<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*--> <!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_default"> <refentryinfo> <title>sd_bus_default</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_default</refentrytitle> <manvolnum>3</manvolnum> </refmeta> <refnamediv> <refname>sd_bus_default</refname> <refname>sd_bus_default_user</refname> <refname>sd_bus_default_system</refname> <refname>sd_bus_open</refname> <refname>sd_bus_open_user</refname> <refname>sd_bus_open_system</refname> <refname>sd_bus_open_system_remote</refname> <refname>sd_bus_open_system_machine</refname> <refpurpose>Acquire a connection to a system or user bus</refpurpose> </refnamediv> <refsynopsisdiv> <funcsynopsis> <funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo> <funcprototype> <funcdef>int <function>sd_bus_default</function></funcdef> <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> <funcprototype> <funcdef>int <function>sd_bus_open</function></funcdef> <paramdef>sd_bus **<parameter>bus</parameter></paramdef> </funcprototype> <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>sd_bus **<parameter>bus</parameter></paramdef> <paramdef>const char *<parameter>host</parameter></paramdef> </funcprototype> <funcprototype> <funcdef>int <function>sd_bus_open_system_machine</function></funcdef> <paramdef>sd_bus **<parameter>bus</parameter></paramdef> <paramdef>const char *<parameter>machine</parameter></paramdef> </funcprototype> </funcsynopsis> </refsynopsisdiv> <refsect1> <title>Description</title> <para><function>sd_bus_default()</function> acquires a bus connection object to the user bus when invoked in user context, or to the system bus otherwise. The connection object is associated with the calling thread. Each time the function is invoked from the same thread, the same object is returned, but its reference count is increased by one, as long as at least one reference is kept. When the last reference to the connection is dropped (using the <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry> call), the connection is terminated. Note that the connection is not automatically terminated when the associated thread ends. It is important to drop the last reference to the bus connection explicitly before the thread ends, as otherwise, the connection will leak. Also, queued but unread or unwritten messages keep the bus referenced, see below.</para> <para><function>sd_bus_default_user()</function> returns a user bus connection object associated with the calling thread. <function>sd_bus_default_system()</function> is similar, but connects to the system bus. Note that <function>sd_bus_default()</function> is identical to these two calls, depending on the execution context.</para> <para><function>sd_bus_open()</function> creates a new, independent bus connection to the user bus when invoked in user context, or the system bus otherwise. <function>sd_bus_open_user()</function> is similar, but connects only to the user bus. <function>sd_bus_open_system()</function> does the same, but connects to the system bus. In contrast to <function>sd_bus_default()</function>, <function>sd_bus_default_user()</function>, and <function>sd_bus_default_system()</function>, these calls return new, independent connection objects that are not associated with the invoking thread and are not shared between multiple invocations. It is recommended to share connections per thread to efficiently make use the available resources. Thus, it is recommended to use <function>sd_bus_default()</function>, <function>sd_bus_default_user()</function> and <function>sd_bus_default_system()</function> to connect to the user or system buses.</para> <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment variable is set (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>), it will be used as the address of the user bus. This variable can contain multiple addresses separated by <literal>;</literal>. If this variable is not set, a suitable default for the default user D-Bus instance will be used.</para> <para>If the <varname>$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 <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>. <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_machine()</function> connects to the system bus in the specified <parameter>machine</parameter>, where <parameter>machine</parameter> is the name of a local container. See <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for more information about the "machine" concept. Note that connections into local containers are only available to privileged processes at this time.</para> <para>These calls allocate a bus connection object and initiate the connection to a well-known bus of some form. An alternative to using these high-level calls is to create an unconnected bus object with <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry> and to connect it with <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>. </para> </refsect1> <refsect1> <title>Reference ownership</title> <para>The functions <function>sd_bus_open()</function>, <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 connection 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>The functions <function>sd_bus_default()</function>, <function>sd_bus_default_user()</function> and <function>sd_bus_default_system()</function> do not necessarily create a new object, but increase the connection reference of an existing connection object by one. Use <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry> to drop the reference.</para> <para>Queued but unwritten/unread messages also keep a reference to their bus connection object. For this reason, even if an application dropped all references to a bus connection, it might not get destroyed right away. Until all incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close the bus connection. When using the default bus logic, it is a good idea to first invoke <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process terminates, and thus its bus connection object should be freed.</para> <para>The life cycle of the default bus connection should be the responsibility of the code that creates/owns the thread the default bus connection object is associated with. Library code should neither call <function>sd_bus_flush()</function> nor <function>sd_bus_close()</function> on default bus objects unless it does so in its own private, self-allocated thread. Library code should not use the default bus object in other threads unless it is clear that the program using it will life cycle the bus connection object and flush and close it before exiting from the thread. In libraries where it is not clear that the calling program will life cycle the bus connection object, it is hence recommended to use <function>sd_bus_open_system()</function> instead of <function>sd_bus_default_system()</function> and related calls.</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>Errors</title> <para>Returned errors may indicate the following problems:</para> <variablelist> <varlistentry> <term><constant>-EINVAL</constant></term> <listitem><para>The specified parameters are invalid.</para></listitem> </varlistentry> <varlistentry> <term><constant>-ENOMEM</constant></term> <listitem><para>Memory allocation failed.</para></listitem> </varlistentry> <varlistentry> <term><constant>-ESOCKTNOSUPPORT</constant></term> <listitem><para>The protocol version required to connect to the selected bus is not supported.</para></listitem> </varlistentry> </variablelist> <para>In addition, any further connection-related errors may be by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para> </refsect1> <refsect1> <title>Notes</title> <para><function>sd_bus_open_user()</function> and the other functions described here are available as a shared library, which can be compiled and linked to with the <constant>libsystemd</constant> <citerefentry project='die-net'><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 project='die-net'><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>