X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=man%2Fdaemon.xml;h=a8bbfc055b62f7f1cff4d0b1f9b37f212bb9244e;hp=3d1e921a165c36fd594bfe726b94f6f7674d8a12;hb=6ecb6cec66739d733e95302031998f517261380c;hpb=64aba792f7d2af56ba5308518a270490f85f3b8b diff --git a/man/daemon.xml b/man/daemon.xml index 3d1e921a1..a8bbfc055 100644 --- a/man/daemon.xml +++ b/man/daemon.xml @@ -1,6 +1,6 @@ + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> - - - - daemon - systemd - - - - Developer - Lennart - Poettering - lennart@poettering.net - - - - - - daemon - 7 - - - - daemon - Writing and Packaging System Daemons - - - - Description - - 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 systemd. - - - SysV Daemons - - 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 - with SysV is essential. - - - Close all open file - descriptors except STDIN, STDOUT, - STDERR (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 - /proc/self/fd, - with a fallback of iterating from file - descriptor 3 to the value returned by - getrlimit() for - RLIMIT_NOFILE. - - Reset all signal - handlers to their default. This is - best done by iterating through the - available signals up to the limit of - _NSIG and resetting them to - SIG_DFL. - - Reset the signal mask - using sigprocmask(). - - Call fork(), - to create a background - process. - - In the child, call - 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. - - 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 - init/PID 1, as all daemons should - be. - - In the daemon process, - connect /dev/null - to STDIN, STDOUT, - STDERR. - - In the daemon process, - reset the umask to 0, so that the file - modes passed to open(), mkdir() and - suchlike directly control the access - mode of the created files and - directories. - - 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. - - In the daemon process, - drop privileges, if possible and - applicable. - - 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 fork() and available - in both processes. - - 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 - accessible. - - - The BSD daemon() function should not be - used, as it does 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 - well as to simplify integration into systems - using systemd. - - - - New-Style Daemons - - 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. - - 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. - - It is recommended for new-style daemons - to implement the following: - - - If SIGTERM is - received, shut down the daemon and - exit cleanly. - - If SIGHUP is received, - reload the configuration files, if - this applies. - - 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 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. - - 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. - - If D-Bus is used, make - your daemon bus-activatable, via - 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. - - 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 parallization 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. - - If applicable a daemon - should notify the init system about - startup completion or status - updates via the sd_notify() - interface. - - Instead of using the - syslog() call to log directly to the - system logger, a new-style daemon may - choose to simply log to STDERR via - fprintf(), which is then forwarded to - syslog by the init system. If log - 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. - - - - - - Bus Activation - - - - Socket Activation - - - - Writing Service Files - - - - Installing Service Files - - - - - - - See Also - - daemon3, - sd_listen_fds3 - - + + + + daemon + systemd + + + + Developer + Lennart + Poettering + lennart@poettering.net + + + + + + daemon + 7 + + + + daemon + Writing and packaging system daemons + + + + Description + + 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 + 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 + + 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. + + + 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 + /proc/self/fd, with a fallback of + iterating from file descriptor 3 to the value returned by + getrlimit() for + RLIMIT_NOFILE. + + Reset all signal handlers to their default. + This is best done by iterating through the available signals + up to the limit of _NSIG and resetting + them to SIG_DFL. + + Reset the signal mask + using + sigprocmask(). + + 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. + + In the child, call fork() + again, to ensure that the daemon can never re-acquire a + terminal again. + + 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 + re-parented to init/PID 1, as all daemons should + be. + + In the daemon process, connect + /dev/null to standard input, output, and + error. + + In the daemon process, reset the umask to 0, + so that the file modes passed to open(), + mkdir() and suchlike directly control the + access mode of the created files and + directories. + + 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. + + In the daemon process, write the daemon PID + (as returned by getpid()) to a PID file, + for example /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. + + In the daemon process, drop privileges, if + possible and applicable. + + 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 + fork() and hence available in both the + original and the daemon process. + + Call exit() in the + original process. The process that invoked the daemon must be + able to 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 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 behavior optional and + configurable via a command line argument to ease debugging as + well as to simplify integration into systems using + systemd. + + + + New-Style Daemons + + 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. + + 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. + + 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 + /dev/null unless otherwise configured. The + umask is reset. + + + It is recommended for new-style daemons to implement the + following: + + + If SIGTERM is received, + shut down the daemon and exit cleanly. + + If SIGHUP is received, + reload the configuration files, if this + applies. + + 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 LSB + recommendations for SysV init + scripts. + + 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. + + 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 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. + + 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. + + If applicable, a daemon should notify the init + system about startup completion or status updates via the + sd_notify3 + interface. + + Instead of using the + syslog() call to log directly to the + system syslog service, a new-style daemon may choose to simply + log to standard error via fprintf(), + 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 + <4> (for log level 4 "WARNING" in the + syslog priority scheme), following a similar style as the + Linux kernel's printk() level system. 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 that 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. + + + + + 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. + + + + 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. + + + + 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. + + + + 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=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"]) + + 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. + + + + + Placing Daemon Data + + It is recommended to follow the general guidelines for + placing package files, as discussed in + file-hierarchy7. + + + + See Also + + systemd1, + sd-daemon3, + sd_listen_fds3, + sd_notify3, + daemon3, + systemd.service5, + file-hierarchy7 + +