Daemons have become pretty confusing on Mac OS X. Originally these were handled by inetd, then xinetd, and now launchd has taken over. Unfortunately, there's a mess of left over stuff hanging around: you'll still find services in /etc/xinet.d and other places.
No longer true as of Snow Leopard. You don't have an xinetd.d directory and although you may still have leftover rc.common and rc.netboot files, they are bnot used. Launchd has taken over.
The goal of launchd is to replace xinetd, cron, and anything you might have stuck in rc.local or StartupItems. It controls system programs, but it can be used for ordinary user processes also.
Anything you want controlled by launchd needs a plist file describing what to do if you want it to start automatically, but you can use it to daemonize a specific command also:
$ launchd -d -- ~/bin/ltest
However, by using configuration files, you can use "launchctl" to more powerfully control your jobs. There's an interactive mode - if you just type "launchctl" you can then control daemons from its shell:
launchd% list com.apple.KernelEventAgent com.apple.mDNSResponder com.apple.nibindd com.apple.periodic-daily com.apple.periodic-monthly com.apple.periodic-weekly com.apple.portmap com.apple.syslogd com.vix.cron org.postfix.master org.xinetd.xinetd com.kerio.watchkms launchd%
See tftp and launchctl on MacOSX for an example of using launchct.
See the man pages for launchd and launchctl for details. You can see system launchd plist files in /System/Library/LaunchDaemons/; use these to understand how you'd write your own.
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