> So I am investigating packing my nice Ocaml code into a library and
> writing C bindings so that I can talk to it. [...]
> In particular, I have found
> none that cover how to obtain pointers to OCaml function return values
> that are not strings or ints, how to store them in C-land, and how to pass
> them back to Ocaml as parameters.

You have two choices: copy the contents of the OCaml data structure to
a C data structure, e.g. an OCaml tuple or record of ints can be
copied to a C struct, field by field; or, treat the OCaml value as an
opaque type on C's side.

The code you posted attempts to do the second approach, but you cannot
just take a Caml "value" and give it to arbitrary C code: the Caml GC
will lose track of the value and reclaim it (or copy it elsewhere) at
the first opportunity.

Instead, you need to wrap the Caml value in a C memory block and
register it with the GC using caml_register_global_root.  Then, an
explicit free function must be provided in the C interface to
unregister with the GC and to free the wrapper.  See the
attached modification of your ht_wrap.c example.  It should put you on
the right tracks.

> 5. And this is how I am attempting to compile it.  Note that error I'm
> getting here is in link.  If I add "-lm" it gets less noisy but still is
> mad at me. Why am I having to add math library?

Because the Caml runtime system needs it.  As Dmitry wrote, the
simplest way to build your example is:

ocamlopt -output-obj -o ht_lib.o ht.ml
ocamlopt -c ht_wrap.c
gcc -o htt ht_test.c ht_lib.o ht_wrap.o -L`ocamlopt -where` -lasmrun -lcurses -lm

Hope this helps,

- Xavier Leroy