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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="sd_listen_fds">
<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>
<refpurpose>Check for file descriptors passed by the init system.</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include "sd-daemon.h"</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>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
environment variables before returning (regardless
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 definition
file. 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>
</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>
<para>This function is provided by the reference
implementation of APIs for new-style daemons and
distributed with the systemd package. The algorithm it
implements is simple, and can easily be reimplemented
in daemons if it is important to support this
interface without using the reference
implementation.</para>
<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>
<para>For details about the algorithm check the
liberally licensed reference implementation sources:
<ulink url="http://cgit.freedesktop.org/systemd/tree/src/sd-daemon.c"/>
resp. <ulink
url="http://cgit.freedesktop.org/systemd/tree/src/sd-daemon.h"/></para>
<para><function>sd_listen_fds()</function> is
implemented in the reference implementation's
<filename>sd-daemon.c</filename> and
<filename>sd-daemon.h</filename> files. These
interfaces are available as shared library, which can
be compiled and linked to with the
<literal>libsystemd-daemon</literal>
<citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
file. Alternatively, applications consuming these APIs
may copy the implementation into their source
tree. For more details about the reference
implementation see
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
<para>If the reference implementation is used as
drop-in files and -DDISABLE_SYSTEMD is set during
compilation this function will always return 0 and
otherwise become a NOP.</para>
</refsect1>
<refsect1>
<title>Environment</title>
<variablelist>
<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>7</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>
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