It's not hard, really, just tedious. Unfortunately,
the Caml IDL tool only generates wrapper code for
calling C from O'Caml, not vice versa, so we
write that code by hand.   In short:

C_API_stub.c - contains our C API. This code handles 
  converting C values to O'Caml values, setting up
  actual arguments to O'Caml functions, then calling
  the O'Caml callback functions.  There's some messy
  memory/heap management code in here too

Caml_API_stub.ml - contains wrapper functions that
  are called from C API.  These functions are registered
  using the Callback module.  They call our real Caml API.
  
We create a shared library that contains our Caml library,
the C_API stub, and the O'Caml runtime system.  (IT's BIG!)
And that gets linked with the user's C program.

I hope that makes sense.
Mary
On Wed, 2005-02-16 at 20:19, skaller wrote:
> On Thu, 2005-02-17 at 11:40, Mary Fernandez wrote:
> 
> > Galax provides APIs for O'Caml, C, and Java and is implemented in
> > O'Caml.
> 
> I'm curious how you provide a CAPI for Ocaml code?
-- 
Mary Fernandez <mff@research.att.com>
AT&T Labs - Research