Registering OCaml values with the C runtime.
This module allows OCaml values to be registered with the C runtime under a symbolic name, so that C code can later call back registered OCaml functions, or raise registered OCaml exceptions.
Callback.register n v registers the value v under the name n. C code can later retrieve a handle to v by calling caml_named_value(n).
Callback.register_exception n exn registers the exception contained in the exception value exn under the name n. C code can later retrieve a handle to the exception by calling caml_named_value(n). The exception value thus obtained is suitable for passing as first argument to raise_constant or raise_with_arg.