-<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
-<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
-"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
- This file is part of systemd.
-
- Copyright 2016 Julian Orth
-
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
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- systemd is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
+ Copyright © 2016 Julian Orth
-->
-<refentry id="sd_bus_get_fd">
+<refentry id="sd_bus_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_bus_get_fd</title>
<productname>systemd</productname>
-
- <authorgroup>
- <author>
- <firstname>Julian</firstname>
- <surname>Orth</surname>
- <email>ju.orth@gmail.com</email>
- </author>
- </authorgroup>
</refentryinfo>
<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 connected to the message bus</refpurpose>
+ <refpurpose>Get the file descriptor, I/O events and time-out to wait for from a message bus object</refpurpose>
</refnamediv>
<refsynopsisdiv>
<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 with the message bus. This descriptor can be used with
- <citerefentry
- project='die-net'><refentrytitle>select</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
- <citerefentry
- project='die-net'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
- or similar functions to wait for incoming messages.
- </para>
-
- <para>
- If the bus was created with the
- <citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>
- function, then the <parameter>input_fd</parameter> used in that call is
- returned.
- </para>
+ <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>
- Returns the file descriptor used for incoming messages from the message
- bus.
- </para>
+ <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>
+
+ <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>
+ </variablelist>
+ </refsect2>
</refsect1>
+ <xi:include href="libsystemd-pkgconfig.xml" />
+
<refsect1>
<title>See Also</title>
<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>
projects / thirdparty / systemd.git / blobdiff