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595 lines
32 KiB
XML
595 lines
32 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 General Public License as published by
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the Free Software Foundation; either version 2 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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General Public License for more details.
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You should have received a copy of the GNU 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="daemon">
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<refentryinfo>
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<title>daemon</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>daemon</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>daemon</refname>
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<refpurpose>Writing and Packaging System Daemons</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<para>A daemon is a service process that runs in the
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background and supervises the system or provides
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functionality to other processes. Traditionally,
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daemons are implemented following a scheme originating
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in SysV Unix. Modern daemons should follow a simpler
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yet more powerful scheme (here called "new-style"
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daemons), as implemented by
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. </para>
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<refsect2>
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<title>SysV Daemons</title>
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<para>When a traditional SysV daemon
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starts, it should execute the following steps
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as part of the initialization. Note that these
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steps are unnecessary for new-style daemons (see below),
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and should only be implemented if compatibility
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with SysV is essential.</para>
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<orderedlist>
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<listitem><para>Close all open file
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descriptors except STDIN, STDOUT,
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STDERR (i.e. the first three file
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descriptors 0, 1, 2). This ensures
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that no accidentally passed file
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descriptor stays around in the daemon
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process. On Linux this is best
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implemented by iterating through
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<filename>/proc/self/fd</filename>,
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with a fallback of iterating from file
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descriptor 3 to the value returned by
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<function>getrlimit()</function> for
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RLIMIT_NOFILE.</para></listitem>
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<listitem><para>Reset all signal
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handlers to their default. This is
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best done by iterating through the
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available signals up to the limit of
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_NSIG and resetting them to
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SIG_DFL.</para></listitem>
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<listitem><para>Reset the signal mask
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using
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<function>sigprocmask()</function>.</para></listitem>
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<listitem><para>Sanitize the
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environment block, removing or
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resetting environment variables that
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might negatively impact daemon
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runtime.</para></listitem>
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<listitem><para>Call <function>fork()</function>,
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to create a background
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process.</para></listitem>
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<listitem><para>In the child, call
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<function>setsid()</function> to
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detach from any terminal and create an
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independent session.</para></listitem>
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<listitem><para>In the child, call
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<function>fork()</function> again, to
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ensure the daemon can never re-aquire
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a terminal again.</para></listitem>
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<listitem><para>Call <function>exit()</function> in the
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first child, so that only the second
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child (the actual daemon process)
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stays around. This ensures that the
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daemon process is reparented to
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init/PID 1, as all daemons should
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be.</para></listitem>
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<listitem><para>In the daemon process,
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connect <filename>/dev/null</filename>
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to STDIN, STDOUT,
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STDERR.</para></listitem>
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<listitem><para>In the daemon process,
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reset the umask to 0, so that the file
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modes passed to <function>open()</function>, <function>mkdir()</function> and
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suchlike directly control the access
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mode of the created files and
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directories.</para></listitem>
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<listitem><para>In the daemon process,
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change the current directory to the
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root directory (/), in order to avoid
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that the daemon involuntarily
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blocks mount points from being
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unmounted.</para></listitem>
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<listitem><para>In the daemon process,
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write the daemon PID (as returned by
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<function>getpid()</function>) to a
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PID file, for example
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<filename>/var/run/foobar.pid</filename>
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(for a hypothetical daemon "foobar"),
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to ensure that the daemon cannot be
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started more than once. This must be
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implemented in race-free fashion so
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that the PID file is only updated when
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at the same time it is verified that
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the PID previously stored in the PID
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file no longer exists or belongs to a
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foreign process. Commonly some kind of
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file locking is employed to implement
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this logic.</para></listitem>
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<listitem><para>In the daemon process,
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drop privileges, if possible and
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applicable.</para></listitem>
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<listitem><para>From the daemon
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process notify the original process
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started that initialization is
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complete. This can be implemented via
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an unnamed pipe or similar
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communication channel that is created
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before the first
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<function>fork()</function> and hence
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available in both the original and the
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daemon process.</para></listitem>
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<listitem><para>Call
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<function>exit()</function> in the
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original process. The process that
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invoked the daemon must be able to
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rely that this
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<function>exit()</function> happens
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after initialization is complete and
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all external communication channels
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established and
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accessible.</para></listitem>
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</orderedlist>
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<para>The BSD <function>daemon()</function> function should not be
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used, as it implements only a subset of these steps.</para>
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<para>A daemon that needs to provide
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compatibility with SysV systems should
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implement the scheme pointed out
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above. However, it is recommended to make this
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behaviour optional and configurable via a
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command line argument, to ease debugging as
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well as to simplify integration into systems
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using systemd.</para>
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</refsect2>
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<refsect2>
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<title>New-Style Daemons</title>
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<para>Modern services for Linux should be
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implemented as new-style daemons. This makes it
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easier to supervise and control them at
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runtime and simplifies their
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implementation.</para>
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<para>For developing a new-style daemon none
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of the initialization steps recommended for
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SysV daemons need to be implemented. New-style
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init systems such as systemd make all of them
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redundant. Moreover, since some of these steps
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interfere with process monitoring, file
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descriptor passing and other functionality of
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the init system it is recommended not to
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execute them when run as new-style
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service.</para>
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<para>Note that new-style init systems
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guarantee execution of daemon processes in
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clean process contexts: it is guaranteed that
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the environment block is sanitized, that the
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signal handlers and mask is reset and that no
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left-over file descriptors are passed. Daemons
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will be executed in their own session, and
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STDIN/STDOUT/STDERR connected to
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<filename>/dev/null</filename> unless
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otherwise configured. The umask is reset.</para>
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<para>It is recommended for new-style daemons
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to implement the following:</para>
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<orderedlist>
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<listitem><para>If SIGTERM is
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received, shut down the daemon and
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exit cleanly.</para></listitem>
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<listitem><para>If SIGHUP is received,
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reload the configuration files, if
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this applies.</para></listitem>
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<listitem><para>Provide a correct exit
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code from the main daemon process, as
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this is used by the init system to
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detect service errors and problems. It
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is recommended to follow the exit code
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scheme as defined in the <ulink
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url="http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
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recommendations for SysV init
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scripts</ulink>.</para></listitem>
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<listitem><para>As much as possible,
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rely on systemd's functionality to
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limit the access of the daemon to
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files, services and other
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resources. i.e. rely on systemd's
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resource limit control instead of
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implementing your own, rely on
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systemd's privilege dropping code
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instead of implementing it in the
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daemon, and similar. See
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for the available
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controls.</para></listitem>
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<listitem><para>If possible and
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applicable expose the daemon's control
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interface via the D-Bus IPC system and
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grab a bus name as last step of
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initialization.</para></listitem>
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<listitem><para>If D-Bus is used, make
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your daemon bus-activatable, via
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supplying a D-Bus service activation
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configuration file. This has multiple
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advantages: your daemon may be started
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lazily on-demand; it may be started in
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parallel to other daemons requiring it
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-- which maximizes parallelization and
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boot-up speed; your daemon can be
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restarted on failure, without losing
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any bus requests, as the bus queues
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requests for activatable services. See
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below for details.</para></listitem>
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<listitem><para>If your daemon
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provides services to other local
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processes or remote clients via a
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socket, it should be made
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socket-activatable following the
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scheme pointed out below. Like D-Bus
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activation this enables on-demand
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starting of services as well as it
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allows improved parallelization of
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service start-up. Also, for state-less
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protocols (such as syslog, DNS) a
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daemon implementing socket-based
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activation can be restarted without
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losing a single request. See below for
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details.</para></listitem>
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<listitem><para>If applicable a daemon
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should notify the init system about
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startup completion or status updates
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via the
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<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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interface.</para></listitem>
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<listitem><para>Instead of using the
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<function>syslog()</function> call to log directly to the
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system logger, a new-style daemon may
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choose to simply log to STDERR via
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<function>fprintf()</function>, which is then forwarded to
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syslog by the init system. If log
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priorities are necessary these can be
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encoded by prefixing individual log
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lines with strings like "<4>"
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(for log priority 4 "WARNING" in the
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syslog priority scheme), following a
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similar style as the Linux kernel's
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<function>printk()</function> priority system. In fact,
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using this style of logging also
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enables the init system to optionally
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direct all application logging to the
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kernel log buffer (kmsg), as
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accessible via
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<citerefentry><refentrytitle>dmesg</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
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kind of logging may be enabled by
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setting
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<varname>StandardError=syslog</varname>
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in the service unit file. For details
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see
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<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
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</orderedlist>
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<para>These recommendations are similar but
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not identical to the <ulink
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url="http://developer.apple.com/mac/library/documentation/MacOSX/Conceptual/BPSystemStartup/Articles/LaunchOnDemandDaemons.html#//apple_ref/doc/uid/TP40001762-104738">Apple
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MacOS X Daemon Requirements</ulink>.</para>
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</refsect2>
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<refsect2>
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<title>Socket-Based Activation</title>
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</refsect2>
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<refsect2>
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<title>Bus-Based Activation</title>
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</refsect2>
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<refsect2>
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<title>Path-Based Activation</title>
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</refsect2>
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<refsect2>
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<title>Writing Systemd Unit Files</title>
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<para>When writing systemd unit files, it is
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recommended to consider the following
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suggestions:</para>
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<orderedlist>
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<listitem><para>If possible do not use
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the <varname>Type=forking</varname>
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setting in service files. But if you
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do, make sure to set the PID file path
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using <varname>PIDFile=</varname>. See
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<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details.</para></listitem>
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<listitem><para>If your daemon
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registers a D-Bus name on the bus,
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make sure to use
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<varname>Type=dbus</varname> if
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possible.</para></listitem>
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<listitem><para>Make sure to set a
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good human-readable description string
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with
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<varname>Description=</varname>.</para></listitem>
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<listitem><para>Do not disable
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<varname>DefaultDependencies=</varname>,
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unless you really know what you do and
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your unit is involved in early boot or
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late system shutdown.</para></listitem>
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<listitem><para>Normally, little if
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any dependencies should need to
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be defined explicitly. However, if you
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do configure explicit dependencies, only refer to
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unit names listed on
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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or names introduced by your own
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package to keep the unit file
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operating
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system-independent.</para></listitem>
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<listitem><para>Make sure to include
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an <literal>[Install]</literal> section including
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installation information for the unit
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file. See
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details. To activate your service
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on boot make sure to add a
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<varname>WantedBy=multi-user.target</varname>
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or
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<varname>WantedBy=graphical.target</varname> directive.</para></listitem>
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</orderedlist>
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</refsect2>
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<refsect2>
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<title>Installing Service Files</title>
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<para>At the build installation time
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(e.g. <command>make install</command> during
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package build) packages are recommended to
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install their systemd unit files in the
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directory returned by <command>pkg-config
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systemd
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--variable=systemdsystemnunitdir</command>
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(for system services),
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resp. <command>pkg-config systemd
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--variable=systemdsessionunitdir</command>
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(for session services). This will make the
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services available in the system on explicit
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request but not activate them automatically
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during boot. Optionally, during package
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installation (e.g. <command>rpm -i</command>
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by the administrator) symlinks should be
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created in the systemd configuration
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directories via the
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<citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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tool, to activate them automatically on
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boot.</para>
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<para>Packages using
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<citerefentry><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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are recommended to use a configure script
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excerpt like the following to determine the
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unit installation path during source
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configuration:</para>
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<programlisting>PKG_PROG_PKG_CONFIG
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AC_ARG_WITH([systemdsystemunitdir],
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AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files]),
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[], [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
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AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
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AM_CONDITIONAL(HAVE_SYSTEMD, [test -n "$with_systemdsystemunitdir"])</programlisting>
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<para>This snippet allows automatic
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installation of the unit files on systemd
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machines, and optionally allows their
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installation even on machines lacking
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systemd. (Modification of this snippet for the
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session unit directory is left as excercise to the
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reader.)</para>
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<para>Additionally, to ensure that
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<command>make distcheck</command> continues to
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work, it is recommended to add the following
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to the top-level <filename>Makefile.am</filename>
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file in
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<citerefentry><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based
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projects:</para>
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<programlisting>DISTCHECK_CONFIGURE_FLAGS = \
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--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting>
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<para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para>
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<programlisting>if HAVE_SYSTEMD
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systemdsystemunit_DATA = \
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foobar.socket \
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foobar.service
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endif</programlisting>
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<para>In the
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<citerefentry><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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<filename>.spec</filename> file use a snippet like
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the following to enable/disable the service
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during installation/deinstallation. Consult
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the packaging guidelines of your distribution
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for details and the equivalent for other
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packaging managers:</para>
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<programlisting>%post
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/usr/bin/systemd-install enable foobar.service foobar.socket >/dev/null 2>&1 || :
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%preun
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if [ "$1" -eq 0 ]; then
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/usr/bin/systemd-install disable foobar.service foobar.socket >/dev/null 2>&1 || :
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fi</programlisting>
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</refsect2>
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<refsect2>
|
|
<title>Porting Existing Daemons</title>
|
|
|
|
<para>Since new-style init systems such as
|
|
systemd are compatible with traditional SysV
|
|
init systems it is not strictly necessary to
|
|
port existing daemons to the new
|
|
style. However doing this offers additional
|
|
functionality to the daemons as well as it
|
|
simplifies integration into new-style init
|
|
systems.</para>
|
|
|
|
<para>To port an existing SysV compatible
|
|
daemon the following steps are
|
|
recommended:</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>If not already
|
|
implemented, add an optional command
|
|
line switch to the daemon to disable
|
|
daemonization. This is useful not only
|
|
for using the daemon in new-style init
|
|
systems, but also to ease debugging.</para></listitem>
|
|
|
|
<listitem><para>If the daemon offers
|
|
interfaces to other software running
|
|
on the local system via local AF_UNIX
|
|
sockets, consider implementing
|
|
socket-based activation (see
|
|
above). Usually a minimal patch is
|
|
sufficient to implement this: Extend
|
|
the socket creation in the daemon code
|
|
so that
|
|
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
is checked for already passed sockets
|
|
first. If sockets are passed
|
|
(i.e. when
|
|
<function>sd_listen_fds()</function>
|
|
returns a positive value), skip the
|
|
socket createn step and use the passed
|
|
sockets. Secondly, ensure that the
|
|
file-system socket nodes for local
|
|
AF_UNIX sockets used in the
|
|
socket-based activation are not
|
|
removed when the daemon shuts down, if
|
|
sockets have been passed. Third, if
|
|
the daemon normally closes all
|
|
remaining open file descriptors as
|
|
part of its initialization, the
|
|
sockets passed from the init system
|
|
must be spared. Since new-style init
|
|
systems guarantee that no left-over
|
|
file descriptors are passed to
|
|
executed processes, it might be a good
|
|
choice to simply skip the closing of
|
|
all remaining open file descriptors if
|
|
file descriptors are
|
|
passed.</para></listitem>
|
|
|
|
<listitem><para>Write and install a
|
|
systemd unit file for the service (and
|
|
the sockets if socket-based activation
|
|
is used, as well as a path unit file,
|
|
if the daemon processes a spool
|
|
directory), see above for
|
|
details.</para></listitem>
|
|
|
|
<listitem><para>If the daemon exposes
|
|
interfaces via D-Bus, write and
|
|
install a D-Bus activation file for
|
|
the service, see above for
|
|
details.</para></listitem>
|
|
</orderedlist>
|
|
|
|
</refsect2>
|
|
|
|
</refsect1>
|
|
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd-install</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
|
|
</refentry>
|