"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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+ SPDX-License-Identifier: LGPL-2.1+
+
This file is part of systemd.
Copyright 2014 Zbigniew Jędrzejewski-Szmek
<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
- to 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
+ 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 or otherwise the connection will
- be leaked.</para>
+ 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 to the calling thread.
+ 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
<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>,
- <function>sd_bus_default_system()</function> these calls return
+ <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>,
+ <function>sd_bus_default_user()</function> and
<function>sd_bus_default_system()</function> to connect to the
user or system buses.</para>
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_remote()</function> connects to the system bus on
+ the specified host 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, optionally followed by a
+ <literal>:</literal> and a machine name. If the machine name is given, a connection
+ is created to the system bus in the specified container on the remote machine, and
+ otherwise a connection to the system bus on the specified host is created.</para>
- <para><function>sd_bus_open_system_container()</function> connects
+ <para>Note that entering a container is a privileged operation, and will likely only
+ work for the root user on the remote machine.</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
processes at this time.</para>
<para>These calls allocate a bus connection object and initiate
- the connection ot a well-known bus of some form. An alternative to
+ 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>
</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>The functions <function>sd_bus_open()</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
+ 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 by
- one. Use
+ 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>
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