2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
6 <refentry id=
"sd_event_add_io" xmlns:
xi=
"http://www.w3.org/2001/XInclude">
9 <title>sd_event_add_io
</title>
10 <productname>systemd
</productname>
14 <refentrytitle>sd_event_add_io
</refentrytitle>
15 <manvolnum>3</manvolnum>
19 <refname>sd_event_add_io
</refname>
20 <refname>sd_event_source_get_io_events
</refname>
21 <refname>sd_event_source_set_io_events
</refname>
22 <refname>sd_event_source_get_io_revents
</refname>
23 <refname>sd_event_source_get_io_fd
</refname>
24 <refname>sd_event_source_set_io_fd
</refname>
25 <refname>sd_event_source_get_io_fd_own
</refname>
26 <refname>sd_event_source_set_io_fd_own
</refname>
27 <refname>sd_event_source
</refname>
28 <refname>sd_event_io_handler_t
</refname>
30 <refpurpose>Add an I/O event source to an event loop
</refpurpose>
35 <funcsynopsisinfo>#include
<systemd/sd-event.h
></funcsynopsisinfo>
37 <funcsynopsisinfo><token>typedef
</token> struct sd_event_source sd_event_source;
</funcsynopsisinfo>
40 <funcdef>typedef int (*
<function>sd_event_io_handler_t
</function>)
</funcdef>
41 <paramdef>sd_event_source *
<parameter>s
</parameter></paramdef>
42 <paramdef>int
<parameter>fd
</parameter></paramdef>
43 <paramdef>uint32_t
<parameter>revents
</parameter></paramdef>
44 <paramdef>void *
<parameter>userdata
</parameter></paramdef>
48 <funcdef>int
<function>sd_event_add_io
</function></funcdef>
49 <paramdef>sd_event *
<parameter>event
</parameter></paramdef>
50 <paramdef>sd_event_source **
<parameter>source
</parameter></paramdef>
51 <paramdef>int
<parameter>fd
</parameter></paramdef>
52 <paramdef>uint32_t
<parameter>events
</parameter></paramdef>
53 <paramdef>sd_event_io_handler_t
<parameter>handler
</parameter></paramdef>
54 <paramdef>void *
<parameter>userdata
</parameter></paramdef>
58 <funcdef>int
<function>sd_event_source_get_io_events
</function></funcdef>
59 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
60 <paramdef>uint32_t *
<parameter>events
</parameter></paramdef>
64 <funcdef>int
<function>sd_event_source_set_io_events
</function></funcdef>
65 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
66 <paramdef>uint32_t
<parameter>events
</parameter></paramdef>
70 <funcdef>int
<function>sd_event_source_get_io_revents
</function></funcdef>
71 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
72 <paramdef>uint32_t *
<parameter>revents
</parameter></paramdef>
76 <funcdef>int
<function>sd_event_source_get_io_fd
</function></funcdef>
77 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
81 <funcdef>int
<function>sd_event_source_set_io_fd
</function></funcdef>
82 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
83 <paramdef>int
<parameter>fd
</parameter></paramdef>
87 <funcdef>int
<function>sd_event_source_get_io_fd_own
</function></funcdef>
88 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
92 <funcdef>int
<function>sd_event_source_set_io_fd_own
</function></funcdef>
93 <paramdef>sd_event_source *
<parameter>source
</parameter></paramdef>
94 <paramdef>int
<parameter>b
</parameter></paramdef>
101 <title>Description
</title>
103 <para><function>sd_event_add_io()
</function> adds a new I/O event
104 source to an event loop. The event loop object is specified in the
105 <parameter>event
</parameter> parameter, the event source object is
106 returned in the
<parameter>source
</parameter> parameter. The
107 <parameter>fd
</parameter> parameter takes the UNIX file descriptor
108 to watch, which may refer to a socket, a FIFO, a message queue, a
109 serial connection, a character device, or any other file descriptor
110 compatible with Linux
111 <citerefentry project='man-pages'
><refentrytitle>epoll
</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
112 <parameter>events
</parameter> parameter takes a bit mask of events
113 to watch for, a combination of the following event flags:
114 <constant>EPOLLIN
</constant>,
<constant>EPOLLOUT
</constant>,
115 <constant>EPOLLRDHUP
</constant>,
<constant>EPOLLPRI
</constant>,
116 and
<constant>EPOLLET
</constant>, see
117 <citerefentry project='man-pages'
><refentrytitle>epoll_ctl
</refentrytitle><manvolnum>2</manvolnum></citerefentry>
118 for details. The
<parameter>handler
</parameter> shall reference a
119 function to call when the event source is triggered. The
120 <parameter>userdata
</parameter> pointer will be passed to the
121 handler function, and may be chosen freely by the caller. The
122 handler will also be passed the file descriptor the event was seen
123 on, as well as the actual event flags. It's generally a subset of
124 the events watched, however may additionally include
125 <constant>EPOLLERR
</constant> and
<constant>EPOLLHUP
</constant>.
128 <para>By default, an event source will stay enabled
129 continuously (
<constant>SD_EVENT_ON
</constant>), but this may be
131 <citerefentry><refentrytitle>sd_event_source_set_enabled
</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
132 If the handler function returns a negative error code, it will be
133 disabled after the invocation, even if the
134 <constant>SD_EVENT_ON
</constant> mode was requested before. Note
135 that an event source set to
<constant>SD_EVENT_ON
</constant> will
136 fire continuously unless data is read from or written to the file
137 descriptor to reset the mask of events seen.
140 <para>Setting the I/O event mask to watch for to
0 does not mean
141 that the event source won't be triggered anymore, as
142 <constant>EPOLLHUP
</constant> and
<constant>EPOLLERR
</constant>
143 may be triggered even with a zero event mask. To temporarily
144 disable an I/O event source use
145 <citerefentry><refentrytitle>sd_event_source_set_enabled
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
146 with
<constant>SD_EVENT_OFF
</constant> instead.
</para>
148 <para>To destroy an event source object use
149 <citerefentry><refentrytitle>sd_event_source_unref
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
150 but note that the event source is only removed from the event loop
151 when all references to the event source are dropped. To make sure
152 an event source does not fire anymore, even if it is still referenced,
153 disable the event source using
154 <citerefentry><refentrytitle>sd_event_source_set_enabled
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
155 with
<constant>SD_EVENT_OFF
</constant>.
</para>
157 <para>If the second parameter of
158 <function>sd_event_add_io()
</function> is
159 <constant>NULL
</constant> no reference to the event source object
160 is returned. In this case the event source is considered
161 "floating", and will be destroyed implicitly when the event loop
162 itself is destroyed.
</para>
164 <para>It is recommended to use
165 <function>sd_event_add_io()
</function> only in conjunction with
166 file descriptors that have
<constant>O_NONBLOCK
</constant> set, to
167 ensure that all I/O operations from invoked handlers are properly
168 asynchronous and non-blocking. Using file descriptors without
169 <constant>O_NONBLOCK
</constant> might result in unexpected
170 starvation of other event sources. See
171 <citerefentry project='man-pages'
><refentrytitle>fcntl
</refentrytitle><manvolnum>2</manvolnum></citerefentry>
172 for details on enabling
<constant>O_NONBLOCK
</constant> mode.
</para>
174 <para><function>sd_event_source_get_io_events()
</function> retrieves
175 the configured mask of watched I/O events of an event source created
176 previously with
<function>sd_event_add_io()
</function>. It takes
177 the event source object and a pointer to a variable to store the
180 <para><function>sd_event_source_set_io_events()
</function>
181 configures the mask of watched I/O events of an event source created
182 previously with
<function>sd_event_add_io()
</function>. It takes the
183 event source object and the new event mask.
</para>
185 <para><function>sd_event_source_get_io_revents()
</function>
186 retrieves the I/O event mask of currently seen but undispatched
187 events from an event source created previously with
188 <function>sd_event_add_io()
</function>. It takes the event source
189 object and a pointer to a variable to store the event mask
190 in. When called from a handler function on the handler's event
191 source object this will return the same mask as passed to the
192 handler's
<parameter>revents
</parameter> parameter. This call is
193 primarily useful to check for undispatched events of an event
194 source from the handler of an unrelated (possibly higher priority)
195 event source. Note the relation between
196 <function>sd_event_source_get_pending()
</function> and
197 <function>sd_event_source_get_io_revents()
</function>: both
198 functions will report non-zero results when there's an event
199 pending for the event source, but the former applies to all event
200 source types, the latter only to I/O event sources.
</para>
202 <para><function>sd_event_source_get_io_fd()
</function> retrieves
203 the UNIX file descriptor of an event source created previously
204 with
<function>sd_event_add_io()
</function>. It takes the event
205 source object and returns the non-negative file descriptor
206 or a negative error number on error (see below).
</para>
208 <para><function>sd_event_source_set_io_fd()
</function>
209 changes the UNIX file descriptor of an I/O event source created
210 previously with
<function>sd_event_add_io()
</function>. It takes
211 the event source object and the new file descriptor.
</para>
213 <para><function>sd_event_source_set_io_fd_own()
</function> controls whether the file descriptor of the event source
214 shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
215 event source object. By default it is not closed automatically, and the application has to do this on its own. The
216 <parameter>b
</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
217 when the event source is freed, otherwise it is closed.
</para>
219 <para><function>sd_event_source_get_io_fd_own()
</function> may be used to query the current setting of the file
220 descriptor ownership boolean flag as set with
<function>sd_event_source_set_io_fd_own()
</function>. It returns
221 positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
222 negative on error.
</para>
226 <title>Return Value
</title>
228 <para>On success, these functions return
0 or a positive
229 integer. On failure, they return a negative errno-style error
233 <title>Errors
</title>
235 <para>Returned values may indicate the following problems:
</para>
239 <term><constant>-ENOMEM
</constant></term>
241 <listitem><para>Not enough memory to allocate an object.
</para></listitem>
245 <term><constant>-EINVAL
</constant></term>
247 <listitem><para>An invalid argument has been passed.
</para></listitem>
252 <term><constant>-ESTALE
</constant></term>
254 <listitem><para>The event loop is already terminated.
</para></listitem>
259 <term><constant>-ECHILD
</constant></term>
261 <listitem><para>The event loop has been created in a different process.
</para></listitem>
265 <term><constant>-EDOM
</constant></term>
267 <listitem><para>The passed event source is not an I/O event source.
</para></listitem>
273 <xi:include href=
"libsystemd-pkgconfig.xml" />
276 <title>See Also
</title>
279 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
280 <citerefentry><refentrytitle>sd-event
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
281 <citerefentry><refentrytitle>sd_event_new
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
282 <citerefentry><refentrytitle>sd_event_now
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
283 <citerefentry><refentrytitle>sd_event_add_time
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
284 <citerefentry><refentrytitle>sd_event_add_signal
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
285 <citerefentry><refentrytitle>sd_event_add_child
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
286 <citerefentry><refentrytitle>sd_event_add_inotify
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
287 <citerefentry><refentrytitle>sd_event_add_defer
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
288 <citerefentry><refentrytitle>sd_event_source_set_enabled
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
289 <citerefentry><refentrytitle>sd_event_source_set_priority
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
290 <citerefentry><refentrytitle>sd_event_source_set_userdata
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
291 <citerefentry><refentrytitle>sd_event_source_set_description
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
292 <citerefentry><refentrytitle>sd_event_source_get_pending
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
293 <citerefentry project='man-pages'
><refentrytitle>epoll_ctl
</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
294 <citerefentry project='man-pages'
><refentrytitle>epoll
</refentrytitle><manvolnum>7</manvolnum></citerefentry>