Is there a good guide to writing rlm modules

[David Goodenough]

I am a newbie to Radius, and need to implement some function that is not 
easily done by any of the existing modules.

So I want to write my own, and being a Java programmer I chose JRadius
as well we FreeRadius.  I wrote a module but it is not taking my 
acceptance of the user i the authorize step, but rather send a rejection
to the post_auth step.  Obviously I am missing something, so I thought
that some documentation on writing modules would be good reading.

Anyone know where I can find some?

Thanks

David