man/sd_bus_get_fd.xml

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   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1-or-later
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   8   Copyright © 2016 Julian Orth
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  10
  11 <refentry id="sd_bus_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
  12
  13   <refentryinfo>
  14     <title>sd_bus_get_fd</title>
  15     <productname>systemd</productname>
  16   </refentryinfo>
  17
  18   <refmeta>
  19     <refentrytitle>sd_bus_get_fd</refentrytitle>
  20     <manvolnum>3</manvolnum>
  21   </refmeta>
  22
  23   <refnamediv>
  24     <refname>sd_bus_get_fd</refname>
  25     <refname>sd_bus_get_events</refname>
  26     <refname>sd_bus_get_timeout</refname>
  27
  28     <refpurpose>Get the file descriptor, I/O events and timeout to wait for from a message bus
  29     object</refpurpose>
  30   </refnamediv>
  31
  32   <refsynopsisdiv>
  33     <funcsynopsis>
  34       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  35
  36       <funcprototype>
  37         <funcdef>int <function>sd_bus_get_fd</function></funcdef>
  38         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  39       </funcprototype>
  40
  41       <funcprototype>
  42         <funcdef>int <function>sd_bus_get_events</function></funcdef>
  43         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  44       </funcprototype>
  45
  46       <funcprototype>
  47         <funcdef>int <function>sd_bus_get_timeout</function></funcdef>
  48         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  49         <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
  50       </funcprototype>
  51     </funcsynopsis>
  52   </refsynopsisdiv>
  53
  54   <refsect1>
  55     <title>Description</title>
  56
  57     <para><function>sd_bus_get_fd()</function> returns the file descriptor used to communicate from
  58     a message bus object. This descriptor can be used with
  59     <citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  60     or a similar function to wait for I/O events on the specified bus connection object. If the bus
  61     object was configured with the <function>sd_bus_set_fd()</function> function, then the
  62     <parameter>input_fd</parameter> file descriptor used in that call is returned.</para>
  63
  64     <para><function>sd_bus_get_events()</function> returns the I/O events to wait for, suitable for
  65     passing to <function>poll()</function> or a similar call. Returns a combination of
  66     <constant>POLLIN</constant>, <constant>POLLOUT</constant>, … events, or negative on error.
  67     </para>
  68
  69     <para><function>sd_bus_get_timeout()</function> returns the <emphasis>absolute</emphasis> time-out in μs,
  70     from which the relative time-out to pass to <function>poll()</function> (or a similar call) can be
  71     derived, when waiting for events on the specified bus connection. The returned timeout may be zero, in
  72     which case a subsequent I/O polling call should be invoked in non-blocking mode. The returned timeout may
  73     be <constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely, without any
  74     applied timeout. Note that the returned timeout should be considered only a maximum sleeping time. It is
  75     permissible (and even expected) that shorter timeouts are used by the calling program, in case other
  76     event sources are polled in the same event loop. Note that the returned time-value is absolute, based of
  77     <constant>CLOCK_MONOTONIC</constant> and specified in microseconds. When converting this value in order
  78     to pass it as third argument to <function>poll()</function> (which expects relative milliseconds), care
  79     should be taken to convert to a relative time and use a division that rounds up to ensure the I/O polling
  80     operation doesn't sleep for shorter than necessary, which might result in unintended busy looping
  81     (alternatively, use <citerefentry
  82     project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>2</manvolnum></citerefentry> instead
  83     of plain <function>poll()</function>, which understands timeouts with nano-second granularity).</para>
  84
  85     <para>These three functions are useful to hook up a bus connection object with an external or
  86     manual event loop involving <function>poll()</function> or a similar I/O polling call. Before
  87     each invocation of the I/O polling call, all three functions should be invoked: the file
  88     descriptor returned by <function>sd_bus_get_fd()</function> should be polled for the events
  89     indicated by <function>sd_bus_get_events()</function>, and the I/O call should block for that up
  90     to the timeout returned by <function>sd_bus_get_timeout()</function>. After each I/O polling
  91     call the bus connection needs to process incoming or outgoing data, by invoking
  92     <citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
  93     </para>
  94
  95     <para>Note that these functions are only one of three supported ways to implement I/O event
  96     handling for bus connections. Alternatively use
  97     <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  98     to attach a bus connection to an
  99     <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 100     event loop. Or use
 101     <citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 102     as a simple synchronous, blocking I/O waiting call.</para>
 103   </refsect1>
 104
 105   <refsect1>
 106     <title>Return Value</title>
 107
 108     <para>On success, <function>sd_bus_get_fd()</function> returns the file descriptor used for
 109     communication. On failure, it returns a negative errno-style error code.</para>
 110
 111     <para>On success, <function>sd_bus_get_events()</function> returns the I/O event mask to use for
 112     I/O event watching. On failure, it returns a negative errno-style error code.</para>
 113
 114     <para>On success, <function>sd_bus_get_timeout()</function> returns a non-negative integer. On
 115     failure, it returns a negative errno-style error code.</para>
 116
 117     <refsect2>
 118       <title>Errors</title>
 119
 120       <para>Returned errors may indicate the following problems:</para>
 121
 122       <variablelist>
 123         <varlistentry>
 124           <term><constant>-EINVAL</constant></term>
 125
 126           <listitem><para>An invalid bus object was passed.</para></listitem>
 127         </varlistentry>
 128
 129         <varlistentry>
 130           <term><constant>-ECHILD</constant></term>
 131
 132           <listitem><para>The bus connection was allocated in a parent process and is being reused
 133           in a child process after <function>fork()</function>.</para></listitem>
 134         </varlistentry>
 135
 136         <varlistentry>
 137           <term><constant>-ENOTCONN</constant></term>
 138
 139           <listitem><para>The bus connection has been terminated.</para></listitem>
 140         </varlistentry>
 141
 142         <varlistentry>
 143           <term><constant>-EPERM</constant></term>
 144
 145           <listitem><para>Two distinct file descriptors were passed for input and output using
 146           <function>sd_bus_set_fd()</function>, which <function>sd_bus_get_fd()</function> cannot
 147           return.</para></listitem>
 148         </varlistentry>
 149
 150         <varlistentry>
 151           <term><constant>-ENOPKG</constant></term>
 152
 153           <listitem><para>The bus cannot be resolved.</para>
 154
 155           <xi:include href="version-info.xml" xpointer="v246"/></listitem>
 156         </varlistentry>
 157       </variablelist>
 158     </refsect2>
 159   </refsect1>
 160
 161   <xi:include href="libsystemd-pkgconfig.xml" />
 162
 163   <refsect1>
 164     <title>History</title>
 165     <para><function>sd_bus_get_fd()</function> was added in version 231.</para>
 166     <para><function>sd_bus_get_events()</function> and
 167     <function>sd_bus_get_timeout()</function> were added in version 240.</para>
 168   </refsect1>
 169
 170   <refsect1>
 171     <title>See Also</title>
 172
 173     <para><simplelist type="inline">
 174       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 175       <member><citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 176       <member><citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 177       <member><citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 178       <member><citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 179       <member><citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 180       <member><citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 181     </simplelist></para>
 182   </refsect1>
 183
 184 </refentry>