Joel Reymont wrote:
> Is there an example of returning a variant from OCaml into C somewhere?
> It's a regular variant, declared like this
> 
> type morpher_output =
>   | Success of string
>   | Error of string * int * int * int

You can read the section 18.3.4 of 
http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html

Then if still unclear, examine: http://oracaml.sourceforge.net/
or any other C-interface library available on Caml Hump: 
http://caml.inria.fr/cgi-bin/hump.en.cgi?sort=0&browse=65

> The other issue is that I don't know how to deal with strings allocated 
> in OCaml. Will they be  collected automatically?

If you are using caml_copy_string() or caml_alloc_string() they allocate 
strings on OCaml's heap, and will be automatically garbage collected.

> The strings _must_ be allocated in OCaml but I can guarantee that they 
> will not be modified in the C code. Any suggestions on how to deal with 
> this?

Unless I am not understanding your question, the above two functions are 
the way to allocate strings on OCaml's heap.

Serge