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<refentry id="sd_bus_default" xmlns:xi="">
<refpurpose>Acquire a connection to a system or user bus</refpurpose>
<funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
<funcdef>int <function>sd_bus_default</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_default_user</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_default_system</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_open</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_open_with_description</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>description</parameter></paramdef>
<funcdef>int <function>sd_bus_open_user</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_open_user_with_description</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>description</parameter></paramdef>
<funcdef>int <function>sd_bus_open_user_machine</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>machine</parameter></paramdef>
<funcdef>int <function>sd_bus_open_system</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<funcdef>int <function>sd_bus_open_system_with_description</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>description</parameter></paramdef>
<funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>host</parameter></paramdef>
<funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
<paramdef>sd_bus **<parameter>bus</parameter></paramdef>
<paramdef>const char *<parameter>machine</parameter></paramdef>
<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
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_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>
<function>sd_bus_open_user_with_description()</function>, and
<function>sd_bus_open_system_with_description()</function> are similar to
<function>sd_bus_open()</function>, <function>sd_bus_open_user()</function>, and
<function>sd_bus_open_system()</function>, but allow a description string to be set, see
<parameter>description</parameter> may be <constant>NULL</constant>, in which case this function
is equivalent to <function>sd_bus_open()</function>. This description string is used in log
messages about the bus object, and including a "name" for the bus makes them easier to
understand. Some messages are emitted during bus initialization, hence using this function is
preferable to setting the description later with
<function>sd_bus_open_with_description()</function>. The argument is copied internally and will
not be referenced after the function returns.</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 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 port, 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>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,
possibly prefixed by a user name and a separating <literal>@</literal>. If the container name is
specified as the special string <literal>.host</literal> the connection is made to the local system. This
is useful to connect to the local system bus as specific user, e.g. <literal></literal> to
connect to the local system bus as local user <literal>foobar</literal>. If the <literal>@</literal>
syntax is used either the left-hand side or the right-hand side may be omitted (but not both) in which
case the local user name or <literal>.host</literal> is implied. If the <literal>@</literal> syntax is
not used the connection is always made as root user. See
for a description of the address syntax, and
<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><function>sd_bus_open_user_machine()</function> is similar to
<function>sd_bus_open_system_machine()</function>, but connects to the user bus of the root user, or if
the <literal>@</literal> syntax is used, of the specified user.</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
and to connect it with
<title>Reference ownership</title>
<para>The functions <function>sd_bus_open()</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
<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
to drop the reference.</para>
<para>Queued but unwritten/unread messages 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>Normally, slot objects (as created by
<citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
explicitly with
though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
<parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
that the caller is not directly interested in referencing and managing the bus slot object.</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>
<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>
<para>Returned errors may indicate the following problems:</para>
<listitem><para>The specified parameters are invalid.</para></listitem>
<listitem><para>The requested bus type is not available because of invalid environment (for example
the user session bus is not available because <varname>$XDG_RUNTIME_DIR</varname> is not set).
<listitem><para>Memory allocation failed.</para></listitem>
<listitem><para>The protocol version required to connect to the selected bus is not
<para>In addition, other connection-related errors may be returned. See
<xi:include href="libsystemd-pkgconfig.xml" />
<title>See Also</title>
<citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,