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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd_event_add_io" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_event_add_io</title>
<productname>systemd</productname>
</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_get_io_fd_own</refname>
<refname>sd_event_source_set_io_fd_own</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>ret</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>ret</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>
<funcprototype>
<funcdef>int <function>sd_event_source_get_io_fd_own</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_event_source_set_io_fd_own</function></funcdef>
<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
<paramdef>int <parameter>b</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. Note that not all file descriptors are compatible with epoll, for example regular file or
directories are not. If this function is called with a file descriptor that does not support epoll,
<constant>-EPERM</constant> is returned (also see below). In most cases such file descriptors may be
treated as always-readable or always-writable, so that IO event watching is unnecessary.</para>
<para>The <parameter>handler</parameter> is a function to call when the event source is triggered or
<constant>NULL</constant>. 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>. The
handler may return negative to signal an error (see below), other return values are ignored. If
<parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
used.</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 either be disabled after the invocation,
even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
terminate, see
<citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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 will not 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>If the <parameter>handler</parameter> to <function>sd_event_add_io()</function> is
<constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
the exit code parameter to
<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
<para>Note that this call does not take possession of the file descriptor passed in, ownership (and thus
the duty to close it when it is no longer needed) remains with the caller. However, with the
<function>sd_event_source_set_io_fd_own()</function> call (see below) the event source may optionally
take ownership of the file descriptor after the event source has been created. In that case the file
descriptor is closed automatically as soon as the event source is released.</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 as parameters. If the event source owned the previous file descriptor, that
is if <function>sd_event_source_set_io_fd_own()</function> had been called for the event source with a
non-zero value, then the previous file descriptor will be closed and the event source will also take the
ownership of the new file descriptor on success.</para>
<para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the
event source shall take ownership of the file descriptor. Takes a boolean parameter
<parameter>b</parameter>. When true (nonzero), the file descriptor will be closed automatically when the
event source is freed or when the file descriptor is replaced by
<function>sd_event_source_set_io_fd()</function>. By default, the descriptor is not owned by the event
source, and the application has to do close it on its own if needed.</para>
<para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
negative on error.</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>
<refsect2>
<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, library or module instance.</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>
<varlistentry>
<term><constant>-EPERM</constant></term>
<listitem><para>The passed file descriptor does not support the <citerefentry
project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
API, for example because it is a regular file or directory. See <citerefentry
project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
for details.</para>
<xi:include href="version-info.xml" xpointer="v255"/></listitem>
</varlistentry>
</variablelist>
</refsect2>
</refsect1>
<xi:include href="libsystemd-pkgconfig.xml" />
<refsect1>
<title>History</title>
<para><function>sd_event_io_handler_t()</function>,
<function>sd_event_add_io()</function>,
<function>sd_event_source_get_io_events()</function>,
<function>sd_event_source_set_io_events()</function>,
<function>sd_event_source_get_io_revents()</function>,
<function>sd_event_source_get_io_fd()</function>, and
<function>sd_event_source_set_io_fd()</function> were added in version 229.</para>
<para><function>sd_event_source_get_io_fd_own()</function> and
<function>sd_event_source_set_io_fd_own()</function> were added in version 239.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
<member><citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
|
