X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=man%2Fdaemon.xml;h=7790420c6eb7154cfa80625f24fdb9db43cb0e2b;hp=650e0fa8b1801d069eca9c8e4cfbec0069557766;hb=44ded3abc28620279633f51a05f2416e5aa3e8e2;hpb=160cd5c9aa2301892e13950015de7968c764340d
diff --git a/man/daemon.xml b/man/daemon.xml
index 650e0fa8b..7790420c6 100644
--- a/man/daemon.xml
+++ b/man/daemon.xml
@@ -8,20 +8,20 @@
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU General Public License as published by
- the Free Software Foundation; either version 2 of the License, or
+ under the terms of the GNU Lesser General Public License as published by
+ the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- General Public License for more details.
+ Lesser General Public License for more details.
- You should have received a copy of the GNU General Public License
+ You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see .
-->
-
+
daemon
@@ -44,7 +44,7 @@
daemon
- Writing and Packaging System Daemons
+ Writing and packaging system daemons
@@ -55,8 +55,12 @@
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 systemd.
+ yet more powerful scheme (here called "new-style"
+ daemons), as implemented by
+ systemd1. This
+ manual page covers both schemes, and in
+ particular includes recommendations for daemons that
+ shall be included in the systemd init system.
SysV Daemons
@@ -64,8 +68,8 @@
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,
- and should only be implemented if compatbility
+ steps are unnecessary for new-style daemons (see below),
+ and should only be implemented if compatibility
with SysV is essential.
@@ -75,12 +79,12 @@
descriptors 0, 1, 2). This ensures
that no accidentally passed file
descriptor stays around in the daemon
- process. On Linux this is best
+ process. On Linux, this is best
implemented by iterating through
/proc/self/fd,
with a fallback of iterating from file
descriptor 3 to the value returned by
- getrlimit() for
+ getrlimit() for
RLIMIT_NOFILE.
Reset all signal
@@ -88,30 +92,37 @@
best done by iterating through the
available signals up to the limit of
_NSIG and resetting them to
- SIG_DFL.
+ SIG_DFL.
Reset the signal mask
- using sigprocmask().
+ using
+ sigprocmask().
- Call fork(),
+ Sanitize the
+ environment block, removing or
+ resetting environment variables that
+ might negatively impact daemon
+ runtime.
+
+ Call fork(),
to create a background
process.
In the child, call
- setsid() to detach from any terminal
- and create an independent
- session.
+ setsid() to
+ detach from any terminal and create an
+ independent session.
In the child, call
- fork() again, to ensure the daemon can
- never re-aquire a terminal
- again.
+ fork() again, to
+ ensure that the daemon can never re-acquire
+ a terminal again.
- Call exit() in the
+ Call exit() in the
first child, so that only the second
child (the actual daemon process)
stays around. This ensures that the
- daemon process is reparented to
+ daemon process is re-parented to
init/PID 1, as all daemons should
be.
@@ -122,7 +133,7 @@
In the daemon process,
reset the umask to 0, so that the file
- modes passed to open(), mkdir() and
+ modes passed to open(), mkdir() and
suchlike directly control the access
mode of the created files and
directories.
@@ -134,38 +145,59 @@
blocks mount points from being
unmounted.
+ In the daemon process,
+ write the daemon PID (as returned by
+ getpid()) to a
+ PID file, for example
+ /var/run/foobar.pid
+ (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. Commonly, some kind of
+ file locking is employed to implement
+ this logic.
+
In the daemon process,
drop privileges, if possible and
applicable.
From the daemon
- process notify the original process
+ 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 fork() and available
- in both processes.
+ before the first
+ fork() and hence
+ available in both the original and the
+ daemon process.
- Call exit() in the
+ Call
+ exit() in the
original process. The process that
invoked the daemon must be able to
- rely that this exit() happens after
- initialization is complete and all
- external communication channels
- established and
+ rely on that this
+ exit() happens
+ after initialization is complete and
+ all external communication channels
+ are established and
accessible.
- The BSD daemon() function should not be
- used, as it does only a subset of these steps.
+ The BSD daemon() function should not be
+ used, as it implements only a subset of these steps.
A daemon that needs to provide
compatibility with SysV systems should
implement the scheme pointed out
above. However, it is recommended to make this
- behaviour optional and configurable via a
- command line argument, to ease debugging as
+ behavior optional and configurable via a
+ command line argument to ease debugging as
well as to simplify integration into systems
using systemd.
@@ -179,26 +211,37 @@
runtime and simplifies their
implementation.
- For developing a new-style daemon none
+ 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
+ the init system, it is recommended not to
execute them when run as new-style
service.
+ 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, and
+ STDIN/STDOUT/STDERR connected to
+ /dev/null unless
+ otherwise configured. The umask is reset.
+
It is recommended for new-style daemons
to implement the following:
- If SIGTERM is
+ If SIGTERM is
received, shut down the daemon and
exit cleanly.
- If SIGHUP is received,
+ If SIGHUP is received,
reload the configuration files, if
this applies.
@@ -207,29 +250,42 @@
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 LSB
- recommendations for SysV init scripts
- (http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html).
-
- As much as possible,
- rely on systemd's functionality to
- limit the accces of the daemon to
- files, services and other
- resources. i.e. 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.
+ scheme as defined in the LSB
+ recommendations for SysV init
+ scripts.
If possible and
- applicable expose the daemon's control
+ applicable, expose the daemon's control
interface via the D-Bus IPC system and
grab a bus name as last step of
initialization.
+ For integration in
+ systemd, provide a
+ .service unit
+ file that carries information about
+ starting, stopping and otherwise
+ maintaining the daemon. See
+ systemd.service5
+ for details.
+
+ 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
+ systemd.exec5
+ for the available
+ controls.
+
If D-Bus is used, make
- your daemon bus-activatable, via
+ your daemon bus-activatable by
supplying a D-Bus service activation
configuration file. This has multiple
advantages: your daemon may be started
@@ -237,10 +293,10 @@
parallel to other daemons requiring it
-- which maximizes parallelization and
boot-up speed; your daemon can be
- restarted on failure, without losing
+ restarted on failure without losing
any bus requests, as the bus queues
- requests for activatable
- services.
+ requests for activatable services. See
+ below for details.
If your daemon
provides services to other local
@@ -248,69 +304,645 @@
socket, it should be made
socket-activatable following the
scheme pointed out below. Like D-Bus
- activation this enables on-demand
+ activation, this enables on-demand
starting of services as well as it
- allows improved parallization of
+ allows improved parallelization of
service start-up. Also, for state-less
- protocols (such as syslog, DNS) a
+ protocols (such as syslog, DNS), a
daemon implementing socket-based
activation can be restarted without
- losing a single
- request.
+ losing a single request. See below for
+ details.
- If applicable a daemon
+ If applicable, a daemon
should notify the init system about
- startup completion or status
- updates via the sd_notify()
+ startup completion or status updates
+ via the
+ sd_notify3
interface.
Instead of using the
- syslog() call to log directly to the
- system logger, a new-style daemon may
+ syslog() call to log directly to the
+ system syslog service, a new-style daemon may
choose to simply log to STDERR via
- fprintf(), which is then forwarded to
+ fprintf(), which is then forwarded to
syslog by the init system. If log
- priorities are necessary these can be
+ priorities are necessary, these can be
encoded by prefixing individual log
lines with strings like "<4>"
(for log priority 4 "WARNING" in the
syslog priority scheme), following a
similar style as the Linux kernel's
- printk() priority system. In fact, using
- this style of logging also enables the
- init system to optionally direct all
- application logging to the kernel log
- buffer (kmsg), as accessible via
- dmesg.
+ printk() priority system. In fact,
+ using this style of logging also
+ enables the init system to optionally
+ direct all application logging to the
+ kernel log buffer (kmsg), as
+ accessible via
+ dmesg1. This
+ kind of logging may be enabled by
+ setting
+ StandardError=syslog
+ in the service unit file. For details,
+ see
+ sd-daemon3
+ and
+ systemd.exec5.
+
+ These recommendations are similar but
+ not identical to the Apple
+ MacOS X Daemon Requirements.
+
+
+
+
+ Activation
+
+ 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:
+ bluetoothd.service 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.
+
+
+ Activation on Boot
+
+ Old-style daemons are usually activated
+ exclusively on boot (and manually by the
+ administrator) via SysV init scripts, as
+ detailed in the LSB
+ Linux Standard Base Core
+ Specification. 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.
+
+ In systemd, if the developer or
+ administrator wants to make sure a service or
+ other unit is activated automatically on boot,
+ it is recommended to place a symlink to the
+ unit file in the .wants/
+ directory of either
+ multi-user.target or
+ graphical.target, which
+ are normally used as boot targets at system
+ startup. See
+ systemd.unit5
+ for details about the
+ .wants/ directories, and
+ systemd.special7
+ for details about the two boot targets.
+
- Bus Activation
+ Socket-Based Activation
+
+ 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.
+
+ 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
+ sd_listen_fds3
+ and
+ sd-daemon3. 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.
+
+ systemd implements socket-based
+ activation via .socket
+ units, which are described in
+ systemd.socket5. When
+ configuring socket units for socket-based
+ activation, it is essential that all listening
+ sockets are pulled in by the special target
+ unit sockets.target. It
+ is recommended to place a
+ WantedBy=sockets.target
+ directive in the [Install]
+ section to automatically add such a
+ dependency on installation of a socket
+ unit. Unless
+ DefaultDependencies=no is
+ set, the necessary ordering dependencies are
+ implicitly created for all socket units. For
+ more information about
+ sockets.target, see
+ systemd.special7. It
+ is not necessary or recommended to place any
+ additional dependencies on socket units (for
+ example from
+ multi-user.target or
+ suchlike) when one is installed in
+ sockets.target.
- Socket Activation
+ Bus-Based Activation
+
+ 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
+ SystemdService= 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
+ org.freedesktop.RealtimeKit.service,
+ make sure to set
+ SystemdService=rtkit-daemon.service
+ in that file to bind it to the systemd
+ service
+ rtkit-daemon.service. This
+ is needed to make sure that the daemon is
+ started in a race-free fashion when activated
+ via multiple mechanisms simultaneously.
- Writing Service Files
+ Device-Based Activation
+
+ 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
+ systemd. 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
+ SYSTEMD_WANTS=
+ property. See
+ systemd.device5
+ for details. Often, it is nicer to pull in
+ services from devices only indirectly via
+ dedicated targets. Example: Instead of pulling
+ in bluetoothd.service
+ from all the various bluetooth dongles and
+ other hardware available, pull in
+ bluetooth.target from them and
+ bluetoothd.service from
+ that target. This provides for nicer
+ abstraction and gives administrators the
+ option to enable
+ bluetoothd.service via
+ controlling a
+ bluetooth.target.wants/
+ symlink uniformly with a command like
+ enable of
+ systemctl1
+ instead of manipulating the udev
+ ruleset.
- Installing Service Files
+ Path-Based Activation
+
+ 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
+ .path units, as outlined
+ in
+ systemd.path5.
+
+
+
+ Timer-Based Activation
+
+ 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 .timer units, as
+ described in
+ systemd.timer5.
+
+
+
+ Other Forms of Activation
+
+ 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 .socket
+ 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 IP_FREEBIND
+ socket option, as accessible via
+ FreeBind=yes in systemd
+ socket files (see
+ systemd.socket5
+ 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
+ CPUSchedulingPolicy=idle
+ and/or
+ IOSchedulingClass=idle. 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.
+
+ Integration with Systemd
+
+ Writing Systemd Unit Files
+
+ When writing systemd unit files, it is
+ recommended to consider the following
+ suggestions:
+
+
+ If possible, do not use
+ the Type=forking
+ setting in service files. But if you
+ do, make sure to set the PID file path
+ using PIDFile=. See
+ systemd.service5
+ for details.
+
+ If your daemon
+ registers a D-Bus name on the bus,
+ make sure to use
+ Type=dbus in the
+ service file if
+ possible.
+
+ Make sure to set a
+ good human-readable description string
+ with
+ Description=.
+
+ Do not disable
+ DefaultDependencies=,
+ unless you really know what you do and
+ your unit is involved in early boot or
+ late system shutdown.
+
+ 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
+ systemd.special7
+ or names introduced by your own
+ package to keep the unit file
+ operating
+ system-independent.
+
+ Make sure to include
+ an [Install]
+ section including installation
+ information for the unit file. See
+ systemd.unit5
+ for details. To activate your service
+ on boot, make sure to add a
+ WantedBy=multi-user.target
+ or
+ WantedBy=graphical.target
+ directive. To activate your socket on
+ boot, make sure to add
+ WantedBy=sockets.target. Usually,
+ you also want to make sure that when
+ your service is installed, your socket
+ is installed too, hence add
+ Also=foo.socket in
+ your service file
+ foo.service, for
+ a hypothetical program
+ foo.
+
+
+
+
+
+ Installing Systemd Service Files
+
+ At the build installation time
+ (e.g. make install during
+ package build), packages are recommended to
+ install their systemd unit files in the
+ directory returned by pkg-config
+ systemd
+ --variable=systemdsystemunitdir (for
+ system services) or pkg-config
+ systemd
+ --variable=systemduserunitdir
+ (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. rpm -i
+ by the administrator), symlinks should be
+ created in the systemd configuration
+ directories via the enable
+ command of the
+ systemctl1
+ tool to activate them automatically on
+ boot.
+
+ Packages using
+ autoconf1
+ are recommended to use a configure script
+ excerpt like the following to determine the
+ unit installation path during source
+ configuration:
+
+ PKG_PROG_PKG_CONFIG
+AC_ARG_WITH([systemdsystemunitdir],
+ AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files]),
+ [], [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
+if test "x$with_systemdsystemunitdir" != xno; then
+ AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
+fi
+AM_CONDITIONAL(HAVE_SYSTEMD, [test -n "$with_systemdsystemunitdir" -a "x$with_systemdsystemunitdir" != xno ])
+
+ 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.)
+
+ Additionally, to ensure that
+ make distcheck continues to
+ work, it is recommended to add the following
+ to the top-level Makefile.am
+ file in
+ automake1-based
+ projects:
+
+ DISTCHECK_CONFIGURE_FLAGS = \
+ --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
+
+ Finally, unit files should be installed in the system with an automake excerpt like the following:
+
+ if HAVE_SYSTEMD
+systemdsystemunit_DATA = \
+ foobar.socket \
+ foobar.service
+endif
+
+ In the
+ rpm8
+ .spec 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.
+
+ At the top of the file:
+
+ BuildRequires: systemd
+%{?systemd_requires}
+
+ And as scriptlets, further down:
+
+ %post
+%systemd_post foobar.service foobar.socket
+
+%preun
+%systemd_preun foobar.service foobar.socket
+
+%postun
+%systemd_postun
+
+ If the service shall be restarted during
+ upgrades, replace the
+ %postun scriptlet above
+ with the following:
+
+ %postun
+%systemd_postun_with_restart foobar.service
+
+ Note that
+ %systemd_post and
+ %systemd_preun expect the
+ names of all units that are installed/removed
+ as arguments, separated by
+ spaces. %systemd_postun
+ expects no
+ arguments. %systemd_postun_with_restart
+ expects the units to restart as
+ arguments.
+
+ 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:
+
+ %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
+
+ 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
+ chkconfig 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.
+
+
+
+
+ Porting Existing Daemons
+
+ 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.
+
+ To port an existing SysV compatible daemon, the
+ following steps are recommended:
+
+
+ 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.
+
+ If the daemon offers
+ interfaces to other software running on the
+ local system via local AF_UNIX sockets,
+ consider implementing socket-based activation
+ (see above). Usually, a minimal patch is
+ sufficient to implement this: Extend the
+ socket creation in the daemon code so that
+ sd_listen_fds3
+ is checked for already passed sockets
+ first. If sockets are passed (i.e. when
+ sd_listen_fds() returns a
+ positive value), skip the socket creation step
+ and use the passed sockets. Secondly, ensure
+ that the file system socket nodes for local
+ AF_UNIX sockets used in the socket-based
+ activation are not removed when the daemon
+ shuts down, if sockets have been
+ passed. Third, if the daemon normally closes
+ all remaining open file descriptors as part of
+ its initialization, the sockets passed from
+ the init system must be spared. Since
+ new-style init systems guarantee that no
+ left-over file descriptors are passed to
+ executed processes, it might be a good choice
+ to simply skip the closing of all remaining
+ open file descriptors if sockets are
+ passed.
+
+ 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.
+
+ If the daemon exposes
+ interfaces via D-Bus, write and install a
+ D-Bus activation file for the service, see
+ above for details.
+
+
See Also
systemd1,
+ sd-daemon3,
+ sd_listen_fds3,
+ sd_notify3,
daemon3,
- sd_listen_fds3
+ systemd.service5