man/sd_listen_fds.xml

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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>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_listen_fds</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_listen_fds</refname>
    <refname>sd_listen_fds_with_names</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>

      <funcprototype>
        <funcdef>int <function>sd_listen_fds_with_names</function></funcdef>
        <paramdef>int <parameter>unset_environment</parameter></paramdef>
        <paramdef>char*** <parameter>names</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_listen_fds()</function> may be invoked by a daemon to check for file descriptors
    passed by the service manager as part of the socket-based activation logic. It returns the number of
    received file descriptors. If no file descriptors have been received, zero is returned. The first file
    descriptor may be found at file descriptor number 3 (i.e. <constant>SD_LISTEN_FDS_START</constant>), the
    remaining descriptors follow at 4, 5, 6, …, if any.</para>

    <para>The file descriptors passed this way may be closed at will by the processes receiving them: it's up
    to the processes themselves to close them after use or whether to leave them open until the process exits
    (in which case the kernel closes them automatically). Note that the file descriptors received by daemons
    are duplicates of the file descriptors the service manager originally allocated and bound and of which it
    continuously keeps a copy (except if <varname>Accept=yes</varname> is used). This means any socket option
    changes and other changes made to the sockets will be visible to the service manager too. Most
    importantly this means it's generally not a good idea to invoke <citerefentry
    project='man-pages'><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry> on
    such sockets, since it will shut down communication on the file descriptor the service manager holds for
    the same socket too. Also note that if a daemon is restarted (and its associated sockets are not) it will
    receive file descriptors to the very same sockets as the earlier invocations, thus all socket options
    applied then will still apply.</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) — if there's only one such file (see below).  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>

    <para>If the <parameter>unset_environment</parameter> parameter is
    non-zero, <function>sd_listen_fds()</function> will unset the
    <varname>$LISTEN_FDS</varname>, <varname>$LISTEN_PID</varname> and
    <varname>$LISTEN_FDNAMES</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 return zero, but the
    variables are no longer inherited by child processes.</para>

    <para><function>sd_listen_fds_with_names()</function> is like
    <function>sd_listen_fds()</function>, but optionally also returns
    an array of strings with identification names for the passed file
    descriptors, if that is available and the
    <parameter>names</parameter> parameter is non-<constant>NULL</constant>. This
    information is read from the <varname>$LISTEN_FDNAMES</varname>
    variable, which may contain a colon-separated list of names. For
    socket-activated services, these names may be configured with the
    <varname>FileDescriptorName=</varname> setting in socket unit
    files, see
    <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details. For file descriptors pushed into the file descriptor
    store (see above), the name is set via the
    <varname>FDNAME=</varname> field transmitted via
    <function>sd_pid_notify_with_fds()</function>. The primary usecase
    for these names are services which accept a variety of file
    descriptors which are not recognizable with functions like
    <function>sd_is_socket()</function> alone, and thus require
    identification via a name. It is recommended to rely on named file
    descriptors only if identification via
    <function>sd_is_socket()</function> and related calls is not
    sufficient. Note that the names used are not unique in any
    way. The returned array of strings has as many entries as file
    descriptors have been received, plus a final <constant>NULL</constant> pointer
    terminating the array. The caller needs to free the array itself
    and each of its elements with libc's <function>free()</function>
    call after use. If the <parameter>names</parameter> parameter is
    <constant>NULL</constant>, the call is entirely equivalent to
    <function>sd_listen_fds()</function>.</para>

    <para>Under specific conditions, the following automatic file
    descriptor names are returned:

    <table>
      <title>
         <command>Special names</command>
      </title>

      <tgroup cols='2'>
        <thead>
          <row>
            <entry>Name</entry>
            <entry>Description</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry><literal>unknown</literal></entry>
            <entry>The process received no name for the specific file descriptor from the service manager.</entry>
          </row>

          <row>
            <entry><literal>stored</literal></entry>
            <entry>The file descriptor originates in the service manager's per-service file descriptor store, and the <varname>FDNAME=</varname> field was absent when the file descriptor was submitted to the service manager.</entry>
          </row>

          <row>
            <entry><literal>connection</literal></entry>
            <entry>The service was activated in per-connection style using <varname>Accept=yes</varname> in the socket unit file, and the file descriptor is the connection socket.</entry>
          </row>
        </tbody>
      </tgroup>
    </table>
    </para>
  </refsect1>

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

    <para>On failure, these calls 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, <function>sd_listen_fds()</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. <function>sd_listen_fds_with_names()</function> does the
    same but also parses <varname>$LISTEN_FDNAMES</varname> if
    set.</para>
  </refsect1>

  <refsect1>
    <title>Environment</title>

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

        <listitem><para>Set by the service manager for supervised
        processes that use socket-based activation. This environment
        variable specifies the data
        <function>sd_listen_fds()</function> and
        <function>sd_listen_fds_with_names()</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>sd_pid_notify_with_fds</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>