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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="daemon">
+
+ <refentryinfo>
+ <title>daemon</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>daemon</refentrytitle>
+ <manvolnum>7</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>daemon</refname>
+ <refpurpose>Writing and packaging system daemons</refpurpose>
+ </refnamediv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para>A daemon is a service process that runs in the background
+ and supervises the system or provides functionality to other
+ processes. Traditionally, daemons are implemented following a
+ scheme originating in SysV Unix. Modern daemons should follow a
+ simpler yet more powerful scheme (here called "new-style"
+ daemons), as implemented by
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
+ This manual page covers both schemes, and in particular includes
+ recommendations for daemons that shall be included in the systemd
+ init system.</para>
+
+ <refsect2>
+ <title>SysV Daemons</title>
+
+ <para>When a traditional SysV daemon starts, it should execute
+ the following steps as part of the initialization. Note that
+ these steps are unnecessary for new-style daemons (see below),
+ and should only be implemented if compatibility with SysV is
+ essential.</para>
+
+ <orderedlist>
+ <listitem><para>Close all open file descriptors except
+ standard input, output, and error (i.e. the first three file
+ descriptors 0, 1, 2). This ensures that no accidentally passed
+ file descriptor stays around in the daemon process. On Linux,
+ this is best implemented by iterating through
+ <filename>/proc/self/fd</filename>, with a fallback of
+ iterating from file descriptor 3 to the value returned by
+ <function>getrlimit()</function> for
+ <constant>RLIMIT_NOFILE</constant>. </para></listitem>
+
+ <listitem><para>Reset all signal handlers to their default.
+ This is best done by iterating through the available signals
+ up to the limit of <constant>_NSIG</constant> and resetting
+ them to <constant>SIG_DFL</constant>.</para></listitem>
+
+ <listitem><para>Reset the signal mask
+ using
+ <function>sigprocmask()</function>.</para></listitem>
+
+ <listitem><para>Sanitize the environment block, removing or
+ resetting environment variables that might negatively impact
+ daemon runtime.</para></listitem>
+
+ <listitem><para>Call <function>fork()</function>, to create a
+ background process.</para></listitem>
+
+ <listitem><para>In the child, call
+ <function>setsid()</function> to detach from any terminal and
+ create an independent session.</para></listitem>
+
+ <listitem><para>In the child, call <function>fork()</function> again, to ensure that the daemon can
+ never re-acquire a terminal again. (This relevant if the program — and all its dependencies — does
+ not carefully specify `O_NOCTTY` on each and every single `open()` call that might potentially open a
+ TTY device node.)</para></listitem>
+
+ <listitem><para>Call <function>exit()</function> in the first
+ child, so that only the second child (the actual daemon
+ process) stays around. This ensures that the daemon process is
+ re-parented to init/PID 1, as all daemons should
+ be.</para></listitem>
+
+ <listitem><para>In the daemon process, connect
+ <filename>/dev/null</filename> to standard input, output, and
+ error.</para></listitem>
+
+ <listitem><para>In the daemon process, reset the umask to 0,
+ so that the file modes passed to <function>open()</function>,
+ <function>mkdir()</function> and suchlike directly control the
+ access mode of the created files and
+ directories.</para></listitem>
+
+ <listitem><para>In the daemon process, change the current
+ directory to the root directory (/), in order to avoid that
+ the daemon involuntarily blocks mount points from being
+ unmounted.</para></listitem>
+
+ <listitem><para>In the daemon process, write the daemon PID
+ (as returned by <function>getpid()</function>) to a PID file,
+ for example <filename index='false'>/run/foobar.pid</filename> (for a
+ hypothetical daemon "foobar") to ensure that the daemon cannot
+ be started more than once. This must be implemented in
+ race-free fashion so that the PID file is only updated when it
+ is verified at the same time that the PID previously stored in
+ the PID file no longer exists or belongs to a foreign
+ process.</para></listitem>
+
+ <listitem><para>In the daemon process, drop privileges, if
+ possible and applicable.</para></listitem>
+
+ <listitem><para>From the daemon process, notify the original
+ process started that initialization is complete. This can be
+ implemented via an unnamed pipe or similar communication
+ channel that is created before the first
+ <function>fork()</function> and hence available in both the
+ original and the daemon process.</para></listitem>
+
+ <listitem><para>Call <function>exit()</function> in the
+ original process. The process that invoked the daemon must be
+ able to rely on that this <function>exit()</function> happens
+ after initialization is complete and all external
+ communication channels are established and
+ accessible.</para></listitem>
+ </orderedlist>
+
+ <para>The BSD <function>daemon()</function> function should not
+ be used, as it implements only a subset of these steps.</para>
+
+ <para>A daemon that needs to provide compatibility with SysV
+ systems should implement the scheme pointed out above. However,
+ it is recommended to make this behavior optional and
+ configurable via a command line argument to ease debugging as
+ well as to simplify integration into systems using
+ systemd.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>New-Style Daemons</title>
+
+ <para>Modern services for Linux should be implemented as
+ new-style daemons. This makes it easier to supervise and control
+ them at runtime and simplifies their implementation.</para>
+
+ <para>For developing a new-style daemon, none of the initialization steps recommended for SysV daemons
+ need to be implemented. New-style init systems such as systemd make all of them redundant. Moreover,
+ since some of these steps interfere with process monitoring, file descriptor passing, and other
+ functionality of the service manager, it is recommended not to execute them when run as new-style
+ service.</para>
+
+ <para>Note that new-style init systems guarantee execution of daemon processes in a clean process context: it is
+ guaranteed that the environment block is sanitized, that the signal handlers and mask is reset and that no
+ left-over file descriptors are passed. Daemons will be executed in their own session, with standard input
+ connected to <filename>/dev/null</filename> and standard output/error connected to the
+ <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ logging service, unless otherwise configured. The umask is reset.
+ </para>
+
+ <para>It is recommended for new-style daemons to implement the
+ following:</para>
+
+ <orderedlist>
+ <listitem><para>If applicable, the daemon should notify the service manager about startup completion
+ or status updates via the
+ <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+ interface, in particular <varname>READY=1</varname> and <varname>STATUS=…</varname>.
+ </para></listitem>
+
+ <listitem><para>If <constant>SIGTERM</constant> is received, shut down the daemon and exit cleanly.
+ A <varname>STOPPING=1</varname> notification should be sent via
+ <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+ </para></listitem>
+
+ <listitem><para>If <constant>SIGHUP</constant> is received, reload the configuration files, if this
+ applies. This should be combined with notifications via
+ <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>:
+ <varname>RELOADING=1</varname> and <varname>READY=1</varname>.
+ </para></listitem>
+
+ <listitem><para>Provide a correct exit code from the main daemon process, as this is used by the
+ service manager to detect service errors and problems. It is recommended to follow the exit code
+ scheme as defined in the <ulink
+ url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
+ recommendations for SysV init scripts</ulink>.</para></listitem>
+
+ <listitem><para>If possible and applicable, expose the
+ daemon's control interface via the D-Bus IPC system and grab a
+ bus name as last step of initialization.</para></listitem>
+
+ <listitem><para>For integration in systemd, provide a
+ <filename>.service</filename> unit file that carries
+ information about starting, stopping and otherwise maintaining
+ the daemon. See
+ <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details.</para></listitem>
+
+ <listitem><para>As much as possible, rely on the service manager's functionality to limit the access
+ of the daemon to files, services, and other resources, i.e. in the case of systemd, rely on systemd's
+ resource limit control instead of implementing your own, rely on systemd's privilege dropping code
+ instead of implementing it in the daemon, and so on. See
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+ the available controls.</para></listitem>
+
+ <listitem><para>If D-Bus is used, make your daemon
+ bus-activatable by supplying a D-Bus service activation
+ configuration file. This has multiple advantages: your daemon
+ may be started lazily on-demand; it may be started in parallel
+ to other daemons requiring it — which maximizes
+ parallelization and boot-up speed; your daemon can be
+ restarted on failure without losing any bus requests, as the
+ bus queues requests for activatable services. See below for
+ details.</para></listitem>
+
+ <listitem><para>If your daemon provides services to other
+ local processes or remote clients via a socket, it should be
+ made socket-activatable following the scheme pointed out
+ below. Like D-Bus activation, this enables on-demand starting
+ of services as well as it allows improved parallelization of
+ service start-up. Also, for state-less protocols (such as
+ syslog, DNS), a daemon implementing socket-based activation
+ can be restarted without losing a single request. See below
+ for details.</para></listitem>
+
+ <listitem><para>If the service opens sockets or other files on it own, and those file descriptors
+ shall survive a restart, the daemon should store them in the service manager via
+ <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> with
+ <varname>FDSTORE=1</varname>..</para></listitem>
+
+ <listitem><para>Instead of using the <function>syslog()</function> call to log directly to the system
+ syslog service, a new-style daemon may choose to simply log to standard error via
+ <function>fprintf()</function>, which is then forwarded to syslog. If log levels are necessary, these
+ can be encoded by prefixing individual log lines with strings like <literal>&lt;4&gt;</literal> (for
+ log level 4 "WARNING" in the syslog priority scheme), following a similar style as the Linux kernel's
+ <function>printk()</function> level system. For details, see
+ <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
+ <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+
+ <listitem><para>As new-style daemons are invoked without a controlling TTY (but as their own session
+ leaders) care should be taken to always specify <constant>O_NOCTTY</constant> on
+ <citerefentry><refentrytitle>open</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ calls that possibly reference a TTY device node, so that no controlling TTY is accidentally
+ acquired.</para></listitem>
+
+ </orderedlist>
+
+ <para>These recommendations are similar but not identical to the
+ <ulink
+ url="https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html">Apple
+ MacOS X Daemon Requirements</ulink>.</para>
+ </refsect2>
+
+ </refsect1>
+ <refsect1>
+ <title>Activation</title>
+
+ <para>New-style init systems provide multiple additional
+ mechanisms to activate services, as detailed below. It is common
+ that services are configured to be activated via more than one
+ mechanism at the same time. An example for systemd:
+ <filename>bluetoothd.service</filename> might get activated either
+ when Bluetooth hardware is plugged in, or when an application
+ accesses its programming interfaces via D-Bus. Or, a print server
+ daemon might get activated when traffic arrives at an IPP port, or
+ when a printer is plugged in, or when a file is queued in the
+ printer spool directory. Even for services that are intended to be
+ started on system bootup unconditionally, it is a good idea to
+ implement some of the various activation schemes outlined below,
+ in order to maximize parallelization. If a daemon implements a
+ D-Bus service or listening socket, implementing the full bus and
+ socket activation scheme allows starting of the daemon with its
+ clients in parallel (which speeds up boot-up), since all its
+ communication channels are established already, and no request is
+ lost because client requests will be queued by the bus system (in
+ case of D-Bus) or the kernel (in case of sockets) until the
+ activation is completed.</para>
+
+ <refsect2>
+ <title>Activation on Boot</title>
+
+ <para>Old-style daemons are usually activated exclusively on boot (and manually by the administrator)
+ via SysV init scripts, as detailed in the <ulink
+ url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
+ Linux Standard Base Core Specification</ulink>. This method of activation is supported ubiquitously on
+ Linux init systems, both old-style and new-style systems. Among other issues, SysV init scripts have
+ the disadvantage of involving shell scripts in the boot process. New-style init systems generally use
+ updated versions of activation, both during boot-up and during runtime and using more minimal service
+ description files.</para>
+
+ <para>In systemd, if the developer or administrator wants to
+ make sure that a service or other unit is activated
+ automatically on boot, it is recommended to place a symlink to
+ the unit file in the <filename>.wants/</filename> directory of
+ either <filename>multi-user.target</filename> or
+ <filename>graphical.target</filename>, which are normally used
+ as boot targets at system startup. See
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details about the <filename>.wants/</filename> directories,
+ and
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ for details about the two boot targets.</para>
+
+ </refsect2>
+
+ <refsect2>
+ <title>Socket-Based Activation</title>
+
+ <para>In order to maximize the possible parallelization and robustness and simplify configuration and
+ development, it is recommended for all new-style daemons that communicate via listening sockets to use
+ socket-based activation. In a socket-based activation scheme, the creation and binding of the listening
+ socket as primary communication channel of daemons to local (and sometimes remote) clients is moved out
+ of the daemon code and into the service manager. Based on per-daemon configuration, the service manager
+ installs the sockets and then hands them off to the spawned process as soon as the respective daemon is
+ to be started. Optionally, activation of the service can be delayed until the first inbound traffic
+ arrives at the socket to implement on-demand activation of daemons. However, the primary advantage of
+ this scheme is that all providers and all consumers of the sockets can be started in parallel as soon
+ as all sockets are established. In addition to that, daemons can be restarted with losing only a
+ minimal number of client transactions, or even any client request at all (the latter is particularly
+ true for state-less protocols, such as DNS or syslog), because the socket stays bound and accessible
+ during the restart, and all requests are queued while the daemon cannot process them.</para>
+
+ <para>New-style daemons which support socket activation must be able to receive their sockets from the
+ service manager instead of creating and binding them themselves. For details about the programming
+ interfaces for this scheme provided by systemd, see
+ <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
+ <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. For
+ details about porting existing daemons to socket-based activation, see below. With minimal effort, it
+ is possible to implement socket-based activation in addition to traditional internal socket creation in
+ the same codebase in order to support both new-style and old-style init systems from the same daemon
+ binary.</para>
+
+ <para>systemd implements socket-based activation via
+ <filename>.socket</filename> units, which are described in
+ <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ When configuring socket units for socket-based activation, it is
+ essential that all listening sockets are pulled in by the
+ special target unit <filename>sockets.target</filename>. It is
+ recommended to place a
+ <varname>WantedBy=sockets.target</varname> directive in the
+ [Install] section to automatically add such a
+ dependency on installation of a socket unit. Unless
+ <varname>DefaultDependencies=no</varname> is set, the necessary
+ ordering dependencies are implicitly created for all socket
+ units. For more information about
+ <filename>sockets.target</filename>, see
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+ It is not necessary or recommended to place any additional
+ dependencies on socket units (for example from
+ <filename>multi-user.target</filename> or suchlike) when one is
+ installed in <filename>sockets.target</filename>.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>Bus-Based Activation</title>
+
+ <para>When the D-Bus IPC system is used for communication with clients, new-style daemons should use
+ bus activation so that they are automatically activated when a client application accesses their IPC
+ interfaces. This is configured in D-Bus service files (not to be confused with systemd service unit
+ files!). To ensure that D-Bus uses systemd to start-up and maintain the daemon, use the
+ <varname>SystemdService=</varname> directive in these service files to configure the matching systemd
+ service for a D-Bus service. e.g.: For a D-Bus service whose D-Bus activation file is named
+ <filename>org.freedesktop.RealtimeKit.service</filename>, make sure to set
+ <varname>SystemdService=rtkit-daemon.service</varname> in that file to bind it to the systemd service
+ <filename>rtkit-daemon.service</filename>. This is needed to make sure that the daemon is started in a
+ race-free fashion when activated via multiple mechanisms simultaneously.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>Device-Based Activation</title>
+
+ <para>Often, daemons that manage a particular type of hardware
+ should be activated only when the hardware of the respective
+ kind is plugged in or otherwise becomes available. In a
+ new-style init system, it is possible to bind activation to
+ hardware plug/unplug events. In systemd, kernel devices
+ appearing in the sysfs/udev device tree can be exposed as units
+ if they are tagged with the string <literal>systemd</literal>.
+ Like any other kind of unit, they may then pull in other units
+ when activated (i.e. plugged in) and thus implement device-based
+ activation. systemd dependencies may be encoded in the udev
+ database via the <varname>SYSTEMD_WANTS=</varname> property. See
+ <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details. Often, it is nicer to pull in services from devices
+ only indirectly via dedicated targets. Example: Instead of
+ pulling in <filename>bluetoothd.service</filename> from all the
+ various bluetooth dongles and other hardware available, pull in
+ bluetooth.target from them and
+ <filename>bluetoothd.service</filename> from that target. This
+ provides for nicer abstraction and gives administrators the
+ option to enable <filename>bluetoothd.service</filename> via
+ controlling a <filename>bluetooth.target.wants/</filename>
+ symlink uniformly with a command like <command>enable</command>
+ of
+ <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ instead of manipulating the udev ruleset.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>Path-Based Activation</title>
+
+ <para>Often, runtime of daemons processing spool files or
+ directories (such as a printing system) can be delayed until
+ these file system objects change state, or become non-empty.
+ New-style init systems provide a way to bind service activation
+ to file system changes. systemd implements this scheme via
+ path-based activation configured in <filename>.path</filename>
+ units, as outlined in
+ <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>Timer-Based Activation</title>
+
+ <para>Some daemons that implement clean-up jobs that are
+ intended to be executed in regular intervals benefit from
+ timer-based activation. In systemd, this is implemented via
+ <filename>.timer</filename> units, as described in
+ <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+ </refsect2>
+
+ <refsect2>
+ <title>Other Forms of Activation</title>
+
+ <para>Other forms of activation have been suggested and implemented in some systems. However, there are
+ often simpler or better alternatives, or they can be put together of combinations of the schemes above.
+ Example: Sometimes, it appears useful to start daemons or <filename>.socket</filename> units when a
+ specific IP address is configured on a network interface, because network sockets shall be bound to the
+ address. However, an alternative to implement this is by utilizing the Linux
+ <constant>IP_FREEBIND</constant>/<constant>IPV6_FREEBIND</constant> socket option, as accessible via
+ <varname>FreeBind=yes</varname> in systemd socket files (see
+ <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+ details). This option, when enabled, allows sockets to be bound to a non-local, not configured IP
+ address, and hence allows bindings to a particular IP address before it actually becomes available,
+ making such an explicit dependency to the configured address redundant. Another often suggested trigger
+ for service activation is low system load. However, here too, a more convincing approach might be to
+ make proper use of features of the operating system, in particular, the CPU or I/O scheduler of Linux.
+ Instead of scheduling jobs from userspace based on monitoring the OS scheduler, it is advisable to
+ leave the scheduling of processes to the OS scheduler itself. systemd provides fine-grained access to
+ the CPU and I/O schedulers. If a process executed by the service manager shall not negatively impact
+ the amount of CPU or I/O bandwidth available to other processes, it should be configured with
+ <varname>CPUSchedulingPolicy=idle</varname> and/or <varname>IOSchedulingClass=idle</varname>.
+ Optionally, this may be combined with timer-based activation to schedule background jobs during runtime
+ and with minimal impact on the system, and remove it from the boot phase itself.</para>
+ </refsect2>
+
+ </refsect1>
+ <refsect1>
+ <title>Integration with systemd</title>
+
+ <refsect2>
+ <title>Writing systemd Unit Files</title>
+
+ <para>When writing systemd unit files, it is recommended to
+ consider the following suggestions:</para>
+
+ <orderedlist>
+ <listitem><para>If possible, do not use the
+ <varname>Type=forking</varname> setting in service files. But
+ if you do, make sure to set the PID file path using
+ <varname>PIDFile=</varname>. See
+ <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details.</para></listitem>
+
+ <listitem><para>If your daemon registers a D-Bus name on the
+ bus, make sure to use <varname>Type=dbus</varname> in the
+ service file if possible.</para></listitem>
+
+ <listitem><para>Make sure to set a good human-readable
+ description string with
+ <varname>Description=</varname>.</para></listitem>
+
+ <listitem><para>Do not disable
+ <varname>DefaultDependencies=</varname>, unless you really
+ know what you do and your unit is involved in early boot or
+ late system shutdown.</para></listitem>
+
+ <listitem><para>Normally, little if any dependencies should
+ need to be defined explicitly. However, if you do configure
+ explicit dependencies, only refer to unit names listed on
+ <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ or names introduced by your own package to keep the unit file
+ operating system-independent.</para></listitem>
+
+ <listitem><para>Make sure to include an
+ [Install] section including installation
+ information for the unit file. See
+ <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details. To activate your service on boot, make sure to
+ add a <varname>WantedBy=multi-user.target</varname> or
+ <varname>WantedBy=graphical.target</varname> directive. To
+ activate your socket on boot, make sure to add
+ <varname>WantedBy=sockets.target</varname>. Usually, you also
+ want to make sure that when your service is installed, your
+ socket is installed too, hence add
+ <varname>Also=foo.socket</varname> in your service file
+ <filename>foo.service</filename>, for a hypothetical program
+ <filename>foo</filename>.</para></listitem>
+
+ </orderedlist>
+ </refsect2>
+
+ <refsect2>
+ <title>Installing systemd Service Files</title>
+
+ <para>At the build installation time (e.g. <command>make
+ install</command> during package build), packages are
+ recommended to install their systemd unit files in the directory
+ returned by <command>pkg-config systemd
+ --variable=systemdsystemunitdir</command> (for system services)
+ or <command>pkg-config systemd
+ --variable=systemduserunitdir</command> (for user services).
+ This will make the services available in the system on explicit
+ request but not activate them automatically during boot.
+ Optionally, during package installation (e.g. <command>rpm
+ -i</command> by the administrator), symlinks should be created
+ in the systemd configuration directories via the
+ <command>enable</command> command of the
+ <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ tool to activate them automatically on boot.</para>
+
+ <para>Packages using
+ <citerefentry project='die-net'><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ are recommended to use a configure script
+ excerpt like the following to determine the
+ unit installation path during source
+ configuration:</para>
+
+ <programlisting>PKG_PROG_PKG_CONFIG()
+AC_ARG_WITH([systemdsystemunitdir],
+ [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
+ [with_systemdsystemunitdir=auto])
+AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [
+ def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)
+
+ AS_IF([test "x$def_systemdsystemunitdir" = "x"],
+ [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"],
+ [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])])
+ with_systemdsystemunitdir=no],
+ [with_systemdsystemunitdir="$def_systemdsystemunitdir"])])
+AS_IF([test "x$with_systemdsystemunitdir" != "xno"],
+ [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])])
+AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])</programlisting>
+
+ <para>This snippet allows automatic
+ installation of the unit files on systemd
+ machines, and optionally allows their
+ installation even on machines lacking
+ systemd. (Modification of this snippet for the
+ user unit directory is left as an exercise for the
+ reader.)</para>
+
+ <para>Additionally, to ensure that
+ <command>make distcheck</command> continues to
+ work, it is recommended to add the following
+ to the top-level <filename>Makefile.am</filename>
+ file in
+ <citerefentry project='die-net'><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based
+ projects:</para>
+
+ <programlisting>AM_DISTCHECK_CONFIGURE_FLAGS = \
+ --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting>
+
+ <para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para>
+
+ <programlisting>if HAVE_SYSTEMD
+systemdsystemunit_DATA = \
+ foobar.socket \
+ foobar.service
+endif</programlisting>
+
+ <para>In the
+ <citerefentry project='die-net'><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ <filename>.spec</filename> file, use snippets like the following
+ to enable/disable the service during
+ installation/deinstallation. This makes use of the RPM macros
+ shipped along systemd. Consult the packaging guidelines of your
+ distribution for details and the equivalent for other package
+ managers.</para>
+
+ <para>At the top of the file:</para>
+
+ <programlisting>BuildRequires: systemd
+%{?systemd_requires}</programlisting>
+
+ <para>And as scriptlets, further down:</para>
+
+ <programlisting>%post
+%systemd_post foobar.service foobar.socket
+
+%preun
+%systemd_preun foobar.service foobar.socket
+
+%postun
+%systemd_postun</programlisting>
+
+ <para>If the service shall be restarted during upgrades, replace
+ the <literal>%postun</literal> scriptlet above with the
+ following:</para>
+
+ <programlisting>%postun
+%systemd_postun_with_restart foobar.service</programlisting>
+
+ <para>Note that <literal>%systemd_post</literal> and
+ <literal>%systemd_preun</literal> expect the names of all units
+ that are installed/removed as arguments, separated by spaces.
+ <literal>%systemd_postun</literal> expects no arguments.
+ <literal>%systemd_postun_with_restart</literal> expects the
+ units to restart as arguments.</para>
+
+ <para>To facilitate upgrades from a package version that shipped
+ only SysV init scripts to a package version that ships both a
+ SysV init script and a native systemd service file, use a
+ fragment like the following:</para>
+
+ <programlisting>%triggerun -- foobar &lt; 0.47.11-1
+if /sbin/chkconfig --level 5 foobar ; then
+ /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&amp;1 || :
+fi</programlisting>
+
+ <para>Where 0.47.11-1 is the first package version that includes
+ the native unit file. This fragment will ensure that the first
+ time the unit file is installed, it will be enabled if and only
+ if the SysV init script is enabled, thus making sure that the
+ enable status is not changed. Note that
+ <command>chkconfig</command> is a command specific to Fedora
+ which can be used to check whether a SysV init script is
+ enabled. Other operating systems will have to use different
+ commands here.</para>
+ </refsect2>
+ </refsect1>
+
+ <refsect1>
+ <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 so
+ offers additional functionality to the daemons as well as
+ simplifying 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
+ <constant>AF_UNIX</constant> 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 creation step and use
+ the passed sockets. Secondly, ensure that the file system socket nodes for local
+ <constant>AF_UNIX</constant> 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 service manager 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 sockets 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>
+ </refsect1>
+
+ <refsect1>
+ <title>Placing Daemon Data</title>
+
+ <para>It is recommended to follow the general guidelines for
+ placing package files, as discussed in
+ <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</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>,
+ <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>