systemd/man/sd_event_add_io.xml

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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>
  </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. 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 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, 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>
      <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>