man/daemon.xml - main - niceSOFT/systemd

man/daemon.xml
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Mike Yuan
docs,man: more SysV compat removal notes
18 янв 2026, 09:50
18 янв 2026, 09:5073b8207
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/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 is 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>. Among other issues, SysV init scripts have
      the disadvantage of involving shell scripts in the boot process. Support for init scripts has been
      phased out in modern Linux init systems, and should not be relied upon. 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>Notes</title>

    <para>
       All example codes in this page are licensed under <literal>MIT No Attribution</literal>
      (SPDX-License-Identifier: MIT-0).
    </para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>