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sd_bus_get_fd() and related calls are useful for integrating a bus connection into arbitrary event loops. But sd_bus_set_fd() is quite a different beast, it's for using D-Bus over pre-initialized sockets or pairs of fifos or stuff, i.e. very advanced stuff. Let's split this man page in two, in order not to confuse things needlessly. And while we are at it, let's slightly extend the documentation.
177 lines
7.8 KiB
XML
177 lines
7.8 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 relative and specified in microseconds. When converting this value in
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order to pass it as third argument to <function>poll()</function> (which expects milliseconds),
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care should be taken to 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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