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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 I/O
    events to watch the file descriptor 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 I/O event source is triggered. The
    handler function will be passed the
    <parameter>userdata</parameter> pointer, which 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
    seen. It's generally a subset of the events watched, however may
    additionally have <constant>EPOLLERR</constant> and
    <constant>EPOLLHUP</constant> set.</para>

    <para>By default, the I/O 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 I/O event source set to <constant>SD_EVENT_ON</constant> will
    fire continuously unless data is read or written to the file
    descriptor in order 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 when there's still a
    reference to it kept, consider setting the event source to
    <constant>SD_EVENT_OFF</constant> with
    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    <para>If the second parameter of
    <function>sd_event_add_io()</function> is passed as NULL 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
    starving 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 I/O event mask to watch of an I/O 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.</para>

    <para><function>sd_event_source_set_io_events()</function> changes the
    configured I/O event mask to watch of an I/O event source created previously
    with <function>sd_event_add_io()</function>. It takes the event
    source object and the new event mask to set.</para>

    <para><function>sd_event_source_get_io_revents()</function>
    retrieves the I/O event mask of currently seen but undispatched
    events from an I/O 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 I/O event source created previously
    with <function>sd_event_add_io()</function>. It takes the event
    source object and returns the positive file descriptor in the return
    value, 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 to set.</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 errors 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>