luabingen — C Header to LuaJIT FFI + OCaml Bindings

GitHub

Takes a C header file and generates three files:

• *_external.ml — OCaml external declarations (callable from OCaml)
• *_stubs.c — Dummy C stubs for ocamlc linking (never executed)
• *_bindings.lua — LuaJIT FFI bindings + value-converting wrappers

Build & Run

Requires OCaml 4.13+ with the str library.

ocaml -I +str str.cma luabingen.ml [--prefix PREFIX] <header.h>

# Example: strip "RLAPI " prefix from raylib functions
ocaml -I +str str.cma luabingen.ml --prefix "RLAPI " raylib.h

Output

*_external.ml

OCaml external declarations. One per C function:

external init_window : int -> int -> string -> unit = "init_window"
external draw_rectangle : int -> int -> int -> int -> int -> unit = "draw_rectangle"

C type mapping: - int, unsigned → int (tagged OCaml int) - float, double → float (boxed OCaml float) - bool → bool - void → unit - const char * → string - Structs, enums, pointers → int (opaque)

*_stubs.c

Empty C functions — needed only so ocamlc can link the bytecode. The lua_of_ocaml compiler replaces these calls with direct Lua calls.

#include <caml/mlvalues.h>
CAMLprim value init_window(value v1,value v2,value v3) {... return Val_unit;}
CAMLprim value draw_rectangle(value v1,value v2,...) {... return Val_unit;}

*_bindings.lua

LuaJIT FFI declarations and wrapper functions that convert OCaml values to C values before calling the C function:

local ffi = require("ffi")
ffi.cdef([[
  void InitWindow(int w, int h, const char *title);
  void DrawRectangle(int x, int y, int w, int h, Color color);
]])

function init_window(a1, a2, a3)
  return C.InitWindow(ocaml_int(a1), ocaml_int(a2), ocaml_string(a3))
end

OCaml value conversion helpers are included: - ocaml_int(v) — tagged int (/2) to Lua number - ocaml_float(v) — boxed float {253, value} to Lua number - ocaml_string(v) — identity passthrough

Putting It Together

# Generate bindings from header
ocaml -I +str str.cma luabingen.ml --prefix "RLAPI " raylib.h
# -> raylib_external.ml, raylib_stubs.c, raylib_bindings.lua

# Write your OCaml game using the generated externals
echo 'open Raylib_external' > game.ml
echo 'let () = init_window 600 400 "hello"' >> game.ml

# Compile with C stubs, run through lua_of_ocaml
ocamlc -c raylib_stubs.c
ocamlc -c game.ml
ocamlc -custom -o game.byte raylib_stubs.o game.cmo
loo.sh game.byte -o game.lua

# game.lua requires the generated _bindings.lua + raylib shared library
cat raylib_bindings.lua game.lua > main.lua
luajit main.lua