systemd/man/sd_bus_get_fd.xml

<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  SPDX-License-Identifier: LGPL-2.1+

  Copyright © 2016 Julian Orth
-->

<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 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_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
    <citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry> 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 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><function>sd_bus_get_fd()</function> returns the file descriptor used for communication, or a negative
    <varname>errno</varname>-style error code on error.</para>

    <para><function>sd_bus_get_events()</function> returns the I/O event mask to use for I/O event watching, or a
    negative <varname>errno</varname>-style error code on error.</para>

    <para><function>sd_bus_get_timeout()</function> returns zero or positive on success, or a negative
    <varname>errno</varname>-style error code on error.</para>
  </refsect1>

  <refsect1>
    <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>
    </variablelist>
  </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_set_fd</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>