launchd(8) System Manager's Manual launchd(8)

launchdSystem wide and per-user daemon/agent manager

launchd manages processes, both for the system as a whole and for individual users.

The primary and preferred interface to launchd is via the launchctl(1) tool which (among other options) allows the user or administrator to load and unload jobs. Where possible, it is preferable for jobs to launch on demand based on criteria specified in their respective configuration files.

launchd also manages XPC services that are bundled within applications and frameworks on the system.

During boot launchd is invoked by the kernel to run as the first process on the system and to further bootstrap the rest of the system.

You cannot invoke launchd directly.

On Darwin operating systems, the canonical way to launch a daemon is through launchd as opposed to traditional POSIX and POSIX-like mechanisms or mechanisms provided in earlier versions of OS X. These alternate methods should be considered deprecated and not suitable for new projects.

In the launchd lexicon, a daemon is, by definition, a system-wide service of which there is one instance for all clients. An agent is a service that runs on a per-user basis. Daemons should not attempt to display UI or interact directly with a user's login session. Any and all work that involves interacting with a user should be done through agents.

XPC services which are marked with a ServiceType of System are functionally equivalent to daemons and run in the same environment, sharing the same Mach bootstrap namespace. XPC services which are marked with a ServiceType of User are equivalent to agents with the LimitLoadToSessionType key specifying the Background session and run in the same environment, sharing the same Mach bootstrap namespace. See launchd.plist(5) for more information about user sessions.

On Darwin platforms, a user environment includes a specific Mach bootstrap subset, audit session and other characteristics not recognized by POSIX. Therefore, making the appropriate setuid(2) and setgid(2) system calls is not sufficient to completely assume the identity for a given user. Running a service as a launchd agent or a per-user XPC service is the only way to run a process with a complete identity of that user.

Per-user agents provided by the user.
Per-user agents provided by the administrator.
System-wide daemons provided by the administrator.
Per-user agents provided by Apple.
System-wide daemons provided by Apple.

launchctl(1), launchd.plist(5),

The Daemons and Services Programming Guide can be found at the following URL:

25 November, 2013 Darwin