launchd

launchd

Original author(s)	Apple Computer
Initial release	29 April 2005
Development status	Active
Written in	C
Operating system	Mac OS X
Type	System Startup software
License	Apache License 2.0
Website	http://launchd.macosforge.org/

launchd is a unified, open-source service management framework for starting, stopping and managing daemons, applications, processes, and scripts. Written and designed by Dave Zarzycki at Apple, it was introduced with Mac OS X Tiger and is licensed under the Apache License.

The launchd daemon is essentially a replacement for:

• init
• rc
• init.d script
• rc.d script
• SystemStarter (Mac OS X)
• inetd
• xinetd
• atd
• crond
• watchdogd

Apple has stated that it intends to eliminate all of the aforementioned services in favor of launchd.

For Mac OS X v10.4 Apple moved most of the processes handled by the previously mentioned daemons to launchd. By consolidating all the launch services into one program, launchd significantly shortens boot time on slow computers.

Components

There are two main programs in the launchd system: launchd and launchctl.

launchd manages the daemons at both a system and user level. Similar to xinetd, launchd can start daemons on demand. Similar to watchdogd, launchd can monitor daemons to make sure that they keep running. launchd also has replaced init as PID 1 on Mac OS X and as a result it is responsible for starting the system at boot time.

Configuration files define the parameters of services run by launchd. Stored in the LaunchAgents and LaunchDaemons subdirectories of the Library folders, the property list-based files have approximately thirty different keys that can be set.

launchctl is a command line application used to load and unload daemons, start and stop launchd controlled jobs, get system utilization statistics for launchd and its child processes, and set environment settings.

launchd

launchd has two main tasks. The first is to boot the system, and the second is to load and maintain services.

Here is a simplified view of the Mac OS X Tiger system startup on a PowerPC Mac (on an Intel Mac, EFI replaces Open Firmware and boot.efi replaces BootX):

1. Open Firmware activates, initializes the hardware, and then loads BootX.
2. BootX loads the kernel, spins the pinwheel cursor, and loads any needed kernel extensions (kexts).
3. The kernel loads launchd.
4. launchd runs /etc/rc, scans through /System/Library/LaunchAgents and /Library/LaunchDaemons and acts on the plists as needed, and starts the login window.

In step 4, launchd scans through a few different directories for jobs to run. There are two different folders that are scanned. The LaunchDaemons folders contain items that will run as root, generally background processes. The LaunchAgents folders contain jobs, called agent applications, that will run as a user or in the context of userland. These may be scripts or other foreground items, and they can even include a user interface. These directories are all kept in the typical Library folders of Mac OS X.

Launchd is very different from SystemStarter in that it may not actually launch all the daemons at boot time. Key to launchd, and similar to xinetd, is the idea of launch on demand daemons. When launchd scans through the job plists at boot time it reserves and listens on all of the ports requested by those jobs. If so indicated in the plist by the "OnDemand" key, the daemon is not actually loaded at the time. Rather, launchd will listen on the port, start the daemon when needed, and shut it down when it is not. After a daemon is loaded, launchd will keep track of it and make sure it is running if needed. In this way it is like watchdogd, and shares watchdogd's requirement that processes do not attempt to fork or daemonize on their own. If a process goes into the background launchd will lose track of it and attempt to relaunch it.

Consequently, Mac OS X Tiger boots much faster than previous releases. The system only has to register the daemons that are to run, not actually launch them. In fact, the progress bar that appears during boot time is just a placebo application (named WaitingForLoginWindow) that does not really show anything other than the passage of time.

The hardest part to manage during a launchd boot is dependencies. SystemStarter had a very simple system of dependencies that used the "Uses", "Requires", and "Provides" keys in the plist of a startup item. There are two main strategies when creating launch dependencies on Tiger. Using IPC will allow the daemons to talk amongst themselves to work it out, or you can watch files or paths for changes. Using IPC is much more subtle than the SystemStarter's keys and requires more work from the developer, but it may lead to cleaner and quicker startups. The SystemStarter is an option that is still supported at this time, but it has been reduced in importance in Mac OS X Tiger; it may not be available in future OS X versions.

launchctl

One of the major complaints with the other facilities for service control is that they are strewn across the OS with no central way to manage them. Apple fixes this with launchctl.

On its own, launchctl can take commands from the command line, from standard in, or operate in interactive mode. A set of commands can be made permanent when stored in ~/.launchd.conf or /etc/launchd.conf. With superuser privileges, launchctl can be used to make changes on a global scale.

Property list

A property list (plist) is a type of file that Apple uses for program configuration. When launchd scans a folder, or a job is submitted with launchctl, it reads a plist file that describes how the program is to be run.

A list of often used keys follows below. For a full list, see Apple's manpage for launchd.plist.

Key	Type	Description	Required
Label	String	The name of the job. By convention, the job label is the same as the plist file name, without the .plist extension.	Required.
Program	String	A path to an executable. Useful for simple launches.	At least one of Program or ProgramArguments is required
ProgramArguments	Array of Strings	An array of strings representing a unix command. The first string is generally a path to an executable, while latter strings contain options or parameters.	At least one of Program or ProgramArguments is required
UserName	String	The job will be run as the given user, who may not necessarily be the one who submitted it to launchd.	Optional, defaults to root or current user.
OnDemand	Boolean	Deprecated as of 10.5 with the more powerful "KeepAlive" option. A boolean flag that defines if a job runs continuously or not	Optional, defaults to true.
RunAtLoad	Boolean	A boolean flag that defines if a task is launched immediately when the job is loaded into launchd.	Optional, defaults to false.
StartOnMount	Boolean	A boolean flag that defines if a task is launched when a new file system is mounted.	Optional, defaults to false.
QueueDirectories	Array of Strings	Watch a directory for new files. The directory must be empty to begin with, and must be returned to an empty state before QueueDirectories will launch its task again.	Optional
WatchPaths	Array of Strings	Watch a file-system path for changes - this could be a file or folder.	Optional
StartInterval	Integer	Used to schedule a job that runs on a repeating schedule. Specified as the number of seconds to wait between runs.	Optional
StartCalendarInterval	Dictionary of Integers or Array of Dictionary of Integers	Job scheduling. The syntax is similar to cron.	Optional
RootDirectory	String	The job will be chrooted into this directory before execution	Optional
WorkingDirectory	String	The job will be chdired into this directory before execution	Optional
StandardInPath, StandardOutPath, StandardErrorPath	String	Keys to determine files for input and output for the launched process	Optional
LowPriorityIO	Boolean	Tells the kernel that this task is of a low priority when doing file system I/O	Optional
AbandonProcessGroup	Boolean	A boolean flag that defines whether subprocess launched from tasks launched by launchd will be killed when the tasks end. Useful where a short-lived task starts a long-lived subtask, but may result in zombie processes.	Optional, defaults to false

Use outside Mac OS X

A port to FreeBSD was done as part of Google Summer of Code Project in 2005.

The Ubuntu Linux distribution considered using launchd in 2006. However, launchd was rejected as an option because it was released under the Apple Public Source License – which at the time was described as an "inescapable licence problem".

In August 2006, Apple relicensed launchd under the Apache License, Version 2.0 in an effort to make adoption by other open source developers easier.

See also

• Daemontools
• Runit
• eINIT
• Initng
• Operating System service management
• Super-server
• TCP Wrapper
• Upstart
• Service Management Facility
• systemd

External links

• System Startup Programming Topics: Creating launchd Daemons and Agents at developer.apple.com
• Technical Note TN2083: Daemons and Agents at developer.apple.com
• Getting Started with launchd cache on web.archive.org originally at developer.apple.com
• launchd in Depth from AFP548, published July 08 2005. The basis for much of this article.
• launchd: One Program to Rule Them All – Google Tech talk video presentation by the developer of launchd.
• All About launchd Items (and How To Make One Yourself): The creation of property lists for launchd
• launchd site at macosforge.org contains the official subversion repository for the launchd source.
• launchd's source code at Mac OS Forge (click on "trunk", then "Zip Archive" to get the full source)
• launchd - FreeBSD-Wiki, launchd ported to FreeBSD

Online Unix Manual References

• launchd(8): System wide daemon and per-user agent manager – Darwin and Mac OS X System Manager's Manual
• launchd.conf(5) – Darwin and Mac OS X File Formats Manual
• launchd.plist(5) – Darwin and Mac OS X File Formats Manual
• launchctl(1) – Darwin and Mac OS X General Commands Manual