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sd_event_add_io.xml
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<?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">
<!--
This file is part of systemd.
Copyright 2015 Lennart Poettering
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.
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/>.
-->
<refentry id="sd_event_add_io" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_event_add_io</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>[email protected]</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>sd_event_add_io</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_event_add_io</refname>
<refname>sd_event_source_get_io_events</refname>
<refname>sd_event_source_set_io_events</refname>
<refname>sd_event_source_get_io_revents</refname>
<refname>sd_event_source_get_io_fd</refname>
<refname>sd_event_source_set_io_fd</refname>
<refname>sd_event_source</refname>
<refname>sd_event_io_handler_t</refname>
<refpurpose>Add an I/O event source to an event loop</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
<funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
<funcprototype>
<funcdef>typedef int (*<function>sd_event_io_handler_t</function>)</funcdef>
<paramdef>sd_event_source *<parameter>s</parameter></paramdef>
<paramdef>int <parameter>fd</parameter></paramdef>
<paramdef>uint32_t <parameter>revents</parameter></paramdef>
<paramdef>void *<parameter>userdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_add_io</function></funcdef>
<paramdef>sd_event *<parameter>event</parameter></paramdef>
<paramdef>sd_event_source **<parameter>source</parameter></paramdef>
<paramdef>int <parameter>fd</parameter></paramdef>
<paramdef>uint32_t <parameter>events</parameter></paramdef>
<paramdef>sd_event_io_handler_t <parameter>handler</parameter></paramdef>
<paramdef>void *<parameter>userdata</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_io_events</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint32_t *<parameter>events</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_set_io_events</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint32_t <parameter>events</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_io_revents</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>uint32_t *<parameter>revents</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_get_io_fd</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_set_io_fd</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>int <parameter>fd</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_event_add_io()</function> adds a new I/O event
source to an event loop. The event loop object is specified in the
<parameter>event</parameter> parameter, the event source object is
returned in the <parameter>source</parameter> parameter. The
<parameter>fd</parameter> parameter takes the UNIX file descriptor
to watch, which may refer to a socket, a FIFO, a message queue, a
serial connection, a character device, or any other file descriptor
compatible with Linux
<citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
<parameter>events</parameter> parameter takes a bit mask of events
to watch for, a combination of the following event flags:
<constant>EPOLLIN</constant>, <constant>EPOLLOUT</constant>,
<constant>EPOLLRDHUP</constant>, <constant>EPOLLPRI</constant>,
and <constant>EPOLLET</constant>, see
<citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
for details. The <parameter>handler</parameter> shall reference a
function to call when the event source is triggered. The
<parameter>userdata</parameter> pointer will be passed to the
handler function, and may be chosen freely by the caller. The
handler will also be passed the file descriptor the event was seen
on, as well as the actual event flags. It's generally a subset of
the events watched, however may additionally include
<constant>EPOLLERR</constant> and <constant>EPOLLHUP</constant>.
</para>
<para>By default, an event source will stay enabled
continuously (<constant>SD_EVENT_ON</constant>), but this may be
changed with
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
If the handler function returns a negative error code, it will be
disabled after the invocation, even if the
<constant>SD_EVENT_ON</constant> mode was requested before. Note
that an event source set to <constant>SD_EVENT_ON</constant> will
fire continuously unless data is read from or written to the file
descriptor to reset the mask of events seen.
</para>
<para>Setting the I/O event mask to watch for to 0 does not mean
that the event source won't be triggered anymore, as
<constant>EPOLLHUP</constant> and <constant>EPOLLERR</constant>
may be triggered even with a zero event mask. To temporarily
disable an I/O event source use
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
with <constant>SD_EVENT_OFF</constant> instead.</para>
<para>To destroy an event source object use
<citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
but note that the event source is only removed from the event loop
when all references to the event source are dropped. To make sure
an event source does not fire anymore, even if it is still referenced,
disable the event source using
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
with <constant>SD_EVENT_OFF</constant>.</para>
<para>If the second parameter of
<function>sd_event_add_io()</function> is
<constant>NULL</constant> no reference to the event source object
is returned. In this case the event source is considered
"floating", and will be destroyed implicitly when the event loop
itself is destroyed.</para>
<para>It is recommended to use
<function>sd_event_add_io()</function> only in conjunction with
file descriptors that have <constant>O_NONBLOCK</constant> set, to
ensure that all I/O operations from invoked handlers are properly
asynchronous and non-blocking. Using file descriptors without
<constant>O_NONBLOCK</constant> might result in unexpected
starvation of other event sources. See
<citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
<para><function>sd_event_source_get_io_events()</function> retrieves
the configured mask of watched I/O events of an event source created
previously with <function>sd_event_add_io()</function>. It takes
the event source object and a pointer to a variable to store the
mask in.</para>
<para><function>sd_event_source_set_io_events()</function>
configures the mask of watched I/O events of an event source created
previously with <function>sd_event_add_io()</function>. It takes the
event source object and the new event mask.</para>
<para><function>sd_event_source_get_io_revents()</function>
retrieves the I/O event mask of currently seen but undispatched
events from an event source created previously with
<function>sd_event_add_io()</function>. It takes the event source
object and a pointer to a variable to store the event mask
in. When called from a handler function on the handler's event
source object this will return the same mask as passed to the
handler's <parameter>revents</parameter> parameter. This call is
primarily useful to check for undispatched events of an event
source from the handler of an unrelated (possibly higher priority)
event source. Note the relation between
<function>sd_event_source_get_pending()</function> and
<function>sd_event_source_get_io_revents()</function>: both
functions will report non-zero results when there's an event
pending for the event source, but the former applies to all event
source types, the latter only to I/O event sources.</para>
<para><function>sd_event_source_get_io_fd()</function> retrieves
the UNIX file descriptor of an event source created previously
with <function>sd_event_add_io()</function>. It takes the event
source object and returns the non-negative file descriptor
or a negative error number on error (see below).</para>
<para><function>sd_event_source_set_io_fd()</function>
changes the UNIX file descriptor of an I/O event source created
previously with <function>sd_event_add_io()</function>. It takes
the event source object and the new file descriptor.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On success, these functions return 0 or a positive
integer. On failure, they return a negative errno-style error
code.</para>
</refsect1>
<refsect1>
<title>Errors</title>
<para>Returned values may indicate the following problems:</para>
<variablelist>
<varlistentry>
<term><constant>-ENOMEM</constant></term>
<listitem><para>Not enough memory to allocate an object.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EINVAL</constant></term>
<listitem><para>An invalid argument has been passed.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ESTALE</constant></term>
<listitem><para>The event loop is already terminated.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ECHILD</constant></term>
<listitem><para>The event loop has been created in a different process.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EDOM</constant></term>
<listitem><para>The passed event source is not an I/O event source.</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-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>