18.7.2  Registering Caml closures for use in C functions

There are two ways for obtaining closures of OCaml functions.  First, one can pass the OCaml function as an argument to a primitive function.  For example, if the OCaml code contains a declaration like [external apply : (float -> float) -> float -> float = "ocaml_apply"], the C stub can be written as:

  CAMLexport value ocaml_apply(value vf, value vx)
  {
    CAMLparam2(vf, vx);
    CAMLlocal1(vy);

    vy = caml_callback(vf, vx);
    CAMLreturn(vy);
  }

Another possibility is to use the registration mechanism provided by OCaml: it allows to register OCaml functions under some global name, and then C code can retrieve the corresponding closure by this global name.

On the OCaml side, registration is performed by evaluating Callback.register n v. Here, n is the global name (an arbitrary string) and v the OCaml value. For instance:

    let f n = print_string "f is applied to "; print_int n; print_newline()


〚Note that "x" was changed to "n" in the previous line.〛

18.7.2  Obtaining Caml closures for use in C functions

There are two ways to obtain closures of OCaml functions.  First, one can pass the OCaml function as an argument to a primitive function.  For example, if the OCaml code contains the following declaration [external apply : ('a -> 'b) -> 'a -> 'b = "ocaml_apply"], the C stub can be written as:

  CAMLexport value ocaml_apply(value vf, value vx)
  {
    CAMLparam2(vf, vx);
    CAMLlocal1(vy);

    vy = caml_callback(vf, vx);
    CAMLreturn(vy);
  }

Another possibility is to use the registration mechanism provided by OCaml: it allows to register OCaml functions under some global name, which can then by used by the C code to retrieve the corresponding closure.

On the OCaml side, registration is performed by evaluating Callback.register n v. Here, n is the global name (an arbitrary string) and v the OCaml value. For instance:

    let f n = print_string "f is applied to "; print_int n; print_newline()


〚Note that "x" was changed to "n" in the previous line.〛