CREATE TYPE

Can someone point me at some detailed instructions for creating new
datatypes..

I've found quite a few web pages that mention it (in passing) and give brief
examples - but nothing much I can actually work with for my purposes..

Ideally I'd like to use C as the language and the datatype will need an
'extent' (like 'char' can be char(10) - although its nothing like a char
field...)

I'm targetting this at the new 7.4 - so I think i need to use 'version 1'
method (using Datum etc ?)

Also - the datatype itself is a comlex type which stores half a dozen
different integers (Its a modified datetime - storing the year, month, day,
hour, minute, second, but the extent gives it the ability to do YEAR TO DAY,
HOUR TO SECOND etc, so you only get/set the relevant sections).
When 'selected' it would return a variable length string containing the
relevant data, and would be set by passing in a string (some of the data of
which may well be ignored if its outside the extent of the column etc)

Hope thats enough - all pointers greatfully received..
(and free'd when required :)