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<refentry id="sd_bus_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_bus_get_fd</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_bus_get_fd</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_get_fd</refname>
    <refname>sd_bus_set_fd</refname>
    <refname>sd_bus_get_events</refname>
    <refname>sd_bus_get_timeout</refname>

    <refpurpose>Get the file descriptor, I/O events and time-out to wait for from a message bus object</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_bus_get_fd</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_set_fd</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
        <paramdef>int <parameter>input_fd</parameter></paramdef>
        <paramdef>int <parameter>output_fd</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_get_events</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_get_timeout</function></funcdef>
        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
        <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_bus_get_fd()</function> returns the file descriptor used to communicate from a message bus
    object. This descriptor can be used with <citerefentry
    project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry> or a similar
    function to wait for I/O events on the specified bus connection object. If the bus object was configured with the
    <function>sd_bus_set_fd()</function> function, then the <parameter>input_fd</parameter> file descriptor used in
    that call is returned.</para>

    <para><function>sd_bus_set_fd()</function> sets the file descriptors used to communicate from a message bus
    object. Both <parameter>input_fd</parameter> and <parameter>output_fd</parameter> must be valid file descriptors.
    The same file descriptor may be used as both the input and the output file descriptor. This function must be called
    before the bus is started.</para>

    <para><function>sd_bus_get_events()</function> returns the I/O events to wait for, suitable for passing to
    <function>poll()</function> or a similar call. Returns a combination of <constant>POLLIN</constant>,
    <constant>POLLOUT</constant>, … events, or negative on error.</para>

    <para><function>sd_bus_get_timeout()</function> returns the time-out in µs to pass to to
    <function>poll()</function> or a similar call when waiting for events on the specified bus connection. The returned
    time-out may be zero, in which case a subsequent I/O polling call should be invoked in non-blocking mode. The
    returned timeout may be <constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely,
    without any applied time-out. Note that the returned time-out should be considered only a maximum sleeping time. It
    is permissible (and even expected) that shorter time-outs are used by the calling program, in case other event
    sources are polled in the same event loop. Note that the returned time-value is relative and specified in
    microseconds. When converting this value in order to pass it as third argument to <function>poll()</function>
    (which expects milliseconds), care should be taken to use a division that rounds up to ensure the I/O polling
    operation doesn't sleep for shorter than necessary, which might result in unintended busy looping (alternatively,
    use <citerefentry project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    instead of plain <function>poll()</function>, which understands time-outs with nano-second granularity).</para>

    <para>These three functions are useful to hook up a bus connection object with an external or manual event loop
    involving <function>poll()</function> or a similar I/O polling call. Before each invocation of the I/O polling
    call, all three functions should be invoked: the file descriptor returned by <function>sd_bus_get_fd()</function>
    should be polled for the events indicated by <function>sd_bus_get_events()</function>, and the I/O call should
    block for that up to the time-out returned by <function>sd_bus_get_timeout()</function>. After each I/O polling
    call the bus connection needs to process incoming or outgoing data, by invoking
    <citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    <para>Note that these function are only one of three supported ways to implement I/O event handling for bus
    connections. Alternatively use
    <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry> to attach a
    bus connection to an <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    event loop. Or use <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    as a simple synchronous, blocking I/O waiting call.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, <function>sd_bus_get_fd()</function> returns the file descriptor used for communication. On failure,
    it returns a negative errno-style error code.</para>

    <para>On success, <function>sd_bus_set_fd()</function> returns a non-negative integer. On failure, it returns a
    negative errno-style error code.</para>

    <para>On success, <function>sd_bus_get_events()</function> returns the I/O event mask to use for I/O event watching.
    On failure, it returns a negative errno-style error code.</para>

    <para>On success, <function>sd_bus_get_timeout()</function> returns a non-negative integer. On failure, it returns a
    negative errno-style error code.</para>

    <refsect2>
      <title>Errors</title>

      <para>Returned errors may indicate the following problems:</para>

      <variablelist>
        <varlistentry>
          <term><constant>-EINVAL</constant></term>

          <listitem><para>An invalid bus object was passed.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ECHILD</constant></term>

          <listitem><para>The bus connection was allocated in a parent process and is being reused in a child
          process after <function>fork()</function>.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENOTCONN</constant></term>

          <listitem><para>The bus connection has been terminated.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-EPERM</constant></term>

          <listitem><para>Two distinct file descriptors were passed for input and output using
          <function>sd_bus_set_fd()</function>, which <function>sd_bus_get_fd()</function> cannot
          return.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-EBADF</constant></term>

          <listitem><para>An invalid file descriptor was passed to <function>sd_bus_set_fd()</function>.
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-ENOPKG</constant></term>

          <listitem><para>The bus cannot be resolved.</para></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <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_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
