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25060a570c
The documentation of sd_bus_get_timeout wrongfully states that the returned time-value is relative. In fact, it is an absolute value which is based of CLOCK_MONOTONIC. This change corrects that documentation.
178 lines
7.9 KiB
XML
178 lines
7.9 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1-or-later
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Copyright © 2016 Julian Orth
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-->
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<refentry id="sd_bus_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_bus_get_fd</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_bus_get_fd</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_bus_get_fd</refname>
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<refname>sd_bus_get_events</refname>
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<refname>sd_bus_get_timeout</refname>
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<refpurpose>Get the file descriptor, I/O events and timeout to wait for from a message bus
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object</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_bus_get_fd</function></funcdef>
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<paramdef>sd_bus *<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_get_events</function></funcdef>
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<paramdef>sd_bus *<parameter>bus</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_get_timeout</function></funcdef>
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<paramdef>sd_bus *<parameter>bus</parameter></paramdef>
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<paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_bus_get_fd()</function> returns the file descriptor used to communicate from
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a message bus object. This descriptor can be used with
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<citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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or a similar function to wait for I/O events on the specified bus connection object. If the bus
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object was configured with the <function>sd_bus_set_fd()</function> function, then the
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<parameter>input_fd</parameter> file descriptor used in that call is returned.</para>
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<para><function>sd_bus_get_events()</function> returns the I/O events to wait for, suitable for
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passing to <function>poll()</function> or a similar call. Returns a combination of
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<constant>POLLIN</constant>, <constant>POLLOUT</constant>, … events, or negative on error.
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</para>
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<para><function>sd_bus_get_timeout()</function> returns the timeout in µs to pass to
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<function>poll()</function> or a similar call when waiting for events on the specified bus
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connection. The returned timeout may be zero, in which case a subsequent I/O polling call
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should be invoked in non-blocking mode. The returned timeout may be
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<constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely,
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without any applied timeout. Note that the returned timeout should be considered only a
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maximum sleeping time. It is permissible (and even expected) that shorter timeouts are used by
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the calling program, in case other event sources are polled in the same event loop. Note that
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the returned time-value is absolute, based of <constant>CLOCK_MONOTONIC</constant> and specified
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in microseconds. When converting this value in order to pass it as third argument to
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<function>poll()</function> (which expects relative milliseconds), care should be taken to convert
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to a relative time and use a division that rounds up to ensure the I/O polling operation
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doesn't sleep for shorter than necessary, which might result in unintended busy looping
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(alternatively, use
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<citerefentry project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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instead of plain <function>poll()</function>, which understands timeouts with nano-second
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granularity).</para>
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<para>These three functions are useful to hook up a bus connection object with an external or
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manual event loop involving <function>poll()</function> or a similar I/O polling call. Before
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each invocation of the I/O polling call, all three functions should be invoked: the file
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descriptor returned by <function>sd_bus_get_fd()</function> should be polled for the events
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indicated by <function>sd_bus_get_events()</function>, and the I/O call should block for that up
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to the timeout returned by <function>sd_bus_get_timeout()</function>. After each I/O polling
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call the bus connection needs to process incoming or outgoing data, by invoking
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<citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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</para>
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<para>Note that these functions are only one of three supported ways to implement I/O event
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handling for bus connections. Alternatively use
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<citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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to attach a bus connection to an
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<citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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event loop. Or use
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<citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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as a simple synchronous, blocking I/O waiting call.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>sd_bus_get_fd()</function> returns the file descriptor used for
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communication. On failure, it returns a negative errno-style error code.</para>
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<para>On success, <function>sd_bus_get_events()</function> returns the I/O event mask to use for
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I/O event watching. On failure, it returns a negative errno-style error code.</para>
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<para>On success, <function>sd_bus_get_timeout()</function> returns a non-negative integer. On
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failure, it returns a negative errno-style error code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>An invalid bus object was passed.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The bus connection was allocated in a parent process and is being reused
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in a child process after <function>fork()</function>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ENOTCONN</constant></term>
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<listitem><para>The bus connection has been terminated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EPERM</constant></term>
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<listitem><para>Two distinct file descriptors were passed for input and output using
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<function>sd_bus_set_fd()</function>, which <function>sd_bus_get_fd()</function> cannot
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return.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ENOPKG</constant></term>
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<listitem><para>The bus cannot be resolved.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_process</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_wait</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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