[launchd-dev] launchd check-in: Where's the documentation?

[Sidney San Martín]

I can't find any documentation on the launchd checkin process. Using
SampleD, launch.h, and other people's documentation of their
discoveries it's possible to put together a working daemon, but with
no guarantee that you're doing everything right. (Even though launchd
is open source, its APIs need documentation.)

launchd.plist(5) is the first place I even saw the check-in process
mentioned: "The job must check-in to get a copy of the file
descriptors using APIs outlined in launch(3)". launch(3) doesn't exist
(<rdar://problem/7130390>). It seems like the following, at least,
should be documented:

1) What check-in is and when it is necessary
2) The consequences of a job not checking in when launchd expects it
3) What keys in the response dictionary are filled in at runtime
4) How to memory manage launch data returned by check-in functions
(i.e. what needs to be released)
5) Error conditions and how to handle them

What do you think? Has this come up before? Is there some secret store
of documentation I'm missing? Is this API so intuitive that
documentation is redundant?