Re: [Caml-list] Static linking via Ctypes?

[Jeremy Yallop <yallop@gmail.com>]

I wrote:
> Not exactly, since the stub generation support in the next ctypes
> release resolves these issues in a different way.  In the current
> release (0.2.3) setting up foreign calls and linking are both entirely
> dynamic.  Stub generation will make it possible to link statically as
> well.

Milan Stanojević asked:
> Can you tell us how would this work exactly?
> Let's say I have a function "foo" in my C file (that I want to link
> statically) and I want to give use it in ocaml with a ctype (void @->
> returning int)
> What would I need to say in my ocaml program and do I have to tell
> anything to my build system?

With the current release of ctypes we'd bind 'foo' like this:

   let f = foreign "foo" (void @-> returning int)

The 'foreign' function searches for the symbol "foo" and sets up the
call; the return value is an OCaml function that we can call
immediately.

   val f : unit -> int

With stub generation as implemented in the git master branch the call
to 'foreign' changes slightly.  Instead of calling 'foreign' just
once, we're going to call it once for each stage of generation.  The
way we do this is to abstract over 'foreign' with a functor.  (Note
the similarity to the original binding of 'f' above.)

  module Bindings (F : Cstubs.FOREIGN) =
  struct
     let f = F.foreign "foo" (void @-> returning int)
  end

The first application of the functor generates C:

  Cstubs.write_c std_formatter ~prefix:"foo_" (module Bindings)

The output is a wrapper for the C function 'foo' that marshals and
unmarshals arguments and return values as needed.

The second application generates OCaml:

  Cstubs.write_ml std_formatter ~prefix:"foo_" (module Bindings)

The output is an OCaml module that we can pass to 'Bindings' for the
third and final application, which links together the declaration of
'f' and the generated code:

  Bindings(Generated_ML)

As with the dynamic binding, this gives us an OCaml function that we
can call without further ado:

  val f : unit -> int

The build process is reasonably straightforward.  Since we're
generating code, there are more steps than with the dynamic version,
but we're less likely to need obscure linker flags.  A typical
approach is to place the bindings functor in one file --
foo_binding.ml, say:

   module Bindings(F : Cstubs.FOREIGN) =
   struct
     let foo = F.foreign "foo" Ctypes.(void @-> returning int)
   end

and the calls to 'write_c' and 'write_ml' other files.  I'll put the
call to 'write_c' along with code to print out the headers used in
generate_c.ml:

   let () =
     print_endline "#include <ctypes/cstubs_internals.h>";
     print_endline "#include \"foo.h\"";
     Cstubs.write_c Format.std_formatter ~prefix:"foo_"
       (module Foo_binding.Bindings)

The call to 'write_ml' goes in generate_ml.ml:

   let () = Cstubs.write_ml Format.std_formatter ~prefix:"foo_"
     (module Foo_binding.Bindings)

Then we can compile and run the two programs to generate the stub code:

  $ ocamlfind ocamlc -c -package ctypes.stubs \
     foo_binding.ml generate_c.ml generate_ml.ml
  $ ocamlfind ocamlc -o generate_ml.native -linkpkg -package ctypes.stubs \
     foo_binding.cmo generate_ml.cmo
  $ ./generate_ml.native > generated.ml
  $ ocamlfind ocamlc -o generate_c.native -linkpkg -package ctypes.stubs \
     foo_binding.cmo generate_c.cmo
  $ ./generate_c.native > generated_c_stubs.c

We can then compile the stub code and link it in with the bindings
functor and the library containing 'foo':

  $ ocamlfind ocamlc -c -package ctypes.stubs -I `ocamlc -where`/.. \
     generated_c_stubs.c generated.ml
  $ ocamlfind ocamlmklib -o foo -linkpkg -package ctypes.stubs \
     generated_c_stubs.o generated.cmo foo_binding.cmo -L. -lfoo

Finally, we can load the code and call the 'foo' function:

   $ ocaml -short-paths
           OCaml version 4.01.0

   # #use "topfind";;
   [...]
   # #require "ctypes.stubs";;
   [...]
   # #load "foo.cma";;
   # include Foo_binding.Bindings(Generated);;
   val foo : unit -> int = <fun>
   # foo ();;
   foo called
   - : int = 3

The full stub generation interface (which is actually just the
'write_ml' and 'write_c' functions) is here:

   https://github.com/ocamllabs/ocaml-ctypes/blob/1e1634/src/cstubs/cstubs.mli

Jeremy