Systemd/man/sd_listen_fds.xml

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        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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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>