man: sd_unref does not necessarily free the bus

[thirdparty/systemd.git] / man / sd_bus_default.xml

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<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 &lt;systemd/sd-bus.h&gt;</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
    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
    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>

    <para><function>sd_bus_default_user()</function> returns a user
    bus connection object associated to 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>,
    <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_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>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_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>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
    <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    to drop the reference.</para>

    <para>Queued messages also keep a reference to the bus. For this reason, just
    because no application is having a reference to the bus does not mean that
    the bus object will be destroyed. Until all the messages are sent, the bus object
    will stay alive. <function>sd_bus_flush</function> can be used to send all the
    queued messages so they drop their references.</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>