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<span style="float:right">systemd 221</span><hr><div class="refentry"><a name="daemon"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>daemon — Writing and packaging system daemons</p></div><div class="refsect1"><a name="idm47144714447520"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>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
<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>.
This manual page covers both schemes, and in particular includes
recommendations for daemons that shall be included in the systemd
init system.</p><div class="refsect2"><a name="idm47144714445072"></a><h3 id="SysV Daemons">SysV Daemons<a class="headerlink" title="Permalink to this headline" href="#SysV%20Daemons">¶</a></h3><p>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.</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>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
<code class="filename">/proc/self/fd</code>, with a fallback of
iterating from file descriptor 3 to the value returned by
<code class="function">getrlimit()</code> for
<code class="constant">RLIMIT_NOFILE</code>. </p></li><li class="listitem"><p>Reset all signal handlers to their default.
This is best done by iterating through the available signals
up to the limit of <code class="constant">_NSIG</code> and resetting
them to <code class="constant">SIG_DFL</code>.</p></li><li class="listitem"><p>Reset the signal mask
using
<code class="function">sigprocmask()</code>.</p></li><li class="listitem"><p>Sanitize the environment block, removing or
resetting environment variables that might negatively impact
daemon runtime.</p></li><li class="listitem"><p>Call <code class="function">fork()</code>, to create a
background process.</p></li><li class="listitem"><p>In the child, call
<code class="function">setsid()</code> to detach from any terminal and
create an independent session.</p></li><li class="listitem"><p>In the child, call <code class="function">fork()</code>
again, to ensure that the daemon can never re-acquire a
terminal again.</p></li><li class="listitem"><p>Call <code class="function">exit()</code> 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.</p></li><li class="listitem"><p>In the daemon process, connect
<code class="filename">/dev/null</code> to standard input, output, and
error.</p></li><li class="listitem"><p>In the daemon process, reset the umask to 0,
so that the file modes passed to <code class="function">open()</code>,
<code class="function">mkdir()</code> and suchlike directly control the
access mode of the created files and
directories.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>In the daemon process, write the daemon PID
(as returned by <code class="function">getpid()</code>) to a PID file,
for example <code class="filename">/run/foobar.pid</code> (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.</p></li><li class="listitem"><p>In the daemon process, drop privileges, if
possible and applicable.</p></li><li class="listitem"><p>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
<code class="function">fork()</code> and hence available in both the
original and the daemon process.</p></li><li class="listitem"><p>Call <code class="function">exit()</code> in the
original process. The process that invoked the daemon must be
able to rely on that this <code class="function">exit()</code> happens
after initialization is complete and all external
communication channels are established and
accessible.</p></li></ol></div><p>The BSD <code class="function">daemon()</code> function should not
be used, as it implements only a subset of these steps.</p><p>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.</p></div><div class="refsect2"><a name="idm47144709519616"></a><h3 id="New-Style Daemons">New-Style Daemons<a class="headerlink" title="Permalink to this headline" href="#New-Style%20Daemons">¶</a></h3><p>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.</p><p>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 init system, it is recommended not to
execute them when run as new-style service.</p><p>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/output/error connected to
<code class="filename">/dev/null</code> unless otherwise configured. The
umask is reset.
</p><p>It is recommended for new-style daemons to implement the
following:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>If <code class="constant">SIGTERM</code> is received,
shut down the daemon and exit cleanly.</p></li><li class="listitem"><p>If <code class="constant">SIGHUP</code> is received,
reload the configuration files, if this
applies.</p></li><li class="listitem"><p>Provide a correct exit code from the main
daemon process, as this is used by the init system to detect
service errors and problems. It is recommended to follow the
exit code scheme as defined in the <a class="ulink" href="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html" target="_top">LSB
recommendations for SysV init
scripts</a>.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>For integration in systemd, provide a
<code class="filename">.service</code> unit file that carries
information about starting, stopping and otherwise maintaining
the daemon. See
<a href="systemd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd.service</span>(5)</span></a>
for details.</p></li><li class="listitem"><p>As much as possible, rely on the init system'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 similar.
See
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>
for the available controls.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>If applicable, a daemon should notify the init
system about startup completion or status updates via the
<a href="sd_notify.html"><span class="citerefentry"><span class="refentrytitle">sd_notify</span>(3)</span></a>
interface.</p></li><li class="listitem"><p>Instead of using the
<code class="function">syslog()</code> call to log directly to the
system syslog service, a new-style daemon may choose to simply
log to standard error via <code class="function">fprintf()</code>,
which is then forwarded to syslog by the init system. If log
levels are necessary, these can be encoded by prefixing
individual log lines with strings like
"<code class="literal"><4></code>" (for log level 4 "WARNING" in the
syslog priority scheme), following a similar style as the
Linux kernel's <code class="function">printk()</code> level system. For
details, see
<a href="sd-daemon.html"><span class="citerefentry"><span class="refentrytitle">sd-daemon</span>(3)</span></a>
and
<a href="systemd.exec.html"><span class="citerefentry"><span class="refentrytitle">systemd.exec</span>(5)</span></a>.</p></li></ol></div><p>These recommendations are similar but not identical to the
<a class="ulink" href="https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html" target="_top">Apple
MacOS X Daemon Requirements</a>.</p></div></div><div class="refsect1"><a name="idm47144709496560"></a><h2 id="Activation">Activation<a class="headerlink" title="Permalink to this headline" href="#Activation">¶</a></h2><p>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:
<code class="filename">bluetoothd.service</code> 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.</p><div class="refsect2"><a name="idm47144709493632"></a><h3 id="Activation on Boot">Activation on Boot<a class="headerlink" title="Permalink to this headline" href="#Activation%20on%20Boot">¶</a></h3><p>Old-style daemons are usually activated exclusively on
boot (and manually by the administrator) via SysV init scripts,
as detailed in the <a class="ulink" href="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html" target="_top">LSB
Linux Standard Base Core Specification</a>. 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 employ updated
versions of activation, both during boot-up and during runtime
and using more minimal service description files.</p><p>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 <code class="filename">.wants/</code> directory of
either <code class="filename">multi-user.target</code> or
<code class="filename">graphical.target</code>, which are normally used
as boot targets at system startup. See
<a href="systemd.unit.html"><span class="citerefentry"><span class="refentrytitle">systemd.unit</span>(5)</span></a>
for details about the <code class="filename">.wants/</code> directories,
and
<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>
for details about the two boot targets.</p></div><div class="refsect2"><a name="idm47144709486416"></a><h3 id="Socket-Based Activation">Socket-Based Activation<a class="headerlink" title="Permalink to this headline" href="#Socket-Based%20Activation">¶</a></h3><p>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 employ 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 init system. Based on per-daemon
configuration, the init system 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.</p><p>New-style daemons which support socket activation must be
able to receive their sockets from the init system instead of
creating and binding them themselves. For details about the
programming interfaces for this scheme provided by systemd, see
<a href="sd_listen_fds.html"><span class="citerefentry"><span class="refentrytitle">sd_listen_fds</span>(3)</span></a>
and
<a href="sd-daemon.html"><span class="citerefentry"><span class="refentrytitle">sd-daemon</span>(3)</span></a>.
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.</p><p>systemd implements socket-based activation via
<code class="filename">.socket</code> units, which are described in
<a href="systemd.socket.html"><span class="citerefentry"><span class="refentrytitle">systemd.socket</span>(5)</span></a>.
When configuring socket units for socket-based activation, it is
essential that all listening sockets are pulled in by the
special target unit <code class="filename">sockets.target</code>. It is
recommended to place a
<code class="varname">WantedBy=sockets.target</code> directive in the
"<code class="literal">[Install]</code>" section to automatically add such a
dependency on installation of a socket unit. Unless
<code class="varname">DefaultDependencies=no</code> is set, the necessary
ordering dependencies are implicitly created for all socket
units. For more information about
<code class="filename">sockets.target</code>, see
<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>.
It is not necessary or recommended to place any additional
dependencies on socket units (for example from
<code class="filename">multi-user.target</code> or suchlike) when one is
installed in <code class="filename">sockets.target</code>.</p></div><div class="refsect2"><a name="idm47144709474240"></a><h3 id="Bus-Based Activation">Bus-Based Activation<a class="headerlink" title="Permalink to this headline" href="#Bus-Based%20Activation">¶</a></h3><p>When the D-Bus IPC system is used for communication with
clients, new-style daemons should employ 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 <code class="varname">SystemdService=</code>
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
<code class="filename">org.freedesktop.RealtimeKit.service</code>, make
sure to set
<code class="varname">SystemdService=rtkit-daemon.service</code> in that
file to bind it to the systemd service
<code class="filename">rtkit-daemon.service</code>. This is needed to
make sure that the daemon is started in a race-free fashion when
activated via multiple mechanisms simultaneously.</p></div><div class="refsect2"><a name="idm47144714511600"></a><h3 id="Device-Based Activation">Device-Based Activation<a class="headerlink" title="Permalink to this headline" href="#Device-Based%20Activation">¶</a></h3><p>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 "<code class="literal">systemd</code>".
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 <code class="varname">SYSTEMD_WANTS=</code> property. See
<a href="systemd.device.html"><span class="citerefentry"><span class="refentrytitle">systemd.device</span>(5)</span></a>
for details. Often, it is nicer to pull in services from devices
only indirectly via dedicated targets. Example: Instead of
pulling in <code class="filename">bluetoothd.service</code> from all the
various bluetooth dongles and other hardware available, pull in
bluetooth.target from them and
<code class="filename">bluetoothd.service</code> from that target. This
provides for nicer abstraction and gives administrators the
option to enable <code class="filename">bluetoothd.service</code> via
controlling a <code class="filename">bluetooth.target.wants/</code>
symlink uniformly with a command like <span class="command"><strong>enable</strong></span>
of
<a href="systemctl.html"><span class="citerefentry"><span class="refentrytitle">systemctl</span>(1)</span></a>
instead of manipulating the udev ruleset.</p></div><div class="refsect2"><a name="idm47144709453360"></a><h3 id="Path-Based Activation">Path-Based Activation<a class="headerlink" title="Permalink to this headline" href="#Path-Based%20Activation">¶</a></h3><p>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 <code class="filename">.path</code>
units, as outlined in
<a href="systemd.path.html"><span class="citerefentry"><span class="refentrytitle">systemd.path</span>(5)</span></a>.</p></div><div class="refsect2"><a name="idm47144709450432"></a><h3 id="Timer-Based Activation">Timer-Based Activation<a class="headerlink" title="Permalink to this headline" href="#Timer-Based%20Activation">¶</a></h3><p>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
<code class="filename">.timer</code> units, as described in
<a href="systemd.timer.html"><span class="citerefentry"><span class="refentrytitle">systemd.timer</span>(5)</span></a>.</p></div><div class="refsect2"><a name="idm47144709447680"></a><h3 id="Other Forms of Activation">Other Forms of Activation<a class="headerlink" title="Permalink to this headline" href="#Other%20Forms%20of%20Activation">¶</a></h3><p>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 <code class="filename">.socket</code> 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
<code class="constant">IP_FREEBIND</code> socket option, as accessible
via <code class="varname">FreeBind=yes</code> in systemd socket files (see
<a href="systemd.socket.html"><span class="citerefentry"><span class="refentrytitle">systemd.socket</span>(5)</span></a>
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 IO 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 IO schedulers. If a
process executed by the init system shall not negatively impact
the amount of CPU or IO bandwidth available to other processes,
it should be configured with
<code class="varname">CPUSchedulingPolicy=idle</code> and/or
<code class="varname">IOSchedulingClass=idle</code>. 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.</p></div></div><div class="refsect1"><a name="idm47144709441696"></a><h2 id="Integration with Systemd">Integration with Systemd<a class="headerlink" title="Permalink to this headline" href="#Integration%20with%20Systemd">¶</a></h2><div class="refsect2"><a name="idm47144709441056"></a><h3 id="Writing Systemd Unit Files">Writing Systemd Unit Files<a class="headerlink" title="Permalink to this headline" href="#Writing%20Systemd%20Unit%20Files">¶</a></h3><p>When writing systemd unit files, it is recommended to
consider the following suggestions:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>If possible, do not use the
<code class="varname">Type=forking</code> setting in service files. But
if you do, make sure to set the PID file path using
<code class="varname">PIDFile=</code>. See
<a href="systemd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd.service</span>(5)</span></a>
for details.</p></li><li class="listitem"><p>If your daemon registers a D-Bus name on the
bus, make sure to use <code class="varname">Type=dbus</code> in the
service file if possible.</p></li><li class="listitem"><p>Make sure to set a good human-readable
description string with
<code class="varname">Description=</code>.</p></li><li class="listitem"><p>Do not disable
<code class="varname">DefaultDependencies=</code>, unless you really
know what you do and your unit is involved in early boot or
late system shutdown.</p></li><li class="listitem"><p>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
<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>
or names introduced by your own package to keep the unit file
operating system-independent.</p></li><li class="listitem"><p>Make sure to include an
"<code class="literal">[Install]</code>" section including installation
information for the unit file. See
<a href="systemd.unit.html"><span class="citerefentry"><span class="refentrytitle">systemd.unit</span>(5)</span></a>
for details. To activate your service on boot, make sure to
add a <code class="varname">WantedBy=multi-user.target</code> or
<code class="varname">WantedBy=graphical.target</code> directive. To
activate your socket on boot, make sure to add
<code class="varname">WantedBy=sockets.target</code>. Usually, you also
want to make sure that when your service is installed, your
socket is installed too, hence add
<code class="varname">Also=foo.socket</code> in your service file
<code class="filename">foo.service</code>, for a hypothetical program
<code class="filename">foo</code>.</p></li></ol></div></div><div class="refsect2"><a name="idm47144709427104"></a><h3 id="Installing Systemd Service Files">Installing Systemd Service Files<a class="headerlink" title="Permalink to this headline" href="#Installing%20Systemd%20Service%20Files">¶</a></h3><p>At the build installation time (e.g. <span class="command"><strong>make
install</strong></span> during package build), packages are
recommended to install their systemd unit files in the directory
returned by <span class="command"><strong>pkg-config systemd
--variable=systemdsystemunitdir</strong></span> (for system services)
or <span class="command"><strong>pkg-config systemd
--variable=systemduserunitdir</strong></span> (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. <span class="command"><strong>rpm
-i</strong></span> by the administrator), symlinks should be created
in the systemd configuration directories via the
<span class="command"><strong>enable</strong></span> command of the
<a href="systemctl.html"><span class="citerefentry"><span class="refentrytitle">systemctl</span>(1)</span></a>
tool to activate them automatically on boot.</p><p>Packages using
<a href="http://linux.die.net/man/1/autoconf"><span class="citerefentry"><span class="refentrytitle">autoconf</span>(1)</span></a>
are recommended to use a configure script
excerpt like the following to determine the
unit installation path during source
configuration:</p><pre class="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"])</pre><p>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.)</p><p>Additionally, to ensure that
<span class="command"><strong>make distcheck</strong></span> continues to
work, it is recommended to add the following
to the top-level <code class="filename">Makefile.am</code>
file in
<a href="http://linux.die.net/man/1/automake"><span class="citerefentry"><span class="refentrytitle">automake</span>(1)</span></a>-based
projects:</p><pre class="programlisting">DISTCHECK_CONFIGURE_FLAGS = \
--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</pre><p>Finally, unit files should be installed in the system with an automake excerpt like the following:</p><pre class="programlisting">if HAVE_SYSTEMD
systemdsystemunit_DATA = \
foobar.socket \
foobar.service
endif</pre><p>In the
<a href="http://linux.die.net/man/8/rpm"><span class="citerefentry"><span class="refentrytitle">rpm</span>(8)</span></a>
<code class="filename">.spec</code> 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.</p><p>At the top of the file:</p><pre class="programlisting">BuildRequires: systemd
%{?systemd_requires}</pre><p>And as scriptlets, further down:</p><pre class="programlisting">%post
%systemd_post foobar.service foobar.socket
%preun
%systemd_preun foobar.service foobar.socket
%postun
%systemd_postun</pre><p>If the service shall be restarted during upgrades, replace
the "<code class="literal">%postun</code>" scriptlet above with the
following:</p><pre class="programlisting">%postun
%systemd_postun_with_restart foobar.service</pre><p>Note that "<code class="literal">%systemd_post</code>" and
"<code class="literal">%systemd_preun</code>" expect the names of all units
that are installed/removed as arguments, separated by spaces.
"<code class="literal">%systemd_postun</code>" expects no arguments.
"<code class="literal">%systemd_postun_with_restart</code>" expects the
units to restart as arguments.</p><p>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:</p><pre class="programlisting">%triggerun -- foobar < 0.47.11-1
if /sbin/chkconfig --level 5 foobar ; then
/bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || :
fi</pre><p>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
<span class="command"><strong>chkconfig</strong></span> 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.</p></div></div><div class="refsect1"><a name="idm47144709400512"></a><h2 id="Porting Existing Daemons">Porting Existing Daemons<a class="headerlink" title="Permalink to this headline" href="#Porting%20Existing%20Daemons">¶</a></h2><p>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.</p><p>To port an existing SysV compatible daemon, the following
steps are recommended:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>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.</p></li><li class="listitem"><p>If the daemon offers interfaces to other
software running on the local system via local
<code class="constant">AF_UNIX</code> 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
<a href="sd_listen_fds.html"><span class="citerefentry"><span class="refentrytitle">sd_listen_fds</span>(3)</span></a>
is checked for already passed sockets first. If sockets are
passed (i.e. when <code class="function">sd_listen_fds()</code> returns a
positive value), skip the socket creation step and use the
passed sockets. Secondly, ensure that the file system socket
nodes for local <code class="constant">AF_UNIX</code> sockets used in the
socket-based activation are not removed when the daemon shuts
down, if sockets have been passed. Third, if the daemon normally
closes all remaining open file descriptors as part of its
initialization, the sockets passed from the init system must be
spared. Since new-style init systems guarantee that no left-over
file descriptors are passed to executed processes, it might be a
good choice to simply skip the closing of all remaining open
file descriptors if sockets are passed.</p></li><li class="listitem"><p>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.</p></li><li class="listitem"><p>If the daemon exposes interfaces via D-Bus,
write and install a D-Bus activation file for the service, see
above for details.</p></li></ol></div></div><div class="refsect1"><a name="idm47144709391648"></a><h2 id="Placing Daemon Data">Placing Daemon Data<a class="headerlink" title="Permalink to this headline" href="#Placing%20Daemon%20Data">¶</a></h2><p>It is recommended to follow the general guidelines for
placing package files, as discussed in
<a href="file-hierarchy.html"><span class="citerefentry"><span class="refentrytitle">file-hierarchy</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm47144709389616"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
<a href="sd-daemon.html"><span class="citerefentry"><span class="refentrytitle">sd-daemon</span>(3)</span></a>,
<a href="sd_listen_fds.html"><span class="citerefentry"><span class="refentrytitle">sd_listen_fds</span>(3)</span></a>,
<a href="sd_notify.html"><span class="citerefentry"><span class="refentrytitle">sd_notify</span>(3)</span></a>,
<a href="daemon.html"><span class="citerefentry"><span class="refentrytitle">daemon</span>(3)</span></a>,
<a href="systemd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd.service</span>(5)</span></a>,
<a href="file-hierarchy.html"><span class="citerefentry"><span class="refentrytitle">file-hierarchy</span>(7)</span></a>
</p></div></div></body></html>
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