man/sd_bus_get_fd.xml

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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_get_events</refname>
    <refname>sd_bus_get_timeout</refname>

    <refpurpose>Get the file descriptor, I/O events and timeout 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_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_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 timeout in µs to pass to
    <function>poll()</function> or a similar call when waiting for events on the specified bus
    connection. The returned timeout 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 timeout. Note that the returned timeout should be considered only a
    maximum sleeping time. It is permissible (and even expected) that shorter timeouts 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 absolute, based of <constant>CLOCK_MONOTONIC</constant> and specified 
    in microseconds. When converting this value in order to pass it as third argument to 
    <function>poll()</function> (which expects relative milliseconds), care should be taken to convert
    to a relative time and 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>2</manvolnum></citerefentry>
    instead of plain <function>poll()</function>, which understands timeouts 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 timeout 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 functions 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_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>-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><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>