mirror of
https://github.com/systemd/systemd.git
synced 2024-11-24 02:33:36 +08:00
f977356a82
Prompted by: https://lists.freedesktop.org/archives/systemd-devel/2023-January/048714.html
176 lines
8.0 KiB
XML
176 lines
8.0 KiB
XML
<?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-or-later
|
|
|
|
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 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>
|