231 lines
10 KiB
XML
231 lines
10 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="sd_listen_fds"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_listen_fds</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_listen_fds</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_listen_fds</refname>
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<refname>sd_listen_fds_with_names</refname>
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<refname>SD_LISTEN_FDS_START</refname>
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<refpurpose>Check for file descriptors passed by the system manager</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-daemon.h></funcsynopsisinfo>
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<funcsynopsisinfo>#define SD_LISTEN_FDS_START 3</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_listen_fds</function></funcdef>
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<paramdef>int <parameter>unset_environment</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_listen_fds_with_names</function></funcdef>
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<paramdef>int <parameter>unset_environment</parameter></paramdef>
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<paramdef>char*** <parameter>names</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_listen_fds()</function> may be invoked by a
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daemon to check for file descriptors passed by the service manager as
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part of the socket-based activation logic. It returns the number
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of received file descriptors. If no file descriptors have been
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received, zero is returned. The first file descriptor may be found
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at file descriptor number 3
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(i.e. <constant>SD_LISTEN_FDS_START</constant>), the remaining
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descriptors follow at 4, 5, 6, …, if any.</para>
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<para>If a daemon receives more than one file descriptor, they
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will be passed in the same order as configured in the systemd
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socket unit file (see
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<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details). Nonetheless, it is recommended to verify the correct
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socket types before using them. To simplify this checking, the
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functions
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<citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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are provided. In order to maximize flexibility, it is recommended
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to make these checks as loose as possible without allowing
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incorrect setups. i.e. often, the actual port number a socket is
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bound to matters little for the service to work, hence it should
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not be verified. On the other hand, whether a socket is a datagram
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or stream socket matters a lot for the most common program logics
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and should be checked.</para>
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<para>This function call will set the FD_CLOEXEC flag for all
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passed file descriptors to avoid further inheritance to children
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of the calling process.</para>
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<para>If multiple socket units activate the same service, the order
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of the file descriptors passed to its main process is undefined.
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If additional file descriptors have been passed to the service
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manager using
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<citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
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<literal>FDSTORE=1</literal> messages, these file descriptors are
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passed last, in arbitrary order, and with duplicates
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removed.</para>
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<para>If the <parameter>unset_environment</parameter> parameter is
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non-zero, <function>sd_listen_fds()</function> will unset the
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<varname>$LISTEN_FDS</varname>, <varname>$LISTEN_PID</varname> and
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<varname>$LISTEN_FDNAMES</varname> environment variables before
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returning (regardless of whether the function call itself
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succeeded or not). Further calls to
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<function>sd_listen_fds()</function> will then return zero, but the
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variables are no longer inherited by child processes.</para>
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<para><function>sd_listen_fds_with_names()</function> is like
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<function>sd_listen_fds()</function>, but optionally also returns
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an array of strings with identification names for the passed file
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descriptors, if that is available and the
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<parameter>names</parameter> parameter is non-NULL. This
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information is read from the <varname>$LISTEN_FDNAMES</varname>
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variable, which may contain a colon-separated list of names. For
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socket-activated services, these names may be configured with the
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<varname>FileDescriptorName=</varname> setting in socket unit
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files, see
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<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details. For file descriptors pushed into the file descriptor
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store (see above), the name is set via the
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<varname>FDNAME=</varname> field transmitted via
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<function>sd_pid_notify_with_fds()</function>. The primary usecase
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for these names are services which accept a variety of file
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descriptors which are not recognizable with functions like
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<function>sd_is_socket()</function> alone, and thus require
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identification via a name. It is recommended to rely on named file
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descriptors only if identification via
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<function>sd_is_socket()</function> and related calls is not
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sufficient. Note that the names used are not unique in any
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way. The returned array of strings has as many entries as file
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descriptors have been received, plus a final NULL pointer
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terminating the array. The caller needs to free the array itself
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and each of its elements with libc's <function>free()</function>
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call after use. If the <parameter>names</parameter> parameter is
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NULL, the call is entirely equivalent to
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<function>sd_listen_fds()</function>.</para>
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<para>Under specific conditions, the following automatic file
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descriptor names are returned:
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<table>
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<title>
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<command>Special names</command>
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</title>
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<tgroup cols='2'>
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><literal>unknown</literal></entry>
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<entry>The process received no name for the specific file descriptor from the service manager.</entry>
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</row>
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<row>
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<entry><literal>stored</literal></entry>
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<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>
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</row>
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<row>
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<entry><literal>connection</literal></entry>
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<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>
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</row>
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</tbody>
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</tgroup>
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</table>
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</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On failure, these calls returns a negative errno-style error
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code. If
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<varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname> was
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not set or was not correctly set for this daemon and hence no file
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descriptors were received, 0 is returned. Otherwise, the number of
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file descriptors passed is returned. The application may find them
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starting with file descriptor SD_LISTEN_FDS_START, i.e. file
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descriptor 3.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
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<para>Internally, <function>sd_listen_fds()</function> checks
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whether the <varname>$LISTEN_PID</varname> environment variable
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equals the daemon PID. If not, it returns immediately. Otherwise,
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it parses the number passed in the <varname>$LISTEN_FDS</varname>
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environment variable, then sets the FD_CLOEXEC flag for the parsed
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number of file descriptors starting from SD_LISTEN_FDS_START.
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Finally, it returns the parsed
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number. <function>sd_listen_fds_with_names()</function> does the
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same but also parses <varname>$LISTEN_FDNAMES</varname> if
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set.</para>
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</refsect1>
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<refsect1>
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<title>Environment</title>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$LISTEN_PID</varname></term>
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<term><varname>$LISTEN_FDS</varname></term>
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<term><varname>$LISTEN_FDNAMES</varname></term>
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<listitem><para>Set by the service manager for supervised
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processes that use socket-based activation. This environment
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variable specifies the data
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<function>sd_listen_fds()</function> and
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<function>sd_listen_fds_with_names()</function> parses. See
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above for details.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_is_socket_unix</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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