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| <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
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| <!-- |
| SPDX-License-Identifier: LGPL-2.1-or-later |
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| Copyright © 2016 Julian Orth |
| --> |
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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 <systemd/sd-bus.h></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 <emphasis>absolute</emphasis> time-out in µs, |
| from which the relative time-out to pass to <function>poll()</function> (or a similar call) can be |
| derived, 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> |