204 lines
9.9 KiB
XML
204 lines
9.9 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="sd_listen_fds">
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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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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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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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<refpurpose>Check for file descriptors passed by the init system.</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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</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> shall be
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called by a daemon to check for file descriptors
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passed by the init system as part of the socket-based
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activation logic.</para>
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<para>If the <parameter>unset_environment</parameter>
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parameter is non-zero
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<function>sd_listen_fds()</function> will unset the
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<varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
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environment variables before returning (regardless
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whether the function call itself succeeded or
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not). Further calls to
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<function>sd_listen_fds()</function> will then fail,
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but the variables are no longer inherited by child
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processes.</para>
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<para>If a daemon receives more than one file
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descriptor, they will be passed in the same order as
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configured in the systemd socket definition
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file. Nonetheless it is recommended to verify the
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correct socket types before using them. To simplify
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this checking the 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
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recommended to make these checks as loose as possible
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without allowing incorrect setups. i.e. often the
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actual port number a socket is bound to matters little
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for the service to work, hence it should not be
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verified. On the other hand, whether a socket is a
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datagram or stream socket matters a lot for the most
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common program logics and should be checked.</para>
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<para>This function call will set the FD_CLOEXEC flag
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for all passed file descriptors to avoid further
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inheritance to children of the calling process.</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, this call returns a negative
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errno-style error code. If
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<varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
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was not set or was not correctly set for this daemon and
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hence no file descriptors were received, 0 is
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returned. Otherwise the number of file descriptors
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passed is returned. The application may find them
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starting with file descriptor SD_LISTEN_FDS_START,
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i.e. file descriptor 3.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>This function is provided by the reference
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implementation of APIs for new-style daemons and
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distributed with the systemd package. The algorithm it
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implements is simple, and can easily be reimplemented
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in daemons if it is important to support this
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interface without using the reference
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implementation.</para>
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<para>Internally, this function checks whether the
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<varname>$LISTEN_PID</varname> environment variable
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equals the daemon PID. If not, it returns
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immediately. Otherwise it parses the number passed in
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the <varname>$LISTEN_FDS</varname> environment
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variable, then sets the FD_CLOEXEC flag for the parsed
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number of file descriptors starting from
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SD_LISTEN_FDS_START. Finally it returns the parsed
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number.</para>
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<para>For details about the algorithm check the
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liberally licensed reference implementation sources:
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<ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/sd-daemon.c"/>
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resp. <ulink
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url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h"/></para>
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<para><function>sd_listen_fds()</function> is
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implemented in the reference implementation's
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<filename>sd-daemon.c</filename> and
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<filename>sd-daemon.h</filename> files. These
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interfaces are available as shared library, which can
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be compiled and linked to with the
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<literal>libsystemd-daemon</literal>
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<citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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file. Alternatively, applications consuming these APIs
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may copy the implementation into their source
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tree. For more details about the reference
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implementation see
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<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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<para>If the reference implementation is used as
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drop-in files and -DDISABLE_SYSTEMD is set during
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compilation this function will always return 0 and
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otherwise become a NOP.</para>
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</refsect1>
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<refsect1>
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<title>Environment</title>
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<variablelist>
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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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<listitem><para>Set by the init system
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for supervised processes that use
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socket-based activation. This
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environment variable specifies the
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data
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<function>sd_listen_fds()</function>
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parses. See above for
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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>7</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>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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