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318 lines
16 KiB
XML
318 lines
16 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="systemd.special">
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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 systemd. </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,
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and should only be implemented if compatbility
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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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getrlimit() 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 sigprocmask().</para></listitem>
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<listitem><para>Call fork(),
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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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setsid() to detach from any terminal
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and create an independent
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session.</para></listitem>
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<listitem><para>In the child, call
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fork() again, to ensure the daemon can
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never re-aquire a terminal
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again.</para></listitem>
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<listitem><para>Call exit() 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 open(), mkdir() 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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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 fork() and available
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in both processes.</para></listitem>
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<listitem><para>Call exit() 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 exit() happens after
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initialization is complete and all
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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 daemon() function should not be
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used, as it does 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>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 LSB
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recommendations for SysV init scripts
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(http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html).</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 accces 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.</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
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services.</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 parallization 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
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request.</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
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updates via the sd_notify()
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interface.</para></listitem>
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<listitem><para>Instead of using the
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syslog() 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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fprintf(), 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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printk() priority system. In fact, using
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this style of logging also enables the
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init system to optionally direct all
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application logging to the kernel log
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buffer (kmsg), as accessible via
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dmesg.</para></listitem>
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</orderedlist>
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</refsect2>
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<refsect2>
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<title>Bus Activation</title>
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</refsect2>
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<refsect2>
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<title>Socket Activation</title>
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</refsect2>
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<refsect2>
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<title>Writing Service Files</title>
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</refsect2>
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<refsect2>
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<title>Installing Service Files</title>
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</refsect2>
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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>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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