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<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>
