Re: [Caml-list] Dynlink plugin reevaluates modules of main program

["Nicolás Ojeda Bär" <nicolas.ojeda.bar@lexifi.com>]

Dear Frédéric,

The reason for this behaviour is that `ocamlbuild` is linking b.cmx
(and c.cmx for that matter) into your plugin, when in fact you want it
to use the copy of B (and C) linked to your main program.
You can see the right behaviour if you build your plugin by hand, e.g. by doing

  ocamlopt -shared d.cmx d.cmxs

and run a.native.

If I remember correctly you need to use a .mldylib or .mllib file to
tell `ocamlbuild` explicitly which modules to link into your plugin
(in your case, just B). Someone more knowledgeable about `ocamlbuild`
may want to chime in.

Hope it helps!

Best wishes,
Nicolás



On Thu, Nov 8, 2018 at 11:05 AM Frédéric Fort
<frederic.fort@univ-lille.fr> wrote:
>
> Hello,
>
> I have an existing program and would like to allow to extend it's functionalities with plugins.
> If I simplify my code structure it looks as follows:
>  - a.ml : "main module" of the program
>  - b.ml : additional definitions used in a.ml
>  - c.ml : interface for plugins (a collection of function refs)
>  - d.ml : plugin I would like to load
>
> Now, d.ml uses values defined in b.ml. Some of them are of type string ref
> and it seems that the code of b.ml is reevaluated when I call
> Dynlink.loadfile "/path/to/d.cmxs" which resets them to the empty string.
>
> Is there a way to prevent this from happening ?
> Using allow_only and prohibit is not an option, since multiple plugins would each reevaluate C
> and undo each others modifications.
>
> Yours sincerely,
> Frédéric Fort
>
> P.S.: Here follows a minimal working example.
> I compiled it with
> ocamlbuild -use-ocamlfind -lib dynlink a.native
> ocamlbuild -use-ocamlfind d.cmxs
>
> a.ml:
> open Format
>
> let _ =
>   B.str := "abc";
>   printf "%s\n" !B.str;
>   begin
>     try
>       Dynlink.loadfile "./_build/d.cmxs"
>     with Dynlink.Error err ->
>       failwith (Dynlink.error_message err) end;
>   printf "%s\n" !B.str;
>   match !C.f with
>   | Some(f) -> printf "%s\n" (f 0)
>   | None -> ()
>
> b.ml:
> let str = ref ""
>
> c.ml:
> let f : (int -> string) option ref = ref None
>
> d.ml:
> let _ =
>   C.f := Some((fun x -> !B.str^(string_of_int x)))
>
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