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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1+
Copyright © 2016 Julian Orth
-->
<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 time-out 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
<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><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>
</refsect1>
<refsect1>
<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>
</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_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>
</refentry>
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