Linking OCaml for Godot 4.2

Learning
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1

Hi there – first time poster.

Short question: I’d like to link a .ml library program to a standalone C program into a shared library. (If it helps, I would happy to start with just gcc on linux). Is there documentation on how I might do this, or does anyone know?

Long question: I want to create a language interface for OCaml in the Godot game engine. To do this, you need to provide a function with the following signature:

GDExtensionBool
my_entry_function(
  GDInterfaceGetProcAddress p_get_proc_address /* pointer to function */,
  GDExtensionClassLibraryPtr p_lib /* ptr to Godot class library */,
  GDExtensionInitialization* r_initialization 
    /* ptr to a defined struct type that contains pointers to functions
       that do the actual initialization of your plugin at various
       "levels" of bringing up the Godot engine. */
  );

Which then I can tell Godot to load via a text file on startup.

Failure so Far

I am able to link an executable, but not a shared library with dune or manually. I feel like I’m close, however; Godot can find the startup function and dynamically links to the OCaml runtime (I can tell because it used to not!). However, it says it’s not a shared library (which I agree with) and so can’t load it. Alternatively, I can build it as an object file (seen below), and it claims it’s not dynamic (which I also agree with). Here is the actual error message in the second case [from Godot]:

ERROR: Can't open dynamic library: /home/fizzixnerd/src/toycaml/libhello-gdextension.so. Error: /home/fizzixnerd/src/toycaml/libhello-gdextension.so: only ET_DYN and ET_EXEC can be loaded.
   at: open_dynamic_library (drivers/unix/os_unix.cpp:660)
ERROR: GDExtension dynamic library not found: /home/fizzixnerd/src/toycaml/libhello-gdextension.so
   at: open_library (core/extension/gdextension.cpp:719)
ERROR: Failed loading resource: res://hello.gdextension. Make sure resources have been imported by opening the project in the editor at least once.
   at: _load (core/io/resource_loader.cpp:274)
ERROR: Error loading extension: res://hello.gdextension
   at: load_extensions (core/extension/gdextension_manager.cpp:234)
ERROR: Can't open dynamic library: /home/fizzixnerd/src/toycaml/ocaml/godotcaml/c/libhello-gdextension.so. Error: libcamlrun_shared.so: cannot open shared object file: No such file or directory.
   at: open_dynamic_library (drivers/unix/os_unix.cpp:660)
ERROR: GDExtension dynamic library not found: /home/fizzixnerd/src/toycaml/ocaml/godotcaml/c/libhello-gdextension.so
   at: open_library (core/extension/gdextension.cpp:719)
ERROR: Failed loading resource: res://ocaml/godotcaml/c/hello.gdextension. Make sure resources have been imported by opening the project in the editor at least once.
   at: _load (core/io/resource_loader.cpp:274)
ERROR: Error loading extension: res://ocaml/godotcaml/c/hello.gdextension
   at: load_extensions (core/extension/gdextension_manager.cpp:234)

Details of What I Have

I have something that looks like

  • godotcaml
    • dune-project
    • godotcaml.opam
    • bin
      • dune
      • gdextension_interface.h (* a C header I include in the C src below *)
      • hello-gdextension.c (* a C source file that contains the entry point the plugin expects; calls OCaml from here *)
      • main.ml (* Where I do all my real work in OCaml *)

My dune file looks like

(executable
 (name main)
 (modes (native object))
 (libraries godotcaml ctypes.foreign ctypes)
 (foreign_stubs (language c) (names hello-gdextension))
 (flags (:standard -g -cclib "-L/usr/lib/x86_64-linux-gnu/")))

the library godotcaml is the main library interface for OCaml for Godot I am writing; it shouldn’t really figure into this, I don’t think. It can be statically linked in.

dune build creates a file main.exe.o, and I copy that to where Godot can see it as libhello-gdextension.so. Yup.

So it’s not exactly surprising it doesn’t work, but I think I’m pretty close. Any help would be appreciated.

PS is there a matrix or IRC for ocaml I could join?

5 Likes
2

Do you have a demo?

I would start from this How to wrap C functions to OCaml and an official manual OCaml - Interfacing C with OCaml after that.

1 Like
3

I don’t have proper network connectivity to reply in full, but this is definitely doable. You should use (modes shared_object) in Dune to get it to produce a DLL from the OCaml code (and any included C stubs).

Cheers,
Nicolas

1 Like
4

This seems promising. Note the for future explorers, the actual stanza is (modes (native shared_object)), I believe, as the (modes shared_object) seems to complain (while the other does not). I will report back with experimentations. Thanks, Nicolas

5

That did the trick! Here is the output:

Initializing extension from OCaml!
ERROR: Can't open dynamic library: /home/fizzixnerd/src/toycaml/ocaml/godotcaml/c/libhello-gdextension.so. Error: libcamlrun_shared.so: cannot open shared object file: No such file or directory.
   at: open_dynamic_library (drivers/unix/os_unix.cpp:660)
ERROR: GDExtension dynamic library not found: /home/fizzixnerd/src/toycaml/ocaml/godotcaml/c/libhello-gdextension.so
   at: open_library (core/extension/gdextension.cpp:719)
ERROR: Failed loading resource: res://ocaml/godotcaml/c/hello.gdextension. Make sure resources have been imported by opening the project in the editor at least once.
   at: _load (core/io/resource_loader.cpp:274)
ERROR: Error loading extension: res://ocaml/godotcaml/c/hello.gdextension
   at: load_extensions (core/extension/gdextension_manager.cpp:234)
0
Done initializing from OCaml! Returning 1...Error: Can't run project: no main scene defined in the project.
0

I should be able to figure out the rest from here. Cheers again, Nicolas.

6

This thread got its answer already, but if anyone ends up here and doesn’t want to do the whole thing by hand, they could have a look at ctypes reverse bindings which I believe enable such use case.

1 Like
7

Has links to a Matrix Space & IRC. No XMPP MUC tho.