systemd/man/sd_event_add_io.xml
Zbigniew Jędrzejewski-Szmek 9809a788e4 man: add better descriptions of what event handlers do
The meaning of the return value, the default handlers, and loop exiting are now
described.
2022-01-05 15:19:13 +01:00

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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/4.2/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 &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>
<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.</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 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>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.</para>
<para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the event source
shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
event source object. By default it is not closed automatically, and the application has to do this on its own. The
<parameter>b</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
when the event source is freed, otherwise it is closed.</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.</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>
</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-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_inotify</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><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>