one. Use
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<citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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to drop the reference.</para>
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<para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
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if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
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incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
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alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
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references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
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the bus connection. When using the default bus logic, it is a good idea to first invoke
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<function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
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terminates, and thus its bus connection object should be freed.</para>
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<para>Normally, slot objects (as created by
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<citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
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calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
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its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
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floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
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the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
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explicitly with
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<citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
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<parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
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that the caller is not directly interested in referencing and managing the bus slot object.</para>
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<para>The life cycle of the default bus connection should be the
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responsibility of the code that creates/owns the thread the
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default bus connection object is associated with. Library code
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should neither call <function>sd_bus_flush()</function> nor
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<function>sd_bus_close()</function> on default bus objects unless
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it does so in its own private, self-allocated thread. Library code
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should not use the default bus object in other threads unless it
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is clear that the program using it will life cycle the bus
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connection object and flush and close it before exiting from the
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thread. In libraries where it is not clear that the calling
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program will life cycle the bus connection object, it is hence
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recommended to use <function>sd_bus_open_system()</function>
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instead of <function>sd_bus_default_system()</function> and
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related calls.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, these calls return 0 or a positive
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integer. On failure, these calls return a negative
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errno-style error code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>The specified parameters are invalid.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ENOMEM</constant></term>
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<listitem><para>Memory allocation failed.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ESOCKTNOSUPPORT</constant></term>
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<listitem><para>The protocol version required to connect to the selected bus is not
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supported.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>In addition, any further connection-related errors may be by returned. See
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<citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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</refsect2>
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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-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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