man: document that start limiting of GC'ed units doesn't work (#7337)

[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
    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 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>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
    <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>