You shouldn't have to package up a non-caml managed pointer in any
sort of caml structure at all.  Take a look at section 18.6 of the
manual, specifically the definitions of curses_initscr and
curses_wrefresh.  Any pointer that's outside the caml heap (that is,
pretty much any pointer that you're getting from a non-caml function)
can simply be treated as opaque, and you can use Caml's type system to
make sure it's valid (as long as the C code always handles these
pointers correctly.)

I'd write your code like the following, based on that:

Caml code:
type sat_manager
external zchaff_InitManager : unit -> sat_manager
external zchall_ReadCnf : sat_manager -> string -> unit

C++ code:
value zchaff_InitManager(void)
{
  CAMLparam0();
  CAMLreturn((value) SAT_InitManager());
}

void zchaff_ReadCnf(value mng, value filename)
{
  CAMLparam2(mng, filename);
  SAT_Manager solver = (void*)mng;
  cout<<"solver = "<<hex <<solver <<endl;
  assert(solver != NULL);  
  char * fn = String_val(filename);  
  cout<<"file = "<<fn <<endl;
  read_cnf(solver, fn);
  CAMLreturn0;
}

One thing I wonder about, though, is the line:

  SAT_Manager solver = (void*)mng;

shouldn't you be casting to something other than (void*) here?  Not
that I know anything about how the type SAT_Manager is represented.

John.

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