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Concept
Launchd
Summary
launchd is an init and operating system service management daemon created by Apple Inc. as part of macOS to replace its BSD-style init and SystemStarter. There have been efforts to port launchd to FreeBSD and derived systems.
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 macOS 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. launchd itself has no knowledge of these configuration files or any ability to read them - that is the responsibility of "launchctl".
launchctl is a command line application which talks to launchd using IPC and knows how to parse the property list files used to describe launchd jobs, serializing them using a specialized dictionary protocol that launchd understands. launchctl can be 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 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):
Open Firmware activates, initializes the hardware, and then loads BootX.
BootX loads the kernel, spins the pinwheel cursor, and loads any needed kernel extensions (kexts).
The kernel loads launchd.
launchd runs /etc/rc, various scripts which scan through /System/Library/LaunchDaemons and /Library/LaunchDaemons, calling launchctl on the plists as needed, then launchd starts the login window.
Official source
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