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ffd1a3f688
Prompted by https://lists.freedesktop.org/archives/systemd-devel/2018-December/041817.html This also drops all references to select() from our manpages. It's 2018 after all, people should use poll(), or ppoll() or epoll().
161 lines
7.6 KiB
XML
161 lines
7.6 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//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+
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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 time-out to wait for from a message bus 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 a message bus
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object. This descriptor can be used with <citerefentry
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project='man-pages'><refentrytitle>poll</refentrytitle><manvolnum>3</manvolnum></citerefentry> or a similar
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function to wait for I/O events on the specified bus connection object. If the bus object was configured with the
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<citerefentry><refentrytitle>sd_bus_set_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry> function, then
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the <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 passing to
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<function>poll()</function> or a similar call. Returns a combination of <constant>POLLIN</constant>,
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<constant>POLLOUT</constant>, … events, or negative on error.</para>
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<para><function>sd_bus_get_timeout()</function> returns the time-out in µs to pass to to
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<function>poll()</function> or a similar call when waiting for events on the specified bus connection. The returned
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time-out may be zero, in which case a subsequent I/O polling call should be invoked in non-blocking mode. The
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returned timeout may be <constant>UINT64_MAX</constant> in which case the I/O polling call may block indefinitely,
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without any applied time-out. Note that the returned time-out should be considered only a maximum sleeping time. It
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is permissible (and even expected) that shorter time-outs are used by the calling program, in case other event
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sources are polled in the same event loop. Note that the returned time-value is relative and specified in
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microseconds. When converting this value in order to pass it as third argument to <function>poll()</function>
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(which expects milliseconds), care should be taken to use a division that rounds up to ensure the I/O polling
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operation doesn't sleep for shorter than necessary, which might result in unintended busy looping (alternatively,
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use <citerefentry project='man-pages'><refentrytitle>ppoll</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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instead of plain <function>poll()</function>, which understands time-outs with nano-second granularity).</para>
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<para>These three functions are useful to hook up a bus connection object with an external or manual event loop
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involving <function>poll()</function> or a similar I/O polling call. Before each invocation of the I/O polling
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call, all three functions should be invoked: the file descriptor returned by <function>sd_bus_get_fd()</function>
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should be polled for the events indicated by <function>sd_bus_get_events()</function>, and the I/O call should
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block for that up to the time-out 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>.</para>
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<para>Note that these function are only one of three supported ways to implement I/O event handling for bus
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connections. Alternatively use
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<citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry> to attach a
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bus connection to an <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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event loop. Or use <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><function>sd_bus_get_fd()</function> returns the file descriptor used for communication, or a negative
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<varname>errno</varname>-style error code on error.</para>
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<para><function>sd_bus_get_events()</function> returns the I/O event mask to use for I/O event watching, or a
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negative <varname>errno</varname>-style error code on error.</para>
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<para><function>sd_bus_get_timeout()</function> returns zero or positive on success, or a negative
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<varname>errno</varname>-style error code on error.</para>
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</refsect1>
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<refsect1>
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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 in a child process
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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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</variablelist>
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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_set_fd</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 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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