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<refentry id="sd_listen_fds"
  xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_listen_fds</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_listen_fds</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_listen_fds</refname>
    <refname>SD_LISTEN_FDS_START</refname>
    <refpurpose>Check for file descriptors passed by the system manager</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-daemon.h&gt;</funcsynopsisinfo>

      <funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_listen_fds</function></funcdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_listen_fds()</function> shall be called by a
    daemon to check for file descriptors passed by the init system as
    part of the socket-based activation logic.</para>

    <para>If the <parameter>unset_environment</parameter> parameter is
    non-zero, <function>sd_listen_fds()</function> will unset the
    <varname>$LISTEN_FDS</varname> and <varname>$LISTEN_PID</varname>
    environment variables before returning (regardless of whether the
    function call itself succeeded or not). Further calls to
    <function>sd_listen_fds()</function> will then fail, but the
    variables are no longer inherited by child processes.</para>

    <para>If a daemon receives more than one file descriptor, they
    will be passed in the same order as configured in the systemd
    socket unit file (see
    <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details). Nonetheless, it is recommended to verify the correct
    socket types before using them. To simplify this checking, the
    functions
    <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    are provided. In order to maximize flexibility, it is recommended
    to make these checks as loose as possible without allowing
    incorrect setups. i.e. often, the actual port number a socket is
    bound to matters little for the service to work, hence it should
    not be verified. On the other hand, whether a socket is a datagram
    or stream socket matters a lot for the most common program logics
    and should be checked.</para>

    <para>This function call will set the FD_CLOEXEC flag for all
    passed file descriptors to avoid further inheritance to children
    of the calling process.</para>

    <para>If multiple socket units activate the same service the order
    of the file descriptors passed to its main process is undefined.
    If additional file descriptors have been passed to the service
    manager using
    <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
    <literal>FDSTORE=1</literal> messages, these file descriptors are
    passed last, in arbitrary order, and with duplicates
    removed.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On failure, this call returns a negative errno-style error
    code. If
    <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname> was
    not set or was not correctly set for this daemon and hence no file
    descriptors were received, 0 is returned. Otherwise, the number of
    file descriptors passed is returned. The application may find them
    starting with file descriptor SD_LISTEN_FDS_START, i.e. file
    descriptor 3.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>

    <para>Internally, this function checks whether the
    <varname>$LISTEN_PID</varname> environment variable equals the
    daemon PID. If not, it returns immediately. Otherwise, it parses
    the number passed in the <varname>$LISTEN_FDS</varname>
    environment variable, then sets the FD_CLOEXEC flag for the parsed
    number of file descriptors starting from SD_LISTEN_FDS_START.
    Finally, it returns the parsed number.</para>
  </refsect1>

  <refsect1>
    <title>Environment</title>

    <variablelist class='environment-variables'>
      <varlistentry>
        <term><varname>$LISTEN_PID</varname></term>
        <term><varname>$LISTEN_FDS</varname></term>

        <listitem><para>Set by the init system
        for supervised processes that use
        socket-based activation. This
        environment variable specifies the
        data
        <function>sd_listen_fds()</function>
        parses. See above for
        details.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
