<?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">

<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>lennart@poettering.net</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 &lt;systemd/sd-event.h&gt;</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>